This study is conducted in the Kordofan region, central-southern Sudan. The study area is bounded by longitudes 28° 00' and 31° 00' E, and latitudes 11° 30' and 15°00' N and covers an area of about 125835 km 2 . The Kordofan region is located at the southern end of the presentday Sahara. Late Pleistocene-Holocene deposits in Kordofan, recorded sedimentary discontinuities that probably reflect climatic fluctuations during the latest Quaternary. 14 C dating and some archeological findings have been used to date the latest Pleistocene-Holocene succession. In the investigated sections, four stratigraphic units have been recognized, which exhibit eight sedimentological facies.

First of all, I would like to thank my supervisor Prof. Etienne Jaillard for initiating this fantastic research project within the ISTerre and École doctorale : Terre, Univers, Environnement (TUE), of the Université Grenoble Alpes. His broad knowledge and special way to look at data from an interdisciplinary point of view was most inspiring. I really appreciate his efficient work during all stages of this project, significantly improving whatever I wrote and always within an incredible short time. This efficiency was extremely helpful during the last months while my thesis was developing.

The first unit is older that ≈ 10 ky BP and is formed of mottled sandstone or siltstone facies of aeolian origin. The second unit (≈ 10 to 6 ky BP) comprises palustrine and lacustrine facies in the central and northern parts, and fluviatile facies in the South. The third unit ranges from ≈ 6 to 3 ky BP and is restricted to the southern part; it is made of aeolian deposits intercalated with fluviatile or flood plain facies. The fourth unit is younger than 1000 y BP, and is dominated by aeolian red sandstone in the North, and by flood plain facies to the South. Between ≈ 6 and 1 ky BP in the North and between ≈ 3 and 1 ky BP in the South, no deposits are recorded. This hiatus is marked by deflation surfaces in the North, and is interpreted as a period of strong aeolian activity, which prevented deposition, or even eroded part of the sediments deposited between 6 and 3 ky BP.

Several proxies (sedimentology, gastropod sub-fossil shells, pollens, stable isotopes, major element chemistry, clay mineralogy and paleohydrology) have been used to reconstruct the climatic evolution of the region for the past 13 ky, which can be correlated to the well-known evolution of Eastern Sahara during this time-span. Our results indicate that the region has been subjected to arid climate prior to 10 ky BP as evidenced by thick aeolian deposits. Between 10 and 6 ky BP, the region experienced a wet climate as evidenced by the development of pedogenetic calcareous nodules, local deposition of palustrine and lacustrine limestone, abundance of aquatic and semi-aquatic gastropods, high lake levels, and depleted δ 18 O values from gastropod shells and calcareous nodules. After ≈ 6 ky BP, climate evolved to dry conditions in the northern part of the region as indicated by an strong aeolian activity recorded by erosion features, sedimentary hiatus and deflation surfaces, while its southern part remained more humid, as shown by channel and flood plain deposits, and tropical and aquatic pollen taxa. From 1000 yr BP to Present, and probably after 3 ka BP, the region became arid as evidenced by aeolian deposits, the predominance of arid indicator pollens and the enrichment in 18 O of land snail shells.

RÉSUMÉ

Cette étude concerne la région du Kordofan, dans le centre-Sud du Soudan. La région étudiée est comprise entre les méridiens 28°00' et 31°00' E, et les latitudes 11°30' et 15°00' N, et couvre une surface d'environ 125 835 km 2 . Le Kordofan est situé en bordure Sud du Sahara actuel. Les dépôts du Pléistocène supérieur-Holocène du Kordofan, ont enregistré des discontinuités sédimentaires qui reflètent des fluctuations climatiques au cours du Quaternaire terminal. Des datations au 14 C et des collectes archéologiques ont permis de dater la série étudiée. Quatre unités stratigraphiques ont été identifiées, qui comprennent huit faciès sédimentaires.

La première unité est antérieure à ≈ 10 ka BP et est formée de sables ou silts marmorisés d'origine éolienne. La deuxième unité (≈ 10 à 6 ka BP) comprend des faciès palustres et lacustres au Nord et au centre, et des faciès fluviatiles au Sud. La troisième unité, d'âge ≈ 6 à 3 ka BP, n'est présente qu'au Sud; elle est faite de dépôts éoliens intercalés de faciès fluviatiles ou de plaine d'inondation. La quatrième unité est plus jeune que 1000 ans BP, et est dominée par des sables rouges éoliens au Nord, et par des dépôts de plaine d'inondation au Sud. Entre ≈ 6 et 1 ka BP au Nord et entre ≈ 3 et 1 ka BP au Sud, un hiatus est marqué par une surface de déflation au Nord, et est interprété comme une période de forte activité éolienne, qui a empêché le dépôt et a même érodé une partie des sédiments déposés entre 6 et 3 ka BP.

Plusieurs indicateurs (sédimentologie, gastéropodes, pollens, isotopes stables, géochimie des majeurs, minéralogie des argiles et paléo-hydrologie) ont servi à reconstituer l'évolution climatique de la région depuis 13000 ans, qui est cohérente avec l'évolution paléo-climatique de l'Afrique nord-orientale à la même époque. Nos résultats montrent que la région a connu un climat aride avant 10 ka BP, matérialisé par d'épais dépôts éoliens. Entre 10 et 6 ka BP, un climat humide est démontré par le développement de nodules calcaires pédogénétiques, le dépôt local de calcaires palustres et lacustres, l'abondance de gastéropodes aquatiques et semiaquatiques, le haut niveau de lacs, et les valeurs très négatives du δ 18 O des coquilles d'escargots et des nodules calcaires. Après ≈ 6 ka BP, le climat s'assèche dans le Nord de la région étudiée comme le suggèrent la forte activité éolienne enregistrée par un hiatus sédimentaire, des érosions et surfaces de déflation, alors que le Sud reste plus humide, comme le montrent des dépôts fluviatiles (chenaux et plaine d'inondation), et les pollens tropicaux et aquatiques. Après 1000 ans BP, et plus probablement après 3 ka BP, la région devient aride comme en témoignent les dépôts éoliens, la prédominance de pollens de milieu aride et l'enrichissement en 18 O des coquilles d'escargots terrestres.

3.5.4.

Conclusion of the grain size analysis 3.6.

Sedimentary and paleogeographic evolution 3.6.1.

Sedimentary evolution 3.6. 1.1. Sedimentary evolution of the Dilling area 3.6. 1.2. Sedimentary evolution of the Abu Zabad -El Fula area 3.6. 1.3. Sedimentary evolution of the El Obeid -South El Obeid area 3.6. 1.4. Sedimentary evolution of the En Nahud -El Khowei area 3.6. 1.5. Sedimentary evolution of the Sodari -North Bara area 3.6.2.

Sedimentation and paleogeography for the ≈ 13000 -10000 yr BP period 3.6.3. Sedimentation and paleogeography for the ≈ 10000 -6000 cal. yr BP period 3.6.4. Sedimentation and paleogeography for the ≈ 6000 -3000 yr BP period 3.6.5. Sedimentation and paleogeography for the ≈ 3000 -1000 yr BP period Summary of the grain-size percentage for all sections Table 3.9

LIST OF TABLES

Grain size percentage for the red sandstone facies Table 3.10 horizontal grain size distributional percentage for the upper part of the mottled facies Regional climatic zones around Sudan 3 Fig. 1.3 Mean annual rainfall across Kordofan Region 3 Fig. 1.4 Geomorphological map of the study area 5 Fig. 1.5 Digital elevation model of the Kordofan Region 6 Fig. 1.6 Detailed digital elevation model of the study area 6 Fig. 1.7 Relief of the study area 7 Fig. 1.8 Distribution of pre-Quaternary gelogical units in the Kordofan Region Fig. 1.9 General distribution of surficial deposits in the Kordofan Region Fig. 1.10 Current climatic zones of Africa Fig. 1.11 Schematic of the general patterns of winds and pressure over Africa Fig. 1.12 Distribution map of reconstructed lake levels across Africa, 9,000 years ago relative to today Position of dated samples in the El Ga'ah well Fig. 3.16 Time-correlation transect between El Fula and Dilling Fig. 3.17 Time-correlation transect between En Nahud and El Obeid Fig. 3.18 Time-correlation transect between El Ga'ah and Dilling Fig. 3.19 Mottled facies examples Fig. 3.20 Nodular calcrete or calcareous nodules View of the El Ga'ah delta facies Fig. 3.36 Details of the delta facies Fig. 3.37

LIST OF FIGURES

Location map of the sections studied for grain size analysis Fig. 3.38 Lithology of the studied sections and location of the samples analyzed for grain size analysis Fig. 3.39 A NE-SW topographic profile showing the gradient in the study area Fig. 3.40 Cumulative grain size distribution zones of all sample's curves Fig. 3.41 Representative frequency curves of the studied sections Fig. 3.42

Graphical statistic plots against depths for the north Bara3 section Fig. 3. 43 Graphical statistic plots against depths for the north Bara1 section Fig. 3. 44 Graphical statistic plots against depths for the west El Obeid section Fig. 3. 45 Graphical statistic plots with depth for the east Abu Zabad section Fig. 3.46 Representative Cumulative Probability Curves for the whole area Fig. 3.47

Representative Cumulative probability curves for the different sites Fig. 3.48 Grain-size distribution diagram for all samples from all sections Fig. 3. 49 Vertical evolution of grain size percentages and their interpretation Fig. 3.50 NNE-SSW distribution of the grain size percentages and interpretations Mean grain size and SiO2/Al2O3 plots for the West El Obeid section.

I. INTRODUCTION 1.1. Preface

The Quaternary period covers the last two and an half million years of the Earth history and is well known as the period of ice ages. The Quaternary is characterized by repeated cycles of cold -warm climatic oscillations as the Earth's climate was submitted to alternating glacial and interglacial modes. Associated with the climatic fluctuations and advancing and retreating continental ice sheets, there were dramatic responses of the biota, rivers, lakes, oceans and other natural systems. More than 30 glacialinterglacial cycles are recorded in deep-sea sediment records spanning the last 2.6 my. The Quaternary period is divided into two epochs, the Pleistocene (2600 ka BP) and the Holocene (11.7 ka BP to Present). The late Pleistocene-Holocene epoch is marked by rapid short climatic fluctuations. As a result, the complex Late Quaternary environment (500 ka -Present) recorded rapid sedimentation changes. Therefore, the Quaternary sediments are considered a very important archive to determine environmental and climatic information.

Location

The study area is situated in central-southern Sudan, in the Kordofan Region, East of the White Nile valley and SW of Khartoum. It is bounded by longitudes 28°00' and 31°00'E, and latitudes 11°30' and 15°00'N and covers an area of about 125,835 km 2 (Fig. 1.1).

Climate

According to the classification of climates by [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF], Kordofan contains three climatic belts, which correspond to three major north-south zones in the sub-Saharan region of Africa, where the amount of rainfall and the duration of the rainy season increase toward the equator. The northern part of the region is a low-latitude desert, which merges southward into a belt of tropical steppe. Still farther south is a belt of tropical savanna. Figure 1.2 shows the regional climatic zones around Sudan.

The present-day climate of Kordofan region is variable from North to South. It varies between sub-tropical in Southern Kordofan, and poor savanna to arid in Northern part.

The rainy season in Southern Kordofan is longer than in Northern Kordofan. In Southern Kordofan, it begins in April and continues up to October with varying annual rainfall from 800 mm in the South to 400 mm in the North, while in Northern Kordofan the rainy season usually starts at the end of June and continues till October (92% of the total year precipitation in June-September in El Obeid), with approximate annual rainfall of 320 mm per year in El Obeid. The average annual rainfall increases southward, from approximately 200 mm per year in the northern part and approximately 600 mm per year in the southern part of the study area (Fig. 1.3). The difference between North and South is also reflected in the wind directions, with northerly and southerly winds dominating for six months each in Southern Kordofan, while the northerly winds dominate increasingly northward.

The mean annual air temperature of the province is 27°C (80°F), and temperature extremes of 10°C (50°F) and 46°C (115°F) are common to most areas. The monthly average temperature fluctuates between 40°C in summer and 22°C in winter. The mean relative humidity averages about 21% in the dry season, and 75% during the rainy season. The prevailing winds in the winter are from the north, but during the rainy season they are from the southwest. Wind velocities are usually less than 8 kilometers per hour (5 miles per hr) and frequent calms are not uncommon.

Fig. 1.2: Regional climatic zones around Sudan, and location of the study area.

Fig. 1.3: Mean annual rainfall across the Kordofan Region [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF].

Physiography

The physiographic features are variable in the study area, regarding topography, relief, drainage, soils and vegetation. Figure 1.4 summarizes the geomorphologic features in the study area with some extension to the East to include the White Nile. The climate and geology are the most important factors controlling this physiographic variabilities.

1.4.1. Topography Figure 1.5. is a general digital elevation model of the Kordofan region, while Fig. 1.6 provides more details on the study area. The northern part of the study area is characterized by a low lying, gently undulating surface topography, with few scattered, little elevated but sharp inselbergs mostly made of Paleogene rocks. This terrain is covered by extensive qoz (stabilized sand dunes), sand sheets, and N-S orientated sand dunes and stabilized sand dunes. The sand dunes decrease in size and abundance toward the south (Fig. 1.4).

The southern part is characterized by scattered steep massifs and inselbergs made of basement rocks, some of which are as high as 1300 m above sea level (J. Ed Dair). The plains are formed of dark-grey soils locally intercalated with some sands and sand dunes further north.

Relief and drainage system

The study area presents variable relief shapes. The northern part is dominated by topographic depressions of interdunes located between up to 15 m high sand dunes. The size of these interdunes differs according to differences in the dune amplitudes. In the Sodari and El Ga'ah areas, the sand dunes are large and this results in large interdunes, whereas around Bara, as well as north and east of El Obeid, the sand dunes are thinner and result in narrower interdunes, compared to those of Sodari-El Ga'ah area (Fig. 1.7). These dune and iterdune fields are surrounded by sand sheets.

The central part of the study area is generally marked by its flatness. Reliefs are dominated by smooth V-shaped valleys in the El Khowei-En Nahud, while to the East as well as around Khour Abu Habil, the relief is marked by wide wadies. The southern part is marked by deep V-shaped valleys and by some saddles between the mountains.

These variations in the relief result from variation in the drainage system. The Kordofan region lies within the drainage basin of the White Nile River. The northern part of the study area shows a poor drainage system due to the development of the north-south sand dunes forming barriers or dams crosscutting the ancient rivers (Figs. 1.4 and 1.7). Therefore, seasonal rivers (wadies) are quite rare and may form seasonal lakes behind the sand dunes, or vanish rapidly due to the infiltration of the running water in the sand dunes.

The dominance of sand dunes and sheets in the central part of the study area (En Nahud-El Khowei), results in a coarse dendritic drainage system [START_REF] Elmansour | Assessment of the groundwater of En Nuhud basin, West Kordofan State-Sudan[END_REF], with intensive infiltration. The surface water of this area is drained to the South through the El Fula area down to its delta around Muglad.

In the eastern part of the study area, surface water is drained through Khor Abu Habil to the White Nile. The southern part is marked by a well-marked drainage pattern due to more abundant rainfalls and to the clayey soils. Small wadies of this southern part form the tributaries of Khor Abu Habil, or flow southward toward the Sudd area.

Vegetation cover

In a general way, the vegetation type is determined by the climate, relief and type of soil. The variation in rainfall and soil nature from North to South resulted in varied types and density of the vegetation cover. The relief also leads to remarkable variations in the vegetation cover density, since vegetation is denser along the course of the wadies or the depression zones.

In the northern part of the study area, the vegetation cover ranges from sparse growth, droughtresistant grasses and dwarf scrub. The central part is covered by acacia and baobab trees; grassland and bushland are scattered mainly around wadies and khors, and form the bushlandwoodland transition. The southern part is mostly covered by edaphic grassland mosaics with trees.

1.5. Geology 1.5.1. Background [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF] provided a general summary of the geologic units in the Kordofan region. The oldest rocks in the region that now constitute the basement complex were formed in Precambrian times. Following the emplacement of these rocks the region was subjected to a period of prolonged erosion that apparently lasted through most of Paleozoic times. Shallow seas invaded parts of the region in the Late Paleozoic and deposited the sediments of the Nawa Series. Before the close of Paleozoic times, however, the region was uplifted and most of the Nawa sediments were removed by erosion. Only a few isolated remnants of the Nawa Series are now left in Kordofan as evidences of this once-extensive geologic unit.

Deposition of rock-forming materials in the area did not take place again until Mesozoic time when shallow continental seas covered much, if not all, the Province. During this time the clastic sediments of the Nubian Series were laid in continental or nearshore marine environments and over the eroded basement. Near the close of Mesozoic times, the sea receded as the region was again uplifted and then subjected to prolonged subaerial erosion that apparently lasted until Pliocene time. During this interval of erosion most of the Nubian rocks were stripped away and only those occupying the deeper basins in the basement rock surface were preserved. Extensive laterization of the Cretaceous sediments and possibly older rocks occurred during Peleogene time when climatic conditions favorable to laterite formation prevailed in much of the region. Tectonic movements in eastern Africa, probably during Neogene time, resulted in the formation of several structural basins in the Nubian and basement rocks of the Kordofan Province. During Pliocene and early Pleistocene times, these basins were filled with fluvial and lacustrine deposits that now make up the clastic Umm Ruwaba Formation. According to El Shafie et al. ( 2011), the Um Ruwaba Formation ranges from the Pliocene to Pleistocene.

Following deposition of the Umm Ruwaba Formation, several types of surficial deposits were laid down that now form a virtually continuous mantle over the Umm Ruwaba Formation and older rocks of Kordofan. In late Pleistocene time, the southern part of the Province (now Southern Kordofan State), was subjected to widespread and recurrent flooding. The floods, probably emanating from the Nile headwaters to the south, brought clay and silt and deposited these over most of the southern part of the region. Concurrent with flooding in the south, strong northerly winds prevailed in the northern part of the region (Northern Kordofan State now), and denuded the land surface of much of its residual soil cover. A considerable part of the eroded material was deposited as sand dunes over the central part of the region. The residual soils in the north, stabilized sand dunes (Qoz) in the central part of the Province, and clay-plains deposits in the South remain today as evidence of the climatic conditions since late Pleistocene time [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF].

1.5.2. Geological units in the study area

Basement Complex

The name Basement Complex is used to include those igneous, metamorphic and sedimentary rocks that are overlain by horizontal and sub-horizontal, Paleozoic or Mesozoic sedimentary or igneous rocks [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF]. It is considered the most extensive formation in Sudan, excluding the superficial deposits [START_REF] Whiteman | The Geology of the Sudan Republic[END_REF].

Although the basement rocks cover a great part of the study area, they are poorly exposed in the northern parts, while they crop out more extensively in the southern part. The basement outcrops comprise a variety of rock types. Gneisses and migmatites, amphibolites, various types of schists, marbles and calc-silicate rocks, gabbros, syn-to late-tectonic granites, acid volcanic rocks and post-tectonic intrusions are distributed in scattered outcrops [START_REF] Galil | Geology and Mineralization related to anorogenic igneous complex of Northern Nuba Mountains and Northern Kordofan, Sudan[END_REF]. Some isolated outcrops of gneiss and chlorite schist, granite, granodiorite, graphitic schist and syenite are observed in the western parts of the study area (Dawelbeit, 2011;[START_REF] Ginaya | Updating of Groundwater Conditions of Ennahud Basin -North Kordofan, Sudan[END_REF][START_REF] Ginaya | Numerical Modelling of the Groundwater flow in Ennahud Basin, Northern Kordofan -Sudan[END_REF]. Gneisses and migmatites, sericite-graphite schists, chlorite schists, slates and meta-quartzites, granite and granodiorite, schists, slates and quartzites crop out in some small isolated outcrops in the southwestern part of the area (Strojexport, 1976).

Nawa Series (Upper Paleozoic)

The Nawa Series is the least extensive of the sedimentary rock units and has been identified only in the east-central part of Kordofan, south of El Obeid and northeast of Dilling. The series includes well-consolidated micaceous and non-micaceous sandstone, arkose, shale, mudstone, and thin-bedded limestone [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF]. In absence of fossil evidence the Nawa Formation is best classed as Palaeozoic-Mesozoic undifferentiated [START_REF] Whiteman | The Geology of the Sudan Republic[END_REF].

Cretaceous sediments

Cretaceous sediments at first were known as Nubian Sandstone or Nubian Formation. The latter term is applied to those bedded and usually flat-lying conglomerates, grits, sandstones, sandy mudstones and mudstones that rest unconformably on the Basement Complex and Paleozoic sandstone formation (Karakanis 1965). [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF] and [START_REF] Whiteman | The Geology of the Sudan Republic[END_REF] assumed that the Nubian Formation is of continental and/or nearshore marine origin.

In the study area the Nubian Formation is found in some isolated basins in the northeastern, northwestern and central-western parts (Fig. 1.8) with variable thicknesses. The thickness of the Nubian Formation depends on the paleo-relief of the Basement Complex, the thickest deposits being deposited in the depressions (Geotechnika, 1985). Borehole data and geophysics in En Nuhud outlier show a maximum thickness for the Nubian Formation of about 1000-1200 m [START_REF] Elmansour | Crustal architecture, geodynamics and evolution of En Nuhud basin area, Kordofan region, Sudan[END_REF]. Karakanis (1965) divided the Nubian Sandstone Group in Western Kordofan and Darfur into a basal, middle and upper series. The basal series consists of coarse silicified sandstone and conglomerates. The middle series is composed of mudstone and sandstone (mudstones dominate over sandstones). The upper series is composed of poorly sorted sandstone.

In the study area, the Nubian Formation has been subdivided into three lithological facies: (I) Psephitic facies: conglomerates, with predominant grain size > 4.0 mm, (II) Psammitic facies: sandstones and gravely sandstone (0.05 mm < grain size < 4.0 mm, and (III) Pelitic facies: claystone, mudstone and siltstone (grain size < 0.05 mm) [START_REF] Ginaya | Updating of Groundwater Conditions of Ennahud Basin -North Kordofan, Sudan[END_REF].

Fig. 1.8: Distribution of pre-Quaternary gelogical units in the Kordofan Region (from [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF]Elhag et al., 2014).

Laterite (Late Paleogene)

Laterite or ironstone was apparently developed extensively on Nubian sandstones over much of the Kordofan Province during Early and Middle Tertiary time and corresponds to a prolonged period of deep in-place weathering of the host rock under tropical climatic conditions of alternating wet and dry seasons. Erosional remnants of this once extensive laterite deposit occur east and southeast of En Nahud and north of El Fula but are small outcrops. The laterite consists of a highly ferruginous layer of hematitic and limonitic ironstone that locally has an oolitic or vermicular texture. In places, the laterite is hardly distinguishable from the underlying ferruginous Nubian mudstone [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF]).

Umm Ruwaba Formation (Paleogene-Neogene)

The Umm Ruwaba Series consists of lacustrine and fluviatile deposits that, in east-central Kordofan, rest largely on the irregular surface of the basement rocks. In the southwestern part of the Province, however, the deposits are underlain largely by rocks of the Nubian Formation from which in places, they are hardly distinguishable. Outcrops of Umm Ruwaba strata are rare because they are covered in most places by a thin but almost continuous veneer of surficial deposits [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF]. The Umm Ruwaba Formation is composed mostly of mudstone, sandstone and conglomerate. Facies and bedding changes within relatively short distances are very common, the conglomerates are often silty and the mudstones are sandy [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF].

Quaternary deposits (Pleistocene to Recent)

The terms surficial or superficial deposit are the most used in the previous studies to describe the Quaternary deposits. [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF] provided a powerful summary of the Quaternary sediments, indicating that the Kordofan Province is covered by a mantle of unconsolidated surficial deposits. In the northern two-fifths of the Province, this cover consists of coarse residual desert soils and active sand dunes. Adjoining these deposits to the South and extending over most of the central part of the Province is the Qoz sand (largely stabilized sand dunes), sometimes also called the Kordofan sands. Covering the southern part of Kordofan and in places interfingered with the Qoz sand, are the clay-plains deposits (Fig. 1.9). Alluvium in the channels of wadis and khors and slope-wash deposits around the footslopes of the jebels occur locally throughout the Province. Because of their limited areal extent, alluvium and slope-wash deposits are not indicated in Figure 1.9.

Residual desert soils in the north derive from the weathering of the parent rock and are the most extensive of the surficial deposits in the northern part of Kordofan. These materials are unconsolidated, generally less than 7 m thick, and consist largely of unsorted angular and subangular rock fragments ranging in size from sand to boulders. Most of the finer rock material in this area was apparently removed by strong northerly winds and deposited as sand in the central part of the Province [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF].

The Qoz sand deposits, or Kordofan sand [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF], are characteristically red [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF], or varying from pale buff to deep red [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF], and marked by their gently undulating surface [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF], which may indicate typical wind deposited sand [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF]. These Qoz sands are composed of fine to medium sand [START_REF] Ginaya | Updating of Groundwater Conditions of Ennahud Basin -North Kordofan, Sudan[END_REF] made of well-rounded quartz grains [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF]. The Kordofan sand is assumed to represent the southern limit of the windblown sands [START_REF] Elmansour | Assessment of the groundwater of En Nuhud basin, West Kordofan State-Sudan[END_REF]. The Kordofan sand covers large areas in central Sudan, west of the White Nile, especially in the province of Kordofan [START_REF] Elmansour | Assessment of the groundwater of En Nuhud basin, West Kordofan State-Sudan[END_REF], but are less common east of the Nile. This would be related to the distribution of the Nubian Formation, which is much more limited on the right bank [START_REF] Salama | Groundwater Resources of Sudan. Technical report, Directorate for groundwater and Wadi water resources[END_REF].

The clay-plains deposits consist of laminated, loosely compacted clay, silt, sandy clay, and sandy silt formed during successive floods of extensive areas, chiefly in southern Kordofan [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF].

Regarding the origin of superficial deposits, Grabham (in [START_REF] Whiteman | The Geology of the Sudan Republic[END_REF]) assumes that the sands were transported by wind from desert areas floored by granitic rocks, while Andrew (in [START_REF] Whiteman | The Geology of the Sudan Republic[END_REF] accepts their local origin. [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF] proposed the Qoz sands are a product of weathering of the Nubian Formation. [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF] believe that their reddish color and their gently undulating surface reflects their aeolian origin.

Fig. 1.9: General distribution of surficial deposits in the Kordofan Region (after [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF]).

Present-day climate of Africa

Because Africa is such an enormous landmass, stretching from about 35°N to 35°S, the climate differs according to location within the continent [START_REF] Collier | Climate change and Africa[END_REF]. Therefore, Africa can broadly be divided into four climatic zones based on a combination of temperature, precipitation (ppt) and evapotranspiration (Ngaira, 2007) (Fig. 1.10). The zones are:

i) Arid and semi-arid (Sahel region, Kalahari and Namib deserts).

ii) Tropical Savanna grasslands (Sub-Saharan Africa and Central Southern Africa). iii) Equatorial (the Congo region and the East African highlands). iv) Temperate (the South Eastern tip of South Africa) (Ngaira, 2007).

The African climates depend on low altitude pressure and winds over the continent, which are the surface expression of the upper air circulation [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. One of the most important zone of the air circulation is the Congo Basin in Central Africa, which represents the biggest water catchment in Africa. As a consequence, Central Africa is one of the most important climatic regions in the world. During the rainy seasons, the Congo is the wettest place on Earth, and adds 3.5 mm to global sea level each year (Beighley et al., 2011). Seasonal variations in the position of the Inter tropical Convergence Zone (ITCZ) exert a significant control on the seasonal pattern of precipitation maxima across much of Africa. Figure 1.9 shows the major climatic zones and atmospheric circulation regimes for average conditions in July/August and January that allows understanding climatic variability (Nicholson, 2000). Distinct atmospheric circulation systems affect North Africa, West and Central Africa, East Africa, and southern Africa; they are separated in large part by the ITCZ [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. Moreover, the Atlantic and Indian oceans play an important role in shaping the regional climate. During the southern hemisphere summer, differences in surface air pressure between the two oceans drive an eastwest air circulation cell which makes the rainfall more variable over southern Central Africa.

The transport of low-level water vapor from the Atlantic Ocean brings clouds and rain to central Africa throughout the year [START_REF] Creese | Central Africa's climate system[END_REF].

Late Quaternary hydrology of Eastern and Western Sahara

Several important studies either focused on paleohydrology, or related to hydrology, provide good references. These are the study of the paleohydrology of tropical Africa [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF], the study of the White Nile valley [START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF], the White Nile and Lake Albert [START_REF] Williams | Abrupt return of the summer monsoon 15,000 years ago: new supporting evidence from the lower White Nile valley and Lake Albert[END_REF] and the Nile Basin [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF], as well as some other studies carried out in paleolakes of Eastern Sahara (e.g., Pachur and Kröpelin, 1987;[START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF][START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Dawelbeit | Hydrogeological Evaluation of Sug'a El Jamal Basin, North Kordofan State, Sudan[END_REF][START_REF] Bouchette | Hydrodynamics in Holocene Lake Mega-Chad[END_REF]. Nicholson, 1996[START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF].

At a regional scale, [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF] summarized the major hydrological features of Africa based on the general climate, topography and geological features. These features are: (i) large internal drainage basins (e.g., Lake Chad basin) and enormous groundwater reservoirs (e.g., geological formations of the Continental Intercalaire in the Sahara); (ii) large exoreic rivers (Congo, Nile and Niger rivers) and associated wetlands, which have their headwaters in the humid equatorial belt; (iii) great East African Rift lakes that play a significant role in regional climate through water recycling; (iv) other permanent waterbodies concentrated in, or depending on, the equatorial zone (e.g., Lake Chad through the Chari river), or on highlands (e.g., the Ethiopian lakes Ziway-Shala and Abhé) [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. Depending on that, the study of paleohydrology and its implication in the paleoclimatic studies, requires great attention to the lake level fluctuations, record of the flood events in rivers and fluctuation of the groundwater levels.

Holocene Mega-lake Chad (ca. 10°N to 18°N; 12°E to 20°E) was the largest late Quaternary water body in Africa with maximum surface area during the Holocene exceeding 350,000 km 2 . The development of this giant paleo-lake is related to a northward shift of the isohyets interpreted as evidence for an enhanced Monsoon during the African Humid Period [START_REF] Bouchette | Hydrodynamics in Holocene Lake Mega-Chad[END_REF]. The water depth of this giant Mega-Lake was exceeding 150 m at its deepest part in the central northern sub-basin. The lowest elevation point at this lake is less than 200 m asl, while the maximum determined water lake level is 330 m asl [START_REF] Bouchette | Hydrodynamics in Holocene Lake Mega-Chad[END_REF].

Although the River Nile and its two main branches (White and Blue Nile) represent the main and most important hydrological feature in Sudan, there are other more local features. These are represented by some paleolakes west of the White Nile, some active seasonal rivers and wadies, and other fossil rivers and wadies. The most important and already investigated paleolakes are: the West Nubian Lake in NW Sudan [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF][START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF], Selima in north Sudan [START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF], and some small lakes along Wadi Howar in NW Sudan (Pachur and Kröpelin, 1987).

The West Nubia Paleolake is considered the largest hydrographic evidence in Eastern Sahara of the early to mid-Holocene wet phase that affected northern Africa [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF] and its basin represents an area of about 20,000 km 2 [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF]. The maximum water depth estimated in the the West Nubia Paleolake between 9400 and 7500 14 C yr BP was 15 m [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF].

Moreover, beside these paleolakes, paleoclimatic and paleohydrological research undertaken in Eastern Sahara revealed the existence of two major late Pleistocene to mid-Holocene drainage networks that once interrupted the state of internal drainage of this vast region. One network is an about 800 km long watercourse in eastern Libya [START_REF] Pachur | Wadi Howar: Paleoclimatic Evidence from an Extinct River System in the Southeastern Sahara[END_REF], and the second network is the about 400 km long Wadi Howar in northwestern Sudan, which linked the mountains of Western Sudan to the Nile (Pachur and Kröpelin, 1987). Beside, some other wadies are known in southern Libya with present rainfall of 20 mm/year [START_REF] Pachur | Wadi Howar: Paleoclimatic Evidence from an Extinct River System in the Southeastern Sahara[END_REF]. From about 9500 to 4500 years ago, the lower Wadi Howar flowed through an environment characterized by numerous ground water outlets and freshwater lakes (Pachur and Kröpelin, 1987). Wadi Howar constituted the largest tributary to the Nile from the Sahara between the Mediterranean Sea and the Atbara River, and it drained a 400 km wide area linking the present extremely arid northern Sudan (present rainfall, 25 mm/year) to the Nile (Pachur and Kröpelin, 1987).

Latest Quaternary climate of Northeastern Africa

The continental deposits often provide more detailed information about short-term (high frequency) change of climate than most marine records [START_REF] Bradley | Recent Developments in Quaternary Paleoclimatology[END_REF]. The Sahara sediments in particular, represent an important archive for the climate variabilities and consequently the development of the Sahara during the late Pleistocene-Holocene period. Since 1980, the Sahara was the site of important development in paleoclimatology researches throughout a variety of climate proxies that have been recorded in this continental sediments. Most of these researches have focused on the last glacial maximum (LGM)-Holocene period, and very special attention was paid for terminal Pleistocene-Holocene (the last 12 ka) [START_REF] Nicholson | African Environmental and Climatic changes and the general atmospheric circulation in Late Pleistocene and Holocene[END_REF]Pachur and Kröpelin, 1987;[START_REF] Rodrigues | Seasonality in the early Holocene climate of Northwest Sudan: interpretation of Etheria elliptica shell isotopic data[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing[END_REF][START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF][START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF][START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Kuper | Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa's Evolution[END_REF][START_REF] Woodward | The Nile: Evolution, Quaternary River Environments and Material Fluxes[END_REF][START_REF] Hamann | Modern and late Quaternary clay mineral distribution in the area of the SE Mediterranean Sea[END_REF]DeMenocal Williams, 2009;[START_REF] Hoelzmann | Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate[END_REF]Tierney et al., 2011b;[START_REF] Demenocal | Green Sahara: African Humid Periods paced by Earth's orbital changes[END_REF][START_REF] Williams | Climate Change in Deserts (past, present and future)[END_REF].

The period between 20 and 12 ka BP is well documented as a period of dune building in the central Sahara, the Nile basin and East Africa [START_REF] Nicholson | African Environmental and Climatic changes and the general atmospheric circulation in Late Pleistocene and Holocene[END_REF][START_REF] Woodward | The Nile: Evolution, Quaternary River Environments and Material Fluxes[END_REF]Tierney et al., 2011b;[START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF][START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF]2014). This hyper arid phase had great effects and influence on the geomorphology and landscape features of the Sahara region.

The dune building period is followed by a humid phase that extended between ≈ 12 and 6 ky BP. This humid phase is well known as the African Humid Period (AHP) [START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing[END_REF][START_REF] Hamann | Modern and late Quaternary clay mineral distribution in the area of the SE Mediterranean Sea[END_REF][START_REF] Demenocal | Green Sahara: African Humid Periods paced by Earth's orbital changes[END_REF] or lacustrine period [START_REF] Williams | Climate Change in Deserts (past, present and future)[END_REF]. The AHP is well-characterized by numerous large and small lakes [START_REF] Bakker | Late Quaternary Palaeoenvironments of the Sahara Region[END_REF][START_REF] Nicholson | African Environmental and Climatic changes and the general atmospheric circulation in Late Pleistocene and Holocene[END_REF][START_REF] Szabo | Ages of Quaternary pluvial episodes determined by uranium-series and radiocarbon dating of lacustrine deposits of Eastern Sahara[END_REF]DdeMenocal et al., 2000;Cole et al., 2009) (Fig. 1.12). Some of the small lakes are the Dry Selima lake (northern Sudan), the West Nubian Palaeolake (NW Sudan; [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF]Kröpelin and Soulié-Märsche, 1991;[START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF][START_REF] Hoelzmann | Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate[END_REF], Lake Challa in Ethiopia (Tierney et al., 2011b), Lake Tilo in the Ethiopian highlands [START_REF] Telford | Groundwater-Mediated response to Holocene climatic change recorded by the Diatom Stratigraphy of an Ethiopian Crater Lake[END_REF] and Lake Yao in Chad [START_REF] Lézine | A late Holocene pollen and climate record from Lake Yoa, northern Chad[END_REF]. Other palaeolakes formed along the Wadi Howar (Pachur and Kröpelin, 1987). Some lake basins in North Africa were exceptionally large; these so-called megalakes include the Megalake Fezzan (Libya), Megalake Chad, Chotts Megalakes (Algeria) and Megalakes Turkana and Kenya to the southeast [START_REF] Demenocal | Green Sahara: African Humid Periods paced by Earth's orbital changes[END_REF].

Moreover, the AHP is also well documented through the archeological evidences that linked the development of these lakes and the Nile valley. Eastern Sahara (which includes partially Libya, Chad, Egypt and Sudan) represents an important archive for the Holocene climate through the existence of this early to mid-Holocene lakes and development of the human occupation evolution. This human occupation history is well documented over Eastern Sahara in general [START_REF] Kuper | Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa's Evolution[END_REF], in the West Nubia Palaeolake [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF], west of the Nile across the Sahara-Sahel Belt (Mohammed-Ali, 2003), in Egypt and northern Sudan [START_REF] Nicoll | Recent environmental change and prehistoric human activity in Egypt and Northern Sudan[END_REF]), along Wadi Howar (Jesse, 2007), on the eastern margin of the Blue Nile [START_REF] Lario | Paleoenvironmental Evolution of the Blue Nile (central Sudan) During the Early and Mid-Holocene (Mesolithic-Neolithic Transition)[END_REF], and in many other sites in northern and central Sudan. The timing and development of Holocene human occupation in the now hyperarid Sahara has major implications for understanding links between climate change, demography and cultural adaptation [START_REF] Manning | The demographic response to Holocene climate change in the Sahara[END_REF]. In the Atlas, Eastern Sahara and Central Sahara regions, [START_REF] Manning | The demographic response to Holocene climate change in the Sahara[END_REF] demonstrated synchronicity in the population increase and the start of the AHP. The compilation of radiocarbon dates from archeological sites showed that the older dates are in the North, while the bulk of younger dates is located in the South, clearly indicating a movement of prehistoric populations toward the present day Sahelian zone [START_REF] Kuper | Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa's Evolution[END_REF].

Fig. 1.12: Distribution map of reconstructed lake levels across Africa, 9,000 years ago relative to today. Data from Oxford Lake Level Database (COHMAP members, 1988;Street-Perrott et al., 1989) updated with lake-level reconstructions generated in the last twenty years [START_REF] Demenocal | Green Sahara: African Humid Periods paced by Earth's orbital changes[END_REF].

During the late Holocene period (≈ 6 ka BP to present), the Sahara recorded progressively drier condition, although the real start of this dry phase is different across the Sahara in particular, and the African continent in general. This aridification period is well documented in the Eastern Sahara (Tierney and deMenocal, 2013;[START_REF] Manning | The demographic response to Holocene climate change in the Sahara[END_REF] through a variety of evidences: northern Chad experienced a progressive drying in response to weakened insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem [START_REF] Kröpelin | Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years[END_REF]; palynological evidences from Lake Yoa (Chad) indicate gradual shift from moist to arid conditions during the last 6000 yr [START_REF] Lézine | A late Holocene pollen and climate record from Lake Yoa, northern Chad[END_REF]; and desertification and aeolian deflation are recorded during the Middle and Late Holocene in Egypt and northern Sudan [START_REF] Nicoll | Recent environmental change and prehistoric human activity in Egypt and Northern Sudan[END_REF]. In some sites, this dry period has been detected earlier than 6 ka BP, where warm, dry phase between 7000 and 6800 BP has been recognized in the Blue Nile, from sedimentological, flooding and lake level data. This short dry phase in Blue Nile was interrupted by somehow wetter conditions than the present day ones, and continued about 4500 BP when the present arid to hyper-arid conditions commenced [START_REF] Lario | Paleoenvironmental Evolution of the Blue Nile (central Sudan) During the Early and Mid-Holocene (Mesolithic-Neolithic Transition)[END_REF]. In eastern Africa this dry phase has started later ≈ 4.5 ka BP [START_REF] Telford | Groundwater-Mediated response to Holocene climatic change recorded by the Diatom Stratigraphy of an Ethiopian Crater Lake[END_REF].

Statement of the problem and aim of this study

The present study is dealing with the late Quaternary deposits in the Kordofan region from sedimentological and climatic points of view. Most of the previous studies carried out in this region focused on the hydrogeology [START_REF] Karkanis | Hydrochemical Facies of Grounwater in the Western Provinces of Sudan[END_REF][START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF][START_REF] Salama | Groundwater Resources of Sudan. Technical report, Directorate for groundwater and Wadi water resources[END_REF]Strojexport, 1976;[START_REF] Ginaya | Updating of Groundwater Conditions of Ennahud Basin -North Kordofan, Sudan[END_REF]Dawelbeit, 2011;El Hadi, 2012;[START_REF] Elhag | Structures controls on groundwater occurrence and flow in crystalline bedrocks: a case study of the El Obeid area, Western Sudan[END_REF]Elhag et al., 2014;[START_REF] Elmansour | Assessment of the groundwater of En Nuhud basin, West Kordofan State-Sudan[END_REF], geology [START_REF] Whiteman | The Geology of the Sudan Republic[END_REF]Strojexport, 1975;[START_REF] Galil | Geology and Mineralization related to anorogenic igneous complex of Northern Nuba Mountains and Northern Kordofan, Sudan[END_REF][START_REF] Elhag | Structures controls on groundwater occurrence and flow in crystalline bedrocks: a case study of the El Obeid area, Western Sudan[END_REF][START_REF] Elmansour | Crustal architecture, geodynamics and evolution of En Nuhud basin area, Kordofan region, Sudan[END_REF], and many other mineralogical investigation in the Nuba mountains, and hydrocarbon investigation south of El Fula. Very few attention has been paid for the Quaternary by those previous studies, due to less economical value of these sediments, and consequently, all the geophysical exploration and drilling reports describe the Quaternary units as superficial deposits with only very little information about their textural description. [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF] was the first to study in detail these late Quaternary deposits of Kordofan and paid special attention to the distribution of what is called the "Kordofan sand", the geomorphology of the dunes and the origin of the sand. [START_REF] Rodis | Groundwater Geology of Kordofan Province-Sudan, Contribution to the hydrology of Africa and Mediterranean Region[END_REF] provided a map for the surficial deposits of the whole Kordofan region with valuable information about their physical and textural characteristics. However, the lithological variability of the late Quaternary units requires detailed studies, since the "Kordofan sand" is locally interbedded with important units of limestone that had never been mentioned by previous workers, and seems to have experienced intensive pedogenetic processes. Consequently, the sedimentological environment and paleogeographic distribution of these deposits need to be studied. Moreover, the real age of the Quaternary units is still unknown.

On the other hand, although intensive climatic studies have been carried out in the Sahara and especially in Eastern Sahara, the southern limit of the Sahara in Sudan (Kordofan region) has never been studied in order to document the climate evolution during the terminal Pleistocene-Holocene period. However, Late Pleistocene-Holocene deposits in the Kordofan region exhibit lithological discontinuities that probably reflect climatic fluctuations during the late Quaternary. In addition, there are clear lateral and/or vertical variations in the physical properties of the deposits (color, texture, grain size, fossil content …), which suggest large variability in sedimentary and climatic conditions during the late Quaternary.

The present study is focused on the sedimentary record of the climatic evolution of Central Kordofan through several climatic proxies. Therefore, the main objectives of this study are [START_REF]Hyphaene thebaica, 3: Bracharia ramose, 4[END_REF] to determine the age, areal extent and thickness of the Quaternary units, (2) to understand the processes that controlled the deposition and distribution of the Quaternary deposits, and (3) to reconstruct the late Quaternary paleoclimatic evolution of Kordofan region. The results of this study will help us to assess the relationships between climate and sedimentation. In this view, I hoped to address the following questions:

 What were the sedimentation processes of the Late-Quaternary deposits, are there any evidences of fluviatile, aeolian or lacustrine deposits ? If such different kinds of sediment exist; do they show any conformable and/or unconformable contacts ? And what is the age of these successive sedimentary units ?  Do the lithological discontinuities and lateral and/or vertical variations in the sedimentary facies reflect any climatic variations ? If so, what was the role of regional Quaternary climate changes and monsoonal fluctuations, and their relations with the regional context ?

II. METHODOLOGY

Introduction

In order to reconstruct the late-Quaternary sedimentary and climatic evolution of the Kordofan region, it is important to pay great attention in the field to sedimentological features and to know which climatic proxies should be used. Kordofan deposits incorporate and preserve multiproxy evidences in a variety of sedimentary successions. This chapter outlines the approach taken for field observation, selection of representative sites, sample collection, and the practical laboratory investigations of selected paleoclimatic and paleoenvironmental proxies, namely: sedimentological, paleontological (palynology and gastropod sub-fossil shells), geochemical (stable isotopes, major elements and clay mineralogy), hydrological, archeological and carbon dating analyses.

Sites selection

Sites selection is the most important step in conducting such geological research. To investigate the sedimentological and stratigraphic succession in the Kordofan region, sites were selected across the study area depending on a preliminary short field works and Google Earth map survey that show some variations in the geomorphologic features (Fig. 1.4). These geomorphological features provided preliminary indicators of some morphological or lithological variation across the targeted area.

Sites selection was focused on the outcrops that likely contain: (1) well-preserved and well exposed sedimentary successions, (2) organic matter suitable for palynological analysis and carbon dating, although preserved organic matter is scarce in the greatest part of the area, (3) gastropod-rich successions for paleontological and paleobiological studies, (4) clastic sediments suitable for grain size analysis. The selected sites included: banks of wadies and khours (seasonal rivers) that expose sections up to 5 m thick, excavated sections along asphaltic roads up to 7 m deep, active or abandoned quarries and some other sites for surficial survey. In some cases, handmade wells have been caved to obtain adequate sections.

Field work

Two intensive field campaigns in November 2015 and November 2016, in addition to many other short complementary field works, have been carried out. The aims of the field work are to conduct sedimentological investigation (study, measurement and sampling of verical sedimentary profiles), in addition to some archeological and paleohydrological observations.

Sedimentological investigations

The purpose of the field work was to investigate in detail the sedimentary successions across the study area in order to document all the needed information. [START_REF] Tucker | Sedimentary rocks in the field[END_REF] summarized the main aspects of sedimentary rocks to consider in the field, which should be recorded in as much detail as possible. These are:

1. The lithology, that is the composition and/or mineralogy of the sediment;

2. The texture, referring to the features, shape and arrangements of the grains in the sediment, of which the most important aspect to examine in the field is the grain-size; 3. The sedimentary structures, present on bedding surfaces and within beds, some of which record the palaeocurrents that deposited the sediments; 4. The color of the sediments; 5. The geometry and relationships of the beds or rock units, and their lateral and vertical changes; and 6. The nature, distribution and preservation of fossils contained within the sedimentary rocks.

The various attributes of a sedimentary rock are then combined to define a facies, which is the product of a particular depositional environment or of depositional process in that environment.

Facies identification and analysis are the next steps after the field data have been collected [START_REF] Tucker | Sedimentary rocks in the field[END_REF]. In this study, facies identification provided preliminary indications on the climatic evolution, which required confirmations by other analytical climatic proxies.

Sampling

During field work, great attention was paid for samples collection. The collected samples included charcoal, bones and organic matter for carbon dating, organic-rich sediments for palynology, gastropod sub-fossil shells for paleontological investigation, in addition to in situ shells counting in some sections, clastic sediments collection for grain size analysis, clay mineralogy and major elements geochemistry, systematic gastropod sub-fossil shells and calcareous nodules sampling for oxygen and carbon stable isotopes analyses, and archeological remnants when present (Fig. 2.1).

Carbon dating samping

Twenty-six samples for carbon dating were collected across the study area. These samples include: (1) eight samples of bones collected mostly from surficial outcrops of lake shorelines and only one of them collected from a section of wady or khour bank, (2) five charcoal pieces collected from sections of wady or khou banks, and (3) thirteen samples of organic-rich soils and paleosols from sections of wady or khou banks.

Many of these samples were collected randomly (not systematically) due to the lack of material suitable for dating in most parts of the sections.

Palynology sampling

The palynological data obtained in this study proceed from twenty-nine samples from six sections. All samples are from the shaly parts of each section, with focus on the dark fine sediments. The sections rich in clayey strata have been sampled systematically with constant intervals, while those with rare shale levels have been sampled according to the potential suitability of the sediments.

Paleontology sampling

Sampling for paleontology was focused on the collection of gastropod sub-fossil shells from different sites (surficial and in sections). The main purpose of the paleontological sampling was to identify the various observed sub-fossil shells, in order to determine their living environment, and thus, the depositional environment of the surrounding sediment.

Grain size and XRF samplings

Fourty-eight samples from four sections were systematically collected for grain size analysis. Among them, twenty-two samples from two sections were used for XRF analysis of major elements.

All sampled sections are composed of pedogenitized and non pedogenitized levels. From the pedogenitized parts, only the matrix have been collected after removing the pedogenitized features represented by calcareous nodules. Sediment samples range between 400 and 500 g.

Clay mineralogy sampling

Only eight samples were collected along one section, proceeding from different deposits. 300 g of sediments were originally sampled, reduced to a few grams after prepartion (see section 2.4.3.4.).

Stable isotopes sampling

Forty-nine samples were systematically collected from five sections: three sections yielded twenty-four gastropod sub-fossil shells from various species, while other samples are calcareous nodules from two other sections. Radiocarbon 14 C is one of three isotopes of carbon, the others being 12 C and 13 C. By far the most abundant of these is 12 C, which comprises around 98.9% of all naturally occurring carbon atoms. 13 C forms around 1.1% and 14 C one part in 10 10 %. In other words, only about one among one million atoms of carbon is 14 C. Both 12 C and 13 C are stable isotopes, but 14 C is not, and it 'decays' to a stable form of nitrogen, 14 N, through the emission of beta (ß) particles. One ß particle is released from the nucleus for every atom of 14 C that decays [START_REF] Walker | Quaternary dating methods[END_REF].

Radiocarbon dating is the most widely applied dating technique for the Holocene and late Pleistocene period, providing reliable and accurate temporal controls for palaeoenvironmental studies. It was therefore applied to obtain absolute independent chronology of the studied samples.

Radiocarbon dating determines ages of carbonaceous materials using the radio-isotope 14 C. 14 C is produced in the upper atmosphere mainly as the result of the interaction of cosmic ray neutrons with Nitrogen-14 atoms [START_REF] Muscheler | Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10 Be and 14 C records[END_REF][START_REF] Fairbanks | Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230 Th/ 234 U/ 238 U and 14 C dates on pristine corals[END_REF]. 14 C mixes rapidly throughout the atmosphere to form carbon dioxide and enters the global carbon cycle and is assimilated by plants through the photosynthesis process, and by animals through the ingestion of plant tissue [START_REF] Walker | Quaternary dating methods[END_REF]. The majority of 14 C (more than 95%) is absorbed into the oceans as dissolved carbonate, which means that organisms that live in sea water (corals, molluscs, etc.) will also take up 14 C during the course of their life cycle. Although the 14 C in the terrestrial biosphere and in the oceans is constantly decaying, it is continuously replenished from the atmosphere. Hence, the amount of 14 C that is stored in plant and animal tissue and in the world's oceans, the global carbon reservoir, remains approximately constant through time [START_REF] Walker | Quaternary dating methods[END_REF].

Fig. 2.2. Decay curve of radiocarbon. This curve is exponential, not linear. A after each half-life the number of atoms remaining is halved. If there are A 0 atoms of radiocarbon at the beginning of the decay process, then after one half-life there will be A 0 /2 atoms remaining; after two half-lives, there will be A 0 /4, after three, A 0 /8 and so on (after Bowman, 1990;[START_REF] Walker | Quaternary dating methods[END_REF].

Once an organism dies, however, it becomes isolated from the 14 C sources, no further replenishment of 14 C can take place, and the 'radiocarbon clock' runs down by radioactive decay, which occurs in constant rate. Hence by measuring the amount of 14 C that remains in a sample of fossil materials (the residual 14 C content) and comparing this to modern 14 C in standard material, an age can be inferred since the death of the organism. However, the rate at which 14 C decays is needed to know the rate at which 14 C decays. Experimental results have shown that the decay rate of 14 C is 1% every 83 years. However, as with all radioactive isotopes, the decay curve is not linear but exponential (Fig. 2.2). This means that materials significantly older than this can still be dated. The half-life of a 14 C atom is 5730 years, and under normal circumstances, the limit of measurement of 14 C activity (i.e. decay rate) is eight half-lives. This translates into an upper age limit of around 45 000 years. Therefore, samples older than this are usually described as being 'infinite age' and are expressed, for example, as > 45 000 years [START_REF] Walker | Quaternary dating methods[END_REF].

14 C dating of our samples have been carried out [START_REF]Hyphaene thebaica, 3: Bracharia ramose, 4[END_REF] In case where total organic carbon (TOC) is to be analysed, the procedure is limited to the first HCl treatment only.

-In case of bones, extraction of collagen is performed using the procedures originally described by Longin (1971, Nature, 230, 241-242), with further modifications (e.g. Piotrowska N., Goslar T., 2002, Isotopes in Environmental and Health Studies, 38, 1-9). Before extraction, degree of collagen degradation is checked by measuring content of nitrogen and carbon in bone, using analyser Flash EA 1112 Series (ThermoScientific). The samples are regarded suitable for collagen dating, if nitrogen content in bone is not lower than 0.6%, and ratio C/N is not higher than 5. Suitable bones are crushed mechanically to granulation <0.3 mm, the bone powder is then treated with 2M HCl (room temp., 20 min), and 0.1M NaOH (room temp., 1 h). After each step of treatment, the sample is centrifuged and the residuum is collected. Extraction of collagen is processed in HCl (pH = 3, 80°C, 10h), and after centrifugation, residuum is removed. The extracted collagen is then ultrafiltered on pre-cleaned Vivaspin 15 MWCO 30 kD filters (Bronk Ramsey et al. 2004, Radiocarbon, 46, 155-163). Quality of the collagen is ultimately assessed basing on C/N atomic ratio (interval of acceptance: 2.7-3.5) and collagen extraction yield (acceptance threshold: 0.5%). On demand, carbon and nitrogen stable isotopic composition of the collagen can be determined.

In cremated bones, where collagen is too degraded and not suitable for 14 C dating, and geological situation at the site of sample collection excludes precipitation of secondary carbonates, a fraction of structural carbonates is forwarded for dating, while organic fraction is removed by treating bones with 2% NaClO for 48 h and then in 8% CH3COOH for 48h (according to Lanting J.N., Van der Plicht J., 2001. Radiocarbon, 43, 249-254). Then the outer layer of carbonate grains is removed in 2% HCl (1 h) and additionally by quick rinsing with 36% HCl.

-Samples of shells (and other carbonate features) are checked and mechanically cleaned under binocular. The organic coating, if visible, is removed with H2O2 (15-30%) in an ultrasonic bath.

Then the outer carbonate layer (ca. 30%) is removed in 0.5M HCl (if the sample is large enough), the remaining material is treated in 15% H2O2 again (for 10 min in an ultrasonic bath) and the remaining carbonate is leached with concentrated H3PO4 in a vacuum line.

Production of CO 2 and graphitisation

In case of organic samples, CO2 is produced by combusting the sample. Combustion of organic samples is performed in closed (sealed under vacuum) quartz tubes, together with CuO and Ag wool, in 900°C over 10 hours. CO2 from carbonate samples is leached by treating with concentrated orto-phosphoric acid (H3PO4) in a vacuum line.

The obtained gas (CO2 + water vapour) is then dried in a vacuum line, and reduced with hydrogen (H2), using 2 mg of Fe powder as a catalyst. The obtained mixture of carbon and iron is then pressed into special aluminium holder, according to the description provided by Czernik J., Goslar T., 2001, Radiocarbon, 43, 283- Content of 14 C in a sample of carbon is measured using the spectrometer "Compact Carbon AMS" (produced by: National Electrostatics Corporation, USA) described in the paper: Goslar T., Czernik J., Goslar E., 2004, Nuclear Instruments and Methods B, 223-224, 5-11). The measurement is performed by comparing intensities of ionic beams of 14 C, 13 C and 12 C measured for each sample and for standard samples (modern standard: "Oxalic Acid II" and standard of 14 C-free carbon: "background"). In each AMS run, 30-33 samples of unknown age are measured, alternated with measurements of 3-4 samples of modern standard and 1-2 samples of background. In case, where organic samples are dated, the background is represented by coal, while in case of carbonate samples, the background is represented by the sample IAEA C1.

2.4. Different amounts, depending on the TOC% of the samples, are taken in order to obtain, after combustion, a volume of CO2 containing approximately 1 mg of carbon. The sample is burned in the presence of about 500 mg of copper oxide and silver wire for 5 hours at 835°C.

Graphite reduction protocol

CO2 is reduced by hydrogen in the presence of iron powder at 600°C. The mass of iron is equal to 3 times the mass of carbon with a minimum value of 1.5 mg and a maximum value of 4 mg.

Carbon is deposit on the iron powder and the whole is then pressed into a support for measurement by AMS.

Measurement and analysis protocol

The 14 C activity of the sample is calculated by comparing the intensities measured sequentially beams of 14 C, 13 C and 12 C from each sample with those of CO2 standards prepared from the reference oxalic acid HOxII. It is expressed in pMC (percent Modern Carbon) normalized to a delta 13 C of -25 per thousand.

The radiocarbon ages are calculated according to Mook and Van der Plicht (Radiocarbon 14, 1999, p. 227) in correcting the  13 C fractionation calculated from the 13 C/ 12 C ratio measured on ARTEMIS.

The  13 C used includes fractionation occurring both during sample preparation only during SMA measurement. It is also dependent on the transmission of the installation, namely the 13 C/ 12 C measured at the output of the ARTEMIS mass spectrometer compared to the value of 13 C/ 12 C at source output. These values depend on the beam at injection, current and emission.

The  13 C measured by ARTEMIS can not be compared to the  13 C measured on a spectrometer massive. Since  13 C values are corrected to a standard, they do not affect the age measurement of 14 C.

Calibration of 14 C age

In addition to natural variation, the atmospheric 14 C levels have been affected by human activity [START_REF] Walker | Quaternary dating methods[END_REF]. Over the past 250 year, the burning of fossil fuels has liberated large quantities of 12 C into the atmosphere (Houghton et al., 2001[START_REF] Walker | Quaternary dating methods[END_REF]. These quantities of 12 C have diluted 14 C levels and cause some variations in the atmospheric 14 C concentration and consequently profound effects for radiocarbon dating [START_REF] Walker | Quaternary dating methods[END_REF]. A comparison between 14 C and uranium-series dates on fossil coral, indicate that radiocarbon dates are around 3000 years younger than 'true' ages at 15 000 14 C year BP, and around 4000 year younger at 20 000 14 C year BP (Stuiver et al., 1998[START_REF] Walker | Quaternary dating methods[END_REF]. For this reason it is important to convert the radiocarbon date into calendar time or 14 C year BP.

Grain size analysis

Grain size is the most fundamental property of sediment particles, affecting their environment, transport and deposition. Grain size analysis therefore provides important clues to the sediment provenance, transport history and depositional conditions [START_REF] Folk | Brazos River Bar: a study in the significance of grain size parameters[END_REF].

The original aims of this grain size investigation is to determine the origin of the Kordofan sand (aeolian or fluviatile origin), to understand the transport direction, and to have an idea about the transportation mode(s).

Samples for grain size analysis were prepared using standard techniques as described by [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF] to determine grain size distribution. The samples were dry-sieved at a sieve sets ranges from 2.83 to 0.074 mm opening for five minuts, the retained in each sieve is weighted using electronic balance of 0.1g sensitivity. Material smaller than 0.074 mm were not analyzed, instead, a single weight percent is given and plotted as one size then the curves completed according to their directions to include the percentages of the silt and clay contents (material < 0.063 mm) to help in the approximate evaluation of the percentages for all materials.

The cumulative curves of the samples were plotted manually using the free drawing computer program Surfer 11. Graphical statistical parameters (mean grain size MZ, sorting I, skewness SKI, and kurtosis KG) were calculated for each sample following [START_REF] Folk | Brazos River Bar: a study in the significance of grain size parameters[END_REF] (equations 2.1 -2.4). Mean grain size represents the average grain size; sorting (as a standard deviation) refers to the uniformity of the sediment; and skewness measures the asymmetry of the size distribution curve [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF].

M
SK 𝐼 = ∅ 16 + ∅ 84 -∅ 50 2 (∅ 84 -∅ 16 ) + ∅ 5 + ∅ 95 -∅ 50 2 (∅ 95 -∅ 5 ) ………………. (2.3) K 𝐺 = ∅ 95 -∅ 5 2.44 (∅ 75 -∅ 25 ) ………………………………. (2.4)
Where: M Z is the mean size; ϭ I is the inclusive graphical standard deviation; SK I is the inclusive graphic skewness; K G is the graphic kurtosis; and Ø n is the diameter where n% of the sample is greater than this diameter.

Description of sorting, skewness, and kurtosis after [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF] has been used for better understanding of these graphical parameters.

The cumulative percentages of the grain size classification after [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF], were calculated from each curve and plotted in vertical diagrams against their depths to understand their vertical distribution in each section and to have an idea about wind energy fluctuation through time.

Horizontal diagrams were also plotted to understand the spatial variations in wind energy (from NE to SW). As a matter of fact, in areal studies of sand sheets it is useful to know which direction the sand is moving, and to this end an areal study of mean grain size would be of critical importance. One of the ways to proceed would be to lay a sampling grid over the dune field and see if there were any systematic regional changes in grain size [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF]).

Log-probability curves have been prepared according to [START_REF] Visher | Grain size distributions and depositional processes[END_REF], in order to understand the mode(s) of sediments transportation (suspension, saltation, traction) in each sample, and therefore, in each part of the studied sections. Moreover, geographical profiles of the modes of transport across section were used to confirm the variability of the transport energy.

The grain size analysis hase been conducted at the Department of Geology, University of Kordofan, El Obeid, Sudan.

Analysis of paleoclimate proxies

Palynological analysis

Sporomorphs data are used for a variety of Quaternary applications including geochronology, biostratigraphy, palaeoecology, palaeoclimatology and archaeology [START_REF] Traverse | Paleopalynology[END_REF]. [START_REF] Faegri | Textbook of Pollen Analysis[END_REF] indicated that the reliability of palynology as a palaeoenvironmental tool is a function of :

• the robust preservation of pollen grains and their resistance to decay,

• the wide dispersal of pollen grains and their presence in a broad range of environments such as lakes, peat deposits, soils, colluvium, alluvium, estuarine deposits, ocean floor and faecal material,

• the abundant production of pollen grains,

• the relative ease of identification as a result of the distinctive structural pattern of the exine (the outer layer of the wall of a palynomorph), and

• the direct link between the identification of pollen grains and the parent plant [START_REF] Quick | Late Quaternary palaeoenvironments of the southern Cape, South Africa: palynological evidence from three coastal wetlands[END_REF].

It has been established that a combination of a few prominent "indicator" taxa can be characteristic of certain vegetation types and environmental conditions (Scott and Cooremans, 1992). The so-called indicator approach therefore uses the presence (or absence) of indicator taxa, whose modern ecological tolerances are well understood as a basis for reconstructing palaeoenvironments [START_REF] Quick | Late Quaternary palaeoenvironments of the southern Cape, South Africa: palynological evidence from three coastal wetlands[END_REF]. This approach assumes that the limiting conditions in the past were the same as they are today, an assumption thet most probably holds for the time period in question "i.e. the late Pleistocene and Holocene periods" [START_REF] Quick | Late Quaternary palaeoenvironments of the southern Cape, South Africa: palynological evidence from three coastal wetlands[END_REF]. At the beginning of sample preparation, at least tow samples per section were prepared to confirm the existence of palynomorphs and to decide the continuation of the preparation.

The samples have been analyzed using standard palynological preparation techniques established in the Petroleum Laboratories Research and Studies (PLRS), Khartoum, Sudan, by Mr. Abdelbasit Mustafa and Miss. Malaz Mohammed-Ali.

Chemical processing of palynological samples and preparation of slides were carried out at the Petroleum Laboratories Research and Studies (PLRS), Khartoum. The procedure followed in the preparation of palynological slides was largely adapted from the scheme established at PLRS. The following paragraphs outline the steps of this scheme:

Twenty grams of each sample were weighed and crushed to the size of about 2 mm. Samples were then placed in a plastic beaker and thoroughly washed under running water in order to get rid of mud additives and other surface contaminants.

Concentrated hydrochloric acid (35%) was carefully used to remove the carbonates. As there was an abundance of silica in the analyzed samples, about 60 ml of cold concentrated hydrofluoric acid (40%) was carefully added to dissolve the silica and the silicate minerals. Sometimes a mixture of HF and HCl acids in the ratio of 1:3, respectively, wazs used to remove the fluorides that might have formed during the HF treatment.

For quick removal of the acids, samples were placed in a centrifuge to achieve rapid settling of the organic residue. A further concentration of some residues was achieved by sieving using mesh widths between 10-15 μm. The residue was then placed in a clean 50 ml polythene (polyethylene) centrifuge tube. Few drops of HCl were added followed by a solution of zinc chloride (Zn Cl2) of density 2.2 g/cm the tube was then centrifuged at 2200 rotations/second for 15 minutes. The organic part of the residue appeared as a dark layer floating on the top, while the heavy minerals settled at the bottom of the tube. The floating layer was carefully pipetted and washed.

Two or three drops of residue, mixed with few drops of Polyvinyl alcohol (PVA) were evenly spread over a cover slip and placed on a hot plate for 15 minutes to dry. A drop of petropoxy was gently placed in the central portion of each slide and left for ten seconds to get rid of any bubble, which may have formed in the resin. A cover slip was gently lowered allowing the resin to flow to all edges of the slide. The slide was then left for 15 minutes on the hot plate to dry. Two sdes per each sample were made and labeled.

Two slides per samples have been scanned and the various palynomorph types are counted. The counted and identified palynomorphs were classified into major climatic groups and plotted in vertical evolution and geographic distribution, in order to understand the chronological and geographic climate evolution.

Stable isotopes

The stable isotopes of many elements are fractionated during changes in their states of aggregation and by chemical reactions between compounds which the elements occur. The extent of fractionation of two isotopes of the same element is controlled primarily by the difference in their masses and by the temperature of the environment [START_REF] Faure | Isotopes: principles and applications[END_REF].

Forty-nine samples from five sections were analyzed for oxygen and carbon isotopes. Samples of three sections are gastropod shells, while samples from two sections are calcareous nodules.

Preparation of gastropod shells. Initial cleaning of the outer shell surface with normal water removed any adhering sediment and any other contaminations. Shells were then coarsely crushed and the internal surfaces cleaned using normal water followed by ultrasonic cleaning in distilled water for10 min. The ultrasonic cleaning step is repeated three times. Samples are then dried at 60°C for 24 hours. An agate mortar and pestle was used to powder samples and only 1 to 2 mg/sample sent to the laboratory to determine the carbonate mineralogy by X-ray diffraction.

Preparation of calcareous nodules. Initial cleaning of the outer nodule surface with normal water to remove any adhering sediment and any other contaminants. Further advanced preparation was performed at the Institute of Mineralogy and Geochemistry, University of Lausanne by M. Jorge Spangenberg. During this advanced preparation, the calcareous nodule samples were meticulously examined under a binocular microscope to eliminate the organic matter or soil matrix components contaminating the samples, then samples were powdered using an agate mortar samples.

Isotopic analysis of carbon and oxygen was performed at Lausanne Laboratory (Switherlad) by M. Jorge Spangenberg. The stable carbon and oxygen isotope composition of the gastropod shells and calcareous nodules were determined using a Thermo Fisher GasBench II connected to a Delta Plus XL isotope ratiomass spectrometer (IRMS). The stable oxygen isotope ratios are reported in the delta notation as the permil (‰) deviation relative to theVienna PeeDee Belemnite (VPDB) and Vienna Standard Mean Ocean Water standard (VSMOW), while the stable carbon isotope ratios are reported in the delta notation as the permil (‰) deviation relative to theVienna PeeDee Belemnite (VPDB).

Oxygen isotopes

Oxygen (O) is the most abundant chemical element in the crust of the earth. It has three stable isotopes 16 O, 17 O, and 18 O with abundance of 99.726%, 0.038%, and 0.200%, respectively. 18 O is 12.5% heavier than 16 O [START_REF] Faure | Isotopes: principles and applications[END_REF].

The isotopic composition of O and of all other elements whose isotopes are fractionated is usually expressed as the ratio R of the isotopic abundance of the heavy isotope divided by the abundance of the light isotope. Therefore, R is defined in terms of the numbers of atoms rather than in terms of the masses of the isotope. In the case of O, R is defined by the abundance (Equation 2.5). The isotope ratio of the oxygen is measured by mass spectrometry and is expressed relative to standard mean ocean water (SMOW) [START_REF] Faure | Isotopes: principles and applications[END_REF]. However, the standard used for the samples of this study is Vienna Pee Dee Belemnite (VPDB). The use of a standard reduces systematic errors in measurements made on different mass spectrometers and permits R value to be expressed in terms of parameter called delta (δ) as defined in equation (2.6). The δ 18 O parameters may be positive, negative, or zero. A positive δ 18 O value indicates that the sample has a higher 18 O/ 16 O ratio than the standard, which is expressed by stating that sample is enriched in δ 18 O relative to the standard. A negative δ 18 O value indicates that the sample has a lower 18 O/ 16 O ratio than the standard and that sample is depleted in δ 18 O relative to the standard. In other words, the isotopic composition of the oxygen in the sample is always expressed in terms of enrichment or depletion of the heavy isotope ( 18 O).

Since the analyzed samples were collected systematically along sections, then the corresponding δ 18 O values were plotted to show the vertical evolution of the oxygen δ 18 O, which may be then interpreted in terms of climatic evolution through time.

Carbon isotopes

Carbon is the fourth in abundance among the elements of the solar system after H, He and O (Anders and Grevesse, 1989). Carbon has two stable isotopes: 12 C and 13 C with an abundance of 98.90% and 1.10%, respectively. Consequently, the 13 C/ 12 C ratio is 1.083612 and the atomic weight of C is 12.01103. The mass of 13 C is 8.36% greater than that of 12 C, which causes the C isotopes to be fractionated by chemical and biological processes in nature [START_REF] Faure | Isotopes: principles and applications[END_REF].

The isotopic composition of C is expressed by the δ 13 C parameter (Equation 2.7).

δ13 𝐶 = ( ( 13 𝐶 12 𝐶 ⁄ ) 𝑠𝑝𝑙 -( 13 𝐶 12 𝐶 ⁄ ) 𝑠𝑡𝑑 ( 13 𝐶 12 𝐶 ⁄ ) 𝑠𝑡𝑑 ) × 10 3 ‰ ………………………… (2.7)
where the standard is PDB.

Therefore, positive δ 13 C values signify that sample is enriched in 13 C relative to the standard, whereas negative δ 13 C values imply depletion in 13 C. The fractionation factor for C isotopes expressed in equation (2.8) as follows:

α 𝑏 𝑎 (13 𝐶 ) = R a R b ………………………………….. (2.8)
where a and b are C-bearing compounds or phase in isotopic equilibrium at a specified temperature and R is the isotopic 13 C/ 12 C ratio. However, the isotopic composition of C in biogenic carbonate and organic compounds is not a reliable recorder of the environmental temperature because these kinds of compounds do not achieve isotopic equilibrium with the major C reservoir on Earth. When green plants absorb CO2 molecules during photosynthesis, they prefer 12 C over 13 C, which explain why organic matter of photosynthesis plants and herbivorous and carnivorous animals has negative δ 13 C values. On the other hand, the δ 13 C values of marine calcite and aragonite are close to zero because the BDP standard is itself a marine carbonate [START_REF] Faure | Isotopes: principles and applications[END_REF].

The obtained δ 13 C values were plotted in vertical evolution along the targeted sections and then δ 13 C was translated into environmental evolution through time.

Major elements geochemistry

Geochemical analysis of major elements has been applied in this study as a proxy for climate evolution in the study area. The chemical index of alteration (CIA) is the main Climofunction applied in this study. [START_REF] Nesbitt | Early Proterozoic climates and plate motions inferred from major elements chemistry of lutites[END_REF] proposed the use of chemical index of alteration (CIA) as a scale of the degree of rock weathering. It is calculated by taking the molar ratio of Al2O3 to Al2O3 + CaO + Na2O + K2O (alumina, lime, soda and potash) multiplying it by 100 (Equation 2.9). The CIA measures the degree of weathering in the source area of the analyzed sediment [START_REF] Nesbitt | Early Proterozoic climates and plate motions inferred from major elements chemistry of lutites[END_REF][START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF].

𝐶𝐼𝐴 = Al 2 O 3 (Al 2 O 3 + 𝐶𝑎𝑂 * + Na 2 O + K 2 O) × 100 ……………………….. (2.9)
where CaO* is the amount of CaO incorporated in the silicate fraction of the rock [START_REF] Nesbitt | Early Proterozoic climates and plate motions inferred from major elements chemistry of lutites[END_REF][START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF].

According to [START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF], the CaO*content has been calculated using the following methods: a) if the content of CaO was less or equal to the Na2O content, CaO value will be compensated for further calculation, b) if the CaO content is higher, it will be substituted by Na2O value in the above equation. According to [START_REF] Sheldon | Geochemical climofunctions from North American soils and application to paleosols across the Eocene-Oligocene boundary in Oregon[END_REF] the dgree of chemical weathrering in soil due to hydrolysis increases with available precipitation and temperature, the calculated CIA values plotted against corressonding sections.

Since the intensity of weathering depends, in part, on precipitation, [START_REF] Sheldon | Geochemical climofunctions from North American soils and application to paleosols across the Eocene-Oligocene boundary in Oregon[END_REF] developed an empirical relationship relating mean annual precipitation (MAP) to chemical index of alteration without potassium (CIA-K) (Equation 2.10). Because diagenesis can yield elevated potassium (K) concentrations in paleosols, this chemical index omits K [START_REF] Adams | Evaluating the use of weathering indices for determining mean annual precipitation in the ancient stratigraphic record[END_REF]. 𝑃 = 221𝑒 0.0197(𝐶𝐼𝐴-𝐾) ………………………………… (2.10)

In order to have an idea about the temperature, the empirical relationship of [START_REF] Sheldon | Geochemical climofunctions from North American soils and application to paleosols across the Eocene-Oligocene boundary in Oregon[END_REF] between mean annual temperature (MAT) and molecular weathering ratio of K2O+Na2O to Al2O3, have been applied (Equation 2.11). 𝑇 = -18.516(𝑆) + 17. 298 …………………………. (2.11) where S is the molecular weathering ratio (K2O+Na2O/Al2O3).

X-ray fluorescence analysis carried out at the chemical laboratory of the Geological Research Authority of Sudan (GRAS), under supervision of the chief chemist Emtithal Ali.

Samples preparation for X-ray fluorescence analysis

The sediment samples are firstly air-dried, then each sample is sieved through a sieve of 63 µm size. The pressed powder method is applied for sample preparation. Pressed pellets are prepared by pressing loose powder of the sieved sample filled in a hard steel ring, very smooth inside, using a set of dies and pressing machine.

Pellets are generally pressed at 15 to 20 tons per square inch, which produces a flat smooth, dense sample for testing. Once the pellet is held at this pressure for about 30 seconds the pellet is carefully removed. The average thickness of the pellets is about 3 mm. The ring and set of dies are washed thoroughly with first tap water then ethanol acid, and dried to prevent contamination.

Clay mineralogy

Millot (1965) distinguished three principal processes to account for the genesis of clay minerals, which may occur at different points in the geochemical cycle including weathering or soil formation at the earth's surface. These processes are: (a) detrital inheritance whereby, for soils, clay minerals are inherited from pre-existing parent rock or weathered materials; (b) transformation where the essential silicate structure of the clay mineral is maintained to a large extent, but with major change in the interlayer region of the structure; and (c) neoformation, where the clay mineral forms through crystallization of gels or solutions [START_REF] Wilson | The origin and formation of clay minerals in soils: past, present, and feature prospectives[END_REF].

Inherited soil clays may be of an extremely diverse and complex nature, reflecting both the variety of the parent rock as well as the transformation and neoformation processes that may have occurred in previous weathering environments [START_REF] Wilson | The origin and formation of clay minerals in soils: past, present, and feature prospectives[END_REF]. Therefore, the clay mineralogy has been used as climate proxy for the weathering experienced by the source area of the analyzed sediments.

The clay mineral assemblages are controlled by a number of factors (climate, time, parent material, topography, soil-profile type, transport processes, burial diagenesis) (Singer, 1984;Hillier, 1995[START_REF] Kemp | Palaeoclimatic implications of high resolution clay mineral assemblages preceding and across the onset of the Palaeocene-Eocene Thermal Maximum, North Sea Basin[END_REF].

The principle of using the clay mineralogy as a climate proxy is based on the fact that each clay mineral assemblage develops in a preferred environment (physico-chemical conditions), and that predominance of a specific assemblages reflects high degree of the required environmental conditions, leading to climatic information and interpretation. For instance, kaolinite and smectite are considered indicators of humid conditions, possibly tropical climate [START_REF] Kemp | Palaeoclimatic implications of high resolution clay mineral assemblages preceding and across the onset of the Palaeocene-Eocene Thermal Maximum, North Sea Basin[END_REF][START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF], smectite and illite indicate warm climate with alternating wet and dry seasons [START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF], the interstratified smectite illite (I-S) suggests seasonal tropical climate with a prolonged dry season [START_REF] Kemp | Palaeoclimatic implications of high resolution clay mineral assemblages preceding and across the onset of the Palaeocene-Eocene Thermal Maximum, North Sea Basin[END_REF], and illite and chlorite form under physical weathering conditions [START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF].

In this study, the quantitative results of the clay mineralogy have been plotted in vertical evolution and translated into climatic information to describe the weathering conditions in the source area of the analyzed sediments.

Clay mineralogy analysis and interpretation has been performed in the ISTerre laboratory in Grenoble, France, with the priceless help of Dr. Nathaniel Findling and Dr. Bruno Lanson.

Samples preparation and purification of clay.

The objective of the treatment of the samples is to obtain a suspension of the desired size fraction. To do that, firstly the grain size of the samples is reduced by seaving them through sieve of 63 μm size. A suspension of 25-30 g/L is prepared in centrifuge bottles, each bottle being filled with pure water and placed in an ultrasonic bath to enhance particle disaggregation.

Size fractionation by centrifugation with a swinging-bucket rotor

-Particules < 2 μm fraction. To do so the clay suspension (25-30 g/L) is placed in a 500 ml centrifuge pot and centrifuged for 8 minutes at 700 rpm. The top 7 cm of the suspension are collected using a Pasteur pipette whose tip is curved and a water pump (or vacuum pump). This supernatant is kept in a 5 liters beaker. The remaining suspension is diluted with pure water. Centrifuge pots are shaken to put the remaining solid phase back in suspension. Then centrifuge the clay suspension again.

CaCl2 1M (10 ml per liter) is added to flocculate the particles. Decant left overnight. The next day the clear supernatant is removed by siphoning, and the remaining suspension is centrifuged at high speed (4300 rpm for 15 minutes). The clear supernatant is then removed and the < 2 μm clay fraction is resuspended in pure water and kept at 4°C.

-Particules below 0.2 μm. The suspension of the < 2 μm clay fraction (5 g/Lmax) is used in 500 ml centrifuge pots and placed on a shaking table for at least 2 days at 100 rpm. Then the suspension is centrifuged during 40 minutes at 4000 rpm. The top 7 cm of the suspension are collected using a Pasteur pipette and kept in a 5 liters beaker.

The remaining suspension is diluted with pure water. Centrifuge pots are shaken to put the remaining solid phase back in suspension. Ultrasonic bath is used when necessary. Then the clay suspension is centrifuged again for 40 minutes at 4000 rpm to collect again the top 7 cm. CaCl2 1M (10ml per liter) is added to flocculate the particles. Decant left overnight. The next day the clear supernatant is removed by siphoning and the remaining suspension is centrifuged at high speed (4300 rpm for 15 minutes). The clear supernatant is then removed and the < 0.2 μm clay fraction is resuspended in pure water and kept at 4°C.

Mineralogical purifications.

This process includes carbonate removal and removal of organic matter. Sodium acetate buffer solution at pH = 5 is used to remove the carbonate, while Hydrogen peroxide (H2O2) is used to remove the organic matter.

Preparation of homoionic clay separates (Na + or Ca 2+ ). Saturation of a clay separate with

Na/Ca (or other cations) is achieved by washing cycles with the desired solutions NaCl. Centrifugation and removal of the supernatant. The first three cycles are performed for 24 h (room temperature) with concentrated solutions (0.5-1.0 M), and a minimum of three additional cycles are performed with a solution of the desired molarity. Samples are kept on a shaking table during the 24 h contact time so as to optimize the solid-solution contact.

III. STRATIGRAPHY AND SEDIMENTOLOGY

Stratigraphy

A total of twenty-four sections of the Late Quaternary sediments, distributed over five sectors, has been studied in detail in order to build a general stratigraphic picture of the area. Surficial survey was carried out in some sites, where no suitable sections were available for study. The location of the studied sites and their geographic information are given in Figure 3.1 andTable 3.1. The sections will be analyzed according to five areas, corresponding to the Dilling area to the Southeast, the El Fula-Abu Zabad area to the Southwest, the En Nahud-El Khowei area to the West, the El Obeid-South El Obeid in the East, and the North Bara-Sodari sector to the North (Fig. 3.1). The succession in the Dilling area is composed of three main stratigraphic units (Fig. 3.2).

Considering that the lower unit is missing in this sector, the second unit directly overlies the basement rocks, which crops out extensively in this area. The third unit overlies the second one with a conformable contact. The upper unit overlies the third one with an unconformable contact.

Second unit. This unit overlies unconformably foliated granitic rocks. It is formed of breccia or conglomerate comprising reworked rock fragments, as well as some reworked calcareous nodules. Its maximum measured thickness is ≈ 80 cm.

Third unit. The third unit is formed of dark grey, organic-rich, shaly, fine sand sediments, containing some gravels, and in situ developed calcareous nodules. The maximum measured thickness of this unit is ≈ 110 cm.

Fourth unit. The Upper unit overlies unconformably the third unit. A basal conglomerate bearing reworked calcareous nodules is overlain by sandy shales alternating with sand sediments beds, and containing locally calcareous nodules. The contacts between these beds are conformable, either abrupt or progressive. This unit has a maximum measured thickness of ≈ 375 cm.

Age constraints

The organic-rich shaly sand sediments bed of the third unit yielded 14 C ages of 4870 ± 30 and 4133 ± 30 cal. yr BP (Fig. 3.2). Charcoal fragment within the basal conglomerate/breccia of the upper unit yielded a 14 C age of 7682 ± 110 cal. yr BP. Charcoal and wood fragments from the upper part of the Upper unit provided 14 C ages of 759 ± 30, 663 ± 30 and 246 ± 30 cal. yr. BP.

Stratigraphy of the Abu Zabad -El Fula sector

Six sections have been studied in this sector, among which three are located East of the Abu Zabad town, and three others around the El Fula town. In this sector, Late Quaternary sediments are composed of three main stratigraphic units (lower, third, and upper) (Fig. 3.3).

Lower unit. The Lower (first) unit overlies the Cretaceous Nubian Sandstone or the Basement rocks. The contact with the pre-Quaternary rocks is abrupt and unconformable, and is locally underlined by coarse grained conglomerate beds (East of El Fula, East of Abu Zabad). It exhibits mostly mottled silt or fine sand sediments, rich in calcareous nodules and locally in poorly sorted gravels. This unit locally includes conglomerates found either in thick beds (east of El Fula), or in thin lenses in mottled sand sediments (East of Abu Zabad). The maximum measured thickness of this unit is ≈ 450 cm.

Third unit. The third unit overlies the first one with a conformable contact. It exhibits mostly mottled silt or fine sand sediments, the difference with the lower mottled sand is the absence of the calcareous nodules. The maximum measured thickness of this unit is ≈ 120 cm.

Upper unit. This unit overlies conformably the Third unit. It is mostly formed of red, fine sand locally interbedded with sand sediment beds. The latter comprise interbedded layers of coarse sand with lithoclasts, medium sand, fine sandand sandy shale sediments. The contacts between these beds are conformable, either abrupt or progressive. The maximum measured thickness of this unit is ≈ 290 cm.

Age constraints

Only one date have been obtained in this sector, from the lower part of the Lower Unit. The organic matter of this mottled silt sediments bed yielded a 14 C age of 12942 ± 100 cal. yr BP.

Stratigraphy of the El Obeid -South El Obeid sector

This sector extends southward until Khour Es Sonjokaya (section South El Obeid3). Five sections have been studied. Three main stratigraphic units have been identified (lower, third, and upper unit) (Fig. 3.4).

Lower unit. This unit exhibits mostly mottled silt or fine sand sediments rich in calcareous nodules. Structureless beds of the sand sediments have been locally observed. The maximum measured thickness of this unit is ≈ 410 cm. The contact with the pre-Quaternary rocks is abrupt and unconformable, and has been observed only in west El Obeid.

Third unit. This unit also exhibits mostly mottled silt or fine sand and locally planar laminated sand sediments, but without calcareous nodules. Its contact on the Lower unit is conformable.

The maximum measured thickness of this unit is ≈ 390 cm, the thickness increases gradually southward.

Upper unit. The Upper unit unconformably overlies the Third unit in the east and south El Obeid sections, but in west El Obeid the Upper Unit directly overlies the Lower Unit. It is formed mostly of red fine sand sediments locally interbedded with coarser sand sediment beds.

The grain size of the latter varies from coarse sand with lithoclasts, to sandy shale. The contacts between beds are conformable, either abrupt or gradual. The maximum thickness measured for this unit is ≈ 280 cm.

Age constraints

Organic matter from the third unit yielded 14 C ages of 3438 ± 30 and 3331± 35 cal. yr. BP, while charcoal fragments from the base and middle part of the Upper unit yielded 14 C ages of 1088 ± 30 and 905 ± 30 cal. yr. BP, respectively (Fig. 3.4).

Stratigraphy of the En Nahud -El Khowei Sector

Four sections have been studied in this sector. There, the succession comprises three main stratigraphic units, separated by erosional contacts (Fig. 3.5).

Lower unit. This unit exhibits mostly mottled silt or fine sand sediments rich in calcareous nodules. Structureless beds of sand sediments have been observed locally. Its maximum measured thickness is ≈ 270 cm. Its lower contact has not been observed.

Socond unit.

The second unit overlies conformably the Lower unit. It is a lime or sandy limestone bed, the thickness of which varies between ≈ 55 and ≈ 80 cm. The carbonate bed of this unit is marked by the presence of gastropod shells. The lower contact of this unit is usually progressive, whereas the upper contact is abrupt.

Upper unit. The Upper unit unconformably overlies the second unit through an erosional contact. It is formed entirely of red fine sand sediments containing gastropod shells. Its maximum measured thickness is ≈ 155 cm.

Age constraints

Organic matter from the Lower unit yielded a 14 C age of 7968 ± 50 cal. yr BP (Fig. 3.5), while the gastropod-rich limestone bed from the second unit yielded 14 C ages of 9788 ± 80 and 6419 ± 35 cal. yr. BP in the En Nahud area. Both were collected from quarries, the older sample was collected ≈ 20 km north of the En Nahud town, while the younger one is from the southern part of the En Nahud city. The exact stratigraphic location of these samples are unknown, they may either represent the base and top of the limestone unit, or come from two or more distinct limestone levels. These ages are discussed below.

Stratigraphy of the North Bara -Sodari sector

In addition to the surficial survey in many sites in the El Ga'a and Sodari area, the stratigraphy of this sector has been defined according to four sections, three of them are located North of Bara, while the fourth one is a 4 meters deep well, caved in the El Ga'ah area. The Late Quaternary series of this sector comprises three stratigraphic units, separated locally by erosional contacts (Fig. 3.6).

Lower unit. The Lower unit is formed of mottled sand sediments. The sand grains are coarse to fine. The maximum measured thickness of this unit is ≈ 445 cm. Its basal contact has not been observed.

Second unit. This unit overlies unconformably the Lower unit. It is formed of alternating beds of limestone and argillaceous sand sediments. The upper surface of the limestone beds is karsted and/or eroded. The argillaceous sand beds may contain limestone clasts. The contact between the middle and lower units is generally abrupt and conformable, but an erosional, unconformable contact is observed locally in section North Bara 2. The thickness of this unit varies from 0 to ≈ 360 cm, according to the places (Fig. 3.6).

Upper unit. The Upper unit conformably overlies the second unit, or rests directly on the Lower unit through a conglomeratic bed made of iron concretions. It is entirely formed of coarse-to fine-grained red sand sediments. Its maximum measured thickness is ≈ 255 cm.

Age constraints

Three samples of organic matters from organic-rich argillaceous sand sediment beds have been collected in the El Ga'ah well. They belong to the second unit and yielded 14 C ages of 10538 ± 45, 10243 ± 40 and 5941 ± 30 cal. yr BP, respectively. The oldest age has been obtained from the base of the second unit, while the youngest one is from the top of the same unit (Fig. 3.6). In addition, archeological potteries and bone samples have been collected from the paleo-shoreline of north El Ga'ah and north Sodari paleo-lakes, in order to obtain more dates. However, bone samples either were not suitable for dating (lack of collagen for five samples), or yielded very recent ages : 108.47 ± 0.36 yr BP North of El Ga'ah, and 107.86 ± 0.31 yr BP near Sodari. These two samples exhibit present modern carbon (PMC), which means that they are not even suitable for calibration.

Archeological pottery samples have been investigated at the university of Khartoum-Sudan, Archeology department, with the help of the archeologist Dr. Huweida. The collected potteries included the dotted wavy lines decorative, banded decorative, polished and thin, and polished and thick. The dotted wavy line potteries have been classified as Neolithic potteries, whereas other three forms have been determined as historic potteries (Merowetic and Islamic eras). 2000). Reappraisal of radiocarbon dates from central Sudan suggests the following culture succession: Early Khartoum is 9000-7000 BP, Early Neolithic Group is 6900-6450 BP, Middle Neolithic Group is 6400-5800 BP, Late Neolithic Group ages are 5500 to 4700 BP [START_REF] Hassan | Chronology of the Khartoum 'Mesolithic' and 'Neolithic' and related sites in the Sudan: statistical analysis and comparisons with Egypt[END_REF]. Therefore, the age indication of the potteries ornamented by dotted wavy lines (≈ 7000-6000 yr BP) is consistent with our 14 C ages from the Second unit.

Carbon dating

Preview of Carbon dating

The lack or limitation of organic material makes it difficult to date all the stratigraphic units, especially those of light or reddish color. Among the twenty-six samples tested for carbon dating (organic soils and/or paleosols, charcoal, bones; Table 3.2), only seventeen 14 C dates have been considered as dated points in the study area. The remaining nine samples either produced very recent ages, or were not suitable for dating. Among the rejected samples, eight are bones and only one is charcoal. The charcoal sample (Ka 1-1) produced a recent age due to its high percent modern carbon (pMC). The bone samples were rejected due to their high percentage of modern collagene and/or due their very low nitrogene content (< 0.1% N).

The stratigraphic position of the dated samples is given in Figures 3.8 -3.15. The oldest date is 12 942 ± 100 cal. yr BP from the mottled, pale grey Lower unit of the East Abu-Zabad 1 section at depth 650 cm, and the youngest date is 246 ± 30 cal. yr BP from the Dilling section, at depth 40 cm.

Discussion of the 14 C dates

As shown in table 3.2, the dated material includes organic soil, charcoal and limestone. This kind of material, however, has some problems when being dated [START_REF] Walker | Quaternary dating methods[END_REF]. Although all soils contain carbon and can theoretically be dated by radiocarbon, the dynamic nature of the soil system implies that it receives organic matter over a protracted time period, and thus, any radiocarbon date on a soil will be largely a measure of the mean residence time of the various organic fractions within the soil. Therefore, because of the continuous input of organic material into soil, measured radiocarbon ages of soil organic matter or its fractions are generally younger that the true ages of soil (Wang et al., 1996[START_REF] Walker | Quaternary dating methods[END_REF]. Among the samples we dated for this study, a sample from East El Khowei belonging to the Lower unit provided a ≈ 8 kyr BP age (Fig. 3.11), although it is located below the Second unit (limestone and sandy limestone), which yielded ages of ≈10 and ≈ 6 k yr BP around En Nahud and El Ga'ah (Figs. 3.12 and 3.15). This may be due to the contamination by younger material, such as deep roots, which may have introduced young organic matter into older horizons [START_REF] Walker | Quaternary dating methods[END_REF]. Alternatively, deposition of the Second unit around the El Khowei town may have started later than in En Nahud, or the Second unit may correspond to some distinct, heterochronous carbonate beds, the age of which varies from ≈10 and ≈ 6 k cal. yr BP.

Lake sediments are extremely susceptible to contamination by older carbon residue and this leads to a dilution of the 14 C concentration, and hence, to dates that are older than the actual date of sediment deposition [START_REF] Walker | Quaternary dating methods[END_REF]. The same problem is also found in the terrestrial organisms, which may ingest older carbonates (limestone or soil carbonate) during their life time and this older material can be integrated into their shell [START_REF] Walker | Quaternary dating methods[END_REF]. On the other hand, bioturbation caused by organisms in pond or lake beds can lead to disturbance of the sediment sequence and the downward movement of younger sediments, and thus younger age can be recorded from older sediments [START_REF] Walker | Quaternary dating methods[END_REF].

The two dated samples from the En Nahud area were collected from excavated quarries. Therefore, they may represent the base and top of limestone horizon, or they may represent two distinct limestone horizons. We did not observe superimposed carbonate levels in the En Nahud and El Khowei areas, but several limestone beds were observed in the El Ga'ah well. On the other hand, since the samples were collected very close to the surface, one of them may have been contaminated by modern organic matter which may reduce its real age. The * symbol in the En Nahud sample depths indicates that the samples were collected from surficial excavated material. The reworking of sediments may have some effects on the dated samples of charcoal materials.

As a matter of fact, all charcoal samples were collected in fluviatile sediments. Therefore, this material is likely reworked from older deposits, and old dates may be obtained from younger sediments. This process may have affected the date obtained from the North Dilling1 section (Fig. 3.9), where a sample collected from the base of the Upper unit, yielded an age of ≈ 7700 yr BP, which is older than dates from the underlying unit of North Dilling2, dated at ≈ 4900 and ≈ 4100 yr BP (Fig. 3.8). Moreover, deposition may take place in channels caved in older sediment due to local erosion, and yielding young ages in depth. In the East El Obeid section, a sample collected at depth of ≈ 90 cm provided a 14 C age of 40 ± 30 yr. This date has been excluded from the samples presented in table 3.2.

Stratigraphic correlations and synthesis

Although the concept of correlation goes back to the early history of stratigraphy, disagreement has persisted over the exact meaning of the term. Historically, two points of view have prevailed. One view rigidly restricts the meaning of correlation to demonstration of time equivalency, that is, to the demonstration that two bodies of rock were deposited during the same period of time (Dunbar and Rodgers, 1957;Rodgers, 1959). From this point of view, establishing the equivalence of two lithostratigraphic units on the basis of lithologic similarities does not constitute a correlation. A broader interpretation of correlation allows that equivalency may be expressed in lithologic, paleontologic or chronologic terms (Krumbein and Sloss, 1963).

In other words, two bodies of rock can be correlated as belonging to the same lithostratigraphic or biostratigraphic unit even though these units may be of different ages. From a practical point of view, most geologists today accept the latter, broader view of correlation. Petroleum geologists, for example, routinely correlate subsurface units based on lithology, on specific "signatures" recorded by instrumental well logs, or on reflection characteristics on seismic records [START_REF] Boggs | Principle of Sedimentology and Stratigraphy[END_REF]. When dealing with long-lasting units, or regional map surveying, this may be acceptable. In the present work, since (1) the age difference of the Lower unit would represent 70 to 90% of the studied period, (2) the targeted period is very short (mostly Holocene), and (3) the aim of this study is to reconstruct the time evolution of climate, this becomes unacceptable ! Therefore, we preferred to follow time correlation in this study.

According to the presented stratigraphic results in the different areas (Figs. 3.2 -3.6), many areas comprise two stratigraphic units. However, a carbonate unit is present in the En Nahud-El Khowei and Bara-Sodari areas, and deposits younger than the limestone beds are evidenced by 14 C datings in the El Obeid area. Additionally, 14 C dates disclosed a sedimentary hiatus between the second or third unit, and the Upper unit, i.e. between ≈ 6 or 3 kyr BP, and ≈ 1 kyr BP. As a consequence, we drew three time-correlation profiles: an E-W trending El Fula -Dilling (southern part) profile, an E-W oriented En Nahud -El Obeid profile (central part of the study area), and a N-S trending El Ga'a -Dilling profile. Because of the scarcity of dates, these correlations are partly based on the lithological similarities of the units.

El Fula -Dilling transect

The southern profile joins the sections of the El-Fula and Dilling areas, through the Abu-Zabad sections, along a total distance of 180 km. In this area, the four stratigraphic units are recognized partially, since the second unit is missing along Abu Zabad El Fula sector (present in channel deposition lenses ?), and the lower (first) unit is missing in the Dilling area. The base of the lower unit is dated at ≈ 13 000 yr BP in Abu Zabad. The Second unit has not been dated in this transect. The Third unit is dated at ≈ 4900 yr BP in the Dilling area. In the latter area, the charcoal from the basal conglomerate of the Upper unit dated at ≈ 7700 yr BP is interpreted as reworked. The Upper unit in the Dilling area has been dated between ≈ 750 and ≈ 250 yr BP in its upper half part, while its lower part remained undated.

In the Abu Zabad and El Fula areas, the oldest and only available date is the ≈ 13 000 yr BP age obtained from the lower part of the Lower unit. The Third unit in the Abu Zabad and El Fula areas is tentatively correlated with the dated beds of the Dilling area. The Upper unit of the Abu Zabad -El Fula area is correlated with the Upper unit of the Dilling area. In this interpretation, this unit has a maximum age of ≈ 1000 yr BP (see below).

The sedimentary gap between the Lower and Upper unit extends from ≈ 4 to 1 kyr BP in the Dilling area. This gap is marked by erosions and therefore, is shown as a non-deposition and erosional hiatus in the correlation profile (Fig. 3.16).

En Nahud -El Obeid transect

Fig. 3.17: Time-correlation transect between En Nahud and El Obeid.

This transect represents the central part of the study area, joining the En Nahud sections to the West and the El-Obeid sections to the East, through the El Khowei area, with a total distance of 210 km. This transect exhibits the four stratigraphic units, the Second unit being apparently absent in the El Obeid area, and the Third unit being missing in the En Nahud-El Khowei areas.

The Lower unit is correlated with its equivalent of the southern transect, where a maximum age of ≈ 13 kyr BP has been measured. The youngest age of this unit is variable along the transect: it is older than ≈ 10 kyr BP in the En Nahud area, but it may extend up to ≈ 7.5 kyr BP in the El Khowei area. The Second unit encompasses the ≈ 10 to ≈ 6 kyr BP time-span, according to dates obtained in the En Nahud area. The top of the Third unit is dated at ≈ 3 kyr BP in the East El Obeid section. Based on the dates obtained in the South El Obeid and Dilling sections, the upper unit is assumed to be younger than ≈ 1 kyr BP.

The time gap between the Middle and Upper units encompasses the ≈ 6 -1 kyr BP time interval to the West (En Nahud, El Khowei), and the ≈ 3 -1 kyr BP time span to the East. It is interpreted as a hiatus due to erosion occurring before 1 kyr BP, or to non-deposition, or to both processes (Fig. 3.17).

El Ga'ah -Dilling transect

This transect correlates the southern and northern parts of the study area. The total length of this profile is 390 km, with a 90 km long W-E segment between El Ga'ah and North Bara3, and a 300 km long segment from North Bara3 to Dilling. This transect comprises partially four stratigraphic units. The Lower unit is missing to the south, and the Third unit seems to be missing to the North (Fig. 3.18). No age constraints have been found for the Lower unit in the northern area. It is, however, older than the Second unit, i.e. older than 10.5 kyr BP, as indicated by dates from the El Ga'ah section.

In the latter section, the age of the Second unit is consistent with the correlated unit in the En Nahud area, i.e. ≈ 10 to 6 kyr BP. The Third unit is dated between ≈ 3 and 4.9 kyr BP in south El Obeid and Dilling, respectively. The data from South El Obeid and Dilling suggest that the Upper unit is younger than 1000 yr BP.

Therefore, as noted in the Central transect, a hiatus occurred between ≈ 3 and 1 kyr BP in the southeastern area (Dilling, El Obeid), and probably between ≈ 6 and 1 kyr BP in the northwestern areas (Bara, El Ga'ah, Sodari).

Synthesis of the stratigraphy of the Late Quaternary deposits of Kordofan

The stratigraphic succession of the study area exhibits four stratigraphic units, corresponding to four distinct depositional periods, to which one must add a fifth period interpreted as a hiatus. These units show some variation from north to south, since the Lower unit is missing in the South (Dilling area) and the Third unit seems absent in the North. The periods corresponding to these units are: older than 10 kyr BP for the Lower unit (locally older than 7.5 kyr BP), between ≈ 10 and 6 kyr BP for the Second unit, between ≈ 6 and 3 kyr BP for the Third unit, the hiatus period extends from ≈ 6 to 1 kyr BP and from ≈ 3 to 1 kyr BP in the northern and southern parts, respectively, and the fifth period is younger than 1000 year and corresponds to the Upper unit.

Sedimentology

All geologic studies aim at a better understanding of Earth's history. Sedimentary rocks yield many information recorded by various kinds of sedimentological features [START_REF] Boggs | Principle of Sedimentology and Stratigraphy[END_REF].

During field work, great attention has been paid to the sedimentary record.

In addition to facies and lithological variations, the studied sections exhibit many sedimentary figures such as cyclic sedimentation, fining upward succession, unconformity surfaces, bedding pattern, as well as chemical (calcareous nodules) and biological features (bioturbation, root traces), which will be analyzed successively.

Sedimentary Facies

The present sedimentological study is based on the detailed measurement of twenty-four stratigraphic sections (Fig. 3.1 and table 3.1). Height sedimentary facies were recognized in a series of sections studied during field work. These facies are : (1) mottled sand or silty sand, (2) lens shaped conglomerate, (3) alternating sand and silt, (4) sandy carbonate, (5) carbonate, (6) argillaceous sand, (7) red sand and (8) clinoform bearing, coarse clastic facies. Table 3.3 summarizes the main features of these facies.

Mottled sand sediments facies

Description: The mottled sand facies is characterized by mottling, root traces and a noticeable organic matter content. However, there are some differences according to the sites, or even along a same section. For instance, the mottled facies may vary in color from green, to various shades of color, or to pale yellow. In addition, it exhibits intensive development of calcareous nodules in different contents and forms of occurrence (Fig. 3.19). Some differences in grain size have been also observed, varying according to the sites from silty sand to coarse sand, or locally poorly sorted sand. Fine grained mottled facies is restricted to the southern part of the area, whereas coarser grained facies dominates in the northern area.

Distribution:

The mottled sand facies is distributed throughout the study area, except around Dilling.

Interpretation: Mottling or marmorization [START_REF] Freytet | Lacustrine and palustrine carbonate petrography: an overview[END_REF] in soil is primarily associated with soil wetness [START_REF] Ciolkosz | Color and mottling in Pennsylvania soil[END_REF], microorganism population, organic carbon energy source, lack of oxygen, time and temperature (Dobos, 1990, in Ciolkosz andDobos, 1990). Generally, mottling feature occurs when the iron content of sediments is > 2% and if water table fluctuations occur, then ferrous iron moves easily and fixes as ferric iron. The result is a sediment with mottled pink, purple, red, and yellow patches in the accumulation area of ferric iron, and grey or white in the area depleted in iron. Manganese and calcium can also migrate with iron, resulting in a complex fabric of mottled patches and ferruginous nodules. As a consequence, the dominating mottling feature in this facies and its wide occurrence all over the area, indicate that the area experienced wet conditions under low oxidation rates [START_REF] Freytet | Lacustrine and palustrine carbonate petrography: an overview[END_REF].

Facies Distribution Marked Features

Mottled sand or conglomerate

All area except Dilling Mottling, mostly contain calcareous calcretes, root traces, rich in organic matter, pale grey, dark grey and green color.

Lens shaped conglomerate

Widespread in Dilling, El Fula, scattered in Abu Zabad, south El-Obeid, El Khowei Poorly sorted, quartzite and iron oxide clasts, current features: locally planar and trough cross bedding.

Alternating sand and silt

Widely in Dilling, locally south El Obeid, east Abu Zabad, El Fula.

Mostly well sorted but locally poorly sorted; current features: planar cross bedding, trough cross bedding, planar laminae and current ripples, alternation of white sand and dark shale laminae (only in fine-grained beds), locally fining upward, and locally cyclic sedimentation.

Sandy carbonate

Grey argillaceous sand

North Bara, El Ga'a and Sodari Grades of grey and yellow, friable, usually fine-grained

Red sand

All area except Dilling Mostly fine grained, mostly red, rarely reddish brown, no or rare structures, coarse grain size only in northernmost parts

Clinoform bearing coarse clastic

El Ga'ah Mostly coarse sand and gravels, gastropod shells, root molds

The existence of organic matter in soils and sediments is a function of several factors, among which climate is the most important (e.g., [START_REF] Tóth | The effect of climate change on soil Organic Matter decomposition[END_REF]Forry et al., 2014;[START_REF] Priene | Soil organic matter and degradation. Soil conservation and protection for Europe[END_REF]. Moreover, organic matter decays more rapidly at higher temperatures, so soils in warm climates tend to contain less organic matter than those in cool climates (Fact sheet No 3, 2009).

Fine-textured soils tend to have more organic matter than coarse soils; they hold nutrients and water better, thus providing good conditions for plant growth. Coarse soils are better aerated, and the presence of oxygen results in a more rapid decay of organic matter. The wetter a soil is, the less oxygen is available for organic matter to decay, so that it accumulates. Roots are a great contributor to soil organic matter. Grassland produces deep roots that decay deep in the soil. Forest soils in contrast rely primarily on the decay of surface litter for organic matter input. Therefore, the existence of organic matter in mottled facies with variable amount indicates that the environment was wet or saturated especially those marked by dark colors (green and dark grey), while the mottled facies marked by their lighter colors (pale grey, yellow and reddish yellow) experienced less wet condition and became more oxidized than the dark ones.

Additionally, the preservation of root molds, which are tubular voids that outline positions of former, now decayed roots [START_REF] Freytet | Lacustrine and palustrine carbonate petrography: an overview[END_REF], provides good indicator that the area was invaded by a significant vegetation cover.

The most important feature in this mottled facies is the development of calcretes or calcareous nodules. These calcretes present a variety of shape, size and form. Two forms of calcretes have been observed in the study area: nodular and tubular [START_REF] Chen | Calcrete characteristics, distribution and use in mineral exploration[END_REF]. The nodular form is the most abundant and widespread, whereas the tubular form is restricted to some sites.

In the study area, the shape of the nodules is irregular, though leaning to be equal in the three dimensions. It does not contain any biogenic features, but instead it contains locally some iron concretions and quartz grains. The lack of biogenic features indicates their groundwater origin (Alonso-Zarza and Wright, 2010; Alonso-Zarza et al., 201I), related to shallow aquifer systems [START_REF] Netterberg | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF][START_REF] Mann | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF][START_REF] Mann | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF]. The precipitation of carbonate occurs within a previous host rock and around the groundwater table [START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF]. The degree of complexity of individual profiles is largely influenced by the geomorphological setting and hydrological history of the local groundwater systems [START_REF] Arakel | Quaternary vadose Calcretes revisited[END_REF]. The amount of carbonate in a calcrete profile tends to exceed the original pore space of the host. This produces an increase in volume and physical displacement of the original host components.

The development of calcretes occurs in different ways: direct calcium carbonate precipitation [START_REF] Chen | Calcrete characteristics, distribution and use in mineral exploration[END_REF], replacement, and displacement [START_REF] Chen | Calcrete characteristics, distribution and use in mineral exploration[END_REF][START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF].

During replacement, some host components are dissolved and/or removed and are replaced by calcium carbonates, while displacement occurs when the amount of carbonate in a calcrete profile tends to exceed the original pore space of the host. This produces an increase in volume and physical displacement of the original host components [START_REF] Chen | Calcrete characteristics, distribution and use in mineral exploration[END_REF].

Tubular calcretes [START_REF] Chen | Calcrete characteristics, distribution and use in mineral exploration[END_REF] or rhizoliths [START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance[END_REF], root calcretes (Alonso-Zarza and Jones, 2007) and rhizogenic or root formed calcrete [START_REF] Freytet | A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of southern France -Discussion[END_REF] have been found locally, mostly around El Khowei (specifically east El Khowei) and rarely in north Bara. Generally, it is marked by an elongated shape more and is concentrated in specific zones. The typical morphology of tubular calcrete indicates that they formed by replacement of the root organic matter by calcium carbonate, the role of roots being important (Alonzo-Zarza et al., 2010). Moreover, the existence of some biogenic features in the tubular calcrete indicates that it formed in the vadose zone [START_REF] Arakel | Quaternary vadose Calcretes revisited[END_REF], where precipitation of carbonate takes place mostly above the water table and within the sediment [START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF].

As shown in figure 3.22a, the tubular calcretes are more or less vertical with different inclination, and they are larger in size in the upper part than in the lower part. This may be due to the variation in the roots density and to the fact that root size is usually greater in the upper part than in the lower part, nodules being larger in the upper part. Difference in nodule size could also be due to additional re-precipitated carbonate, since re-precipitated carbonate tends to concentrate near the surface and within the voids, due to preferential cohesive bonds between carbonate crystals [START_REF] Chen | Calcrete characteristics, distribution and use in mineral exploration[END_REF]. The channel facies includes the lens-shaped conglomerates and the sediments of poorly sorted sand beds. The former indicates high energy sedimentation, while the latter indicates lower energy regime.

Description: In the Dilling area, the conglomerate is made of poorly sorted, dominantly quartz clasts, with other lithoclasts and some reworked calcareous nodules, while around Abu Zabad and El-Fula it comprises iron concretions and quartz clasts. In some horizons, the iron concretions occur in lenses intercalated with other facies like the mottled facies. The thickest beds of conglomerates (325 cm) are found East of El-Fula. The base of the conglomerate beds is mostly an undulated surface, which separates the conglomerate from the underlying mottled facies or from the basement rocks.

The conglomerate facies is also observed in the El Khowei area, where it contains angular iron concretions of different sizes, ranging between 0.5 and 3.5 cm in size, as well as smaller well rounded quartz grains (Fig. 3.23). Locally, this facies is characterized by well observable current features (e.g., trough and planar cross beddings in Dilling area). Therefore, the iron concretions of the conglomerates of the Abu Zabad, El Fula and El Kohwei are interpreted as reworked from the Nubian sandstone located to the West and North. The topography supports this interpretation, since El Fula and Abu Zabad are located on the southern slope of the Nubian sandstone plateau, while En Nahud and El Khowei are located upon the latter (Fig. 3.24).

3.4.1.2.B. Low regime channel facies (poorly sorted sand sediment beds)

Description: This facies is formed of poorly sorted sand, which exhibits some small lithoclasts within the size-range of fine gravel. Its color varies from pale grey to reddish brown (Fig. 3.25). This facies is marked by current features, such as trough cross bedding and planar cross bedding. Local planar lamination and current ripples are observed in the medium to fine sand beds. Locally these poorly sorted sand beds are lens-shaped (Fig. 3.26). In this case, their basal contact is abrupt and conformable separating it from the underlying high regime channel facies (southern areas), while it is either an erosional unconformable contact when this facies overlies the mottled facies, or abrupt and conformable contact when overlying other alluvial sediments (northern areas). [START_REF] Smith | The Braided Stream Depositional Environment: Comparison of the Platte River with Some Silurian Clastic Rocks, North-Central Appalachians[END_REF].

Alternating sand and silt sediments (flood plain facies)

Description: Lithologically this facies is formed of fine-to medium-grained sand, alternating with sandy shale and fine-grained sand sediments beds. Medium-to coarse-grained sand beds are observed in some sites (e.g. Dilling, south El-Obeid2). The color of sand beds is red to reddish brown, while that of silt or shale beds varies from dark grey to green in the Dilling area, and from reddish-brown to dark grey in northern parts (south El Obeid) (Fig. 3.27).

The alternating beds result in an apparently cyclic sedimentation (Fig. 3.28). Sand sediment beds are locally lens-shaped, as observed south of El Obeid (Fig. 3.26); they are marked by planar or trough cross-bedding, current ripples, planar lamination, and are locally poorly sorted or may present a fining-upward evolution (Fig. 3.29). The base of sand sediment beds in the Dilling area is an abrupt and conformable contact, while South of El Obeid, the sand sediments base is an erosional unconformable contact, eroding the underlying mottled facies. The contacts between the sand and silt sediments are all abrupt and conformable. The fine-grained beds are marked by current ripples and planar laminations. Laminae are mostly 2 to 5 mm thick, but may be 5 mm to 1 cm thick, and rarely reach 1 to 2 cm thick, as observed in the south El-Obeid1 section.

Distribution: This facies is common in the southern part of the study area, around Dilling, El-Fula, east of Abu-Zabad, and south of El-Obeid. Interpretation: The occurrence of planar and trough cross-bedding, planar lamination and current ripples in the sand sediment beds supports a high-to low-energy fluviatile depositional environment [START_REF] Boggs | Principle of Sedimentology and Stratigraphy[END_REF][START_REF] Hewaidy | Stratigraphy, facies analysis and depositional environments of the Upper Unit of Abu Roash "E" member in the Abu Gharadig field, Western Desert, Egypt[END_REF]. As a matter of fact, fluviatile deposits commonly display abundant traction structures, such as planar and trough cross-bedding, upperflow-regime planar bedding and ripple-marked surfaces [START_REF] Boggs | Principle of Sedimentology and Stratigraphy[END_REF].

The alternating beds of sand and silt sediments are assumed to be formed in the flood plain of a river. When the river comes out of its channel, it first deposits the coarse-grained sediments in a high energy regime, and as the flood decreases in intensity, the fine particles are laid down [START_REF] Boggs | Principle of Sedimentology and Stratigraphy[END_REF]. The locally observed fining upward evolution that indicates a gradual decrease in current strength through time, supports the flood plain environment for this facies. This repeated mechanism leads to a cyclic sedimentation. The lens shape of the sand (e.g., south El Obeid1) may be formed due to subsequent erosion, to flood deposits, to the topography of the flood plain, or may represent crevasse splay deposits.

Red sand facies

Description: The granulometry of the sand is dominated by fine grains, but coarser sands are found in the northernmost part of the study area. Generally, this facies is very homogeneous and devoid of any current features. The sediment is usually harder than the flood plain deposits and looks compacted or cemented. The red color is common in all parts, but reddish brown colors are found locally (Fig. 3.30).

Distribution: It is the most widespread facies; it is common in all sites but the Dilling area.

Interpretation: The usual absence of sedimentary features and the prevailing fine grained sand, suggest that this facies has been deposited by aeolian processes. However, in the southern areas, some fine-to medium-grained intercalations of fluviatile sands suggest that the aeolian deposits were spradically reworked by alluvial processes. Note that these southern areas are presently the wettest and steepest zones of the study area, which may explain stronger alluvial processes than in the northern regions. The hardness of the sediment and its reddish color is probably due to its cementation with iron oxide cement. [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF] reported that these deposits in Kordofan, are typical wind deposited sand with well-rounded quartz grains and a color varying from pale buff to deep red. He also noted that the red color is due to iron staining on the surface of the quartz grains. Investigations on mid-Pleistocene red sediments in sub-tropical China, concluded that these deposits were subject to considerable mixing prior to deposition and strong subsequent chemical weathering [START_REF] Hong | Geochemical constraints on provenance of the mid-Pleistocene red earth sediments in subtropical China[END_REF]. Many soils within the tropical, subtropical, equatorial and Mediterranean climatic zones contain a significant amount of clay minerals -dominantly kaolinite -and are frequently characterized by a moderate to high amount of "free" iron oxides. These oxides, which include hydroxides and hydrous oxides, either occur in the form of amorphous gels, discrete particles as coatings on sand grains, or form part of the soil matrix as micro aggregates, cemented aggregates of a few centimeters in diameter, or cemented continuous soil layers. Many of these soils present red colors (Volkoff, 1998).

Sandy carbonate facies

Description: This facies is partially formed of massive, poorly lithified limestone beds and partially of sandy to silty limestone beds. Generally, it is deeply karsted and presents root traces (casts / molds) and minor desiccation cracks. Well preserved gastropod shells are common, and are usually highly concentrated at the top of the bed. Locally however, gastropod shells are concentrated within the facies itself (Fig. 3.31). This facies also occurs locally as clasts reworked in subsequent channel deposits (Fig. 3.32).

Distribution: This facies is widespread in the western-central part of the study area, from the En-Nahud to the El-Khowei areas.

Interpretation: The lithology and sedimentary features of this facies indicate a shallow, subaqueous deposition, frequently exposed to desiccation. They are consequently interpreted as palustrine deposits [START_REF] Freytet | Lacustrine and palustrine carbonate petrography: an overview[END_REF][START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF]Alonso-Zarza et al., 2011). Pure or granular limestone is formed in the bottom of shallow water bodies (Alonso-Zarza, 2003), while silty and/or sandy limestone is formed in less shallower water. However, soils accumulated in the lake shores are sometimes very rich in carbonates and are true palustrine limestones [START_REF] Freytet | Lacustrine and palustrine carbonate petrography: an overview[END_REF]. In palustrine carbonates, precipitation of lime mud occurs in a lacustrine water body, the palustrine carbonates necessarily form on previous lacustrine mud ( [START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF].

Roots induce movement of water and chemical elements (Clothier andGreen, 1997, in Alonso-Zarza, 2003) and act in two different ways: (i) by penetrating the lacustrine mud when the level of the lake drops, and/or (ii) inducing the biochemical precipitation of carbonate around the rhizosphere. In either case, the carbonates formed in these conditions present evidences of active pedogenic processes or long-lasting exposure periods (Alonso-Zarza, 2003). Limestones with root cavities indicate the presence of a well-established vegetation cover and suggest that the root system had to penetrate the recently deposited sediments to reach the water table [START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF].

Desiccation cracks are common in the palustrine facies [START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF]Alonso-Zarza et al., 2011) and are usually due to subsequent sub-aerial exposure (Alonso-Zarza, 2003).

The intensity and duration of the desiccation and root penetration, as well as any later rise in the water table during more humid periods, can cause the reworking, concentration, and coating of the mud fragments (Alonso-Zarza, 2003). Karstification, desiccation and root penetration make easy the fragmentation of this facies and its reworking into subsequent sedimentary deposits (Alonso-Zarza et al., 2011).

Fig. 3.31: Palustrine facies. (A) Lithified whitish-grey limestone, containing gastropod shells within the unit and on its upper surface (En Nahud). (B) White limestone, deeply karsted, with gastropod shells on its top (the black color is due to surficial iron coating) (El Khowei). (C) Grey friable sandy limestone (El Khowei). (D) Pale grey sandy limestone containing calcareous nodules and gastropod shells (West El Khowei).

Carbonate facies

Description: This facies consists of low density, 15 to 45 cm thick, marly or chalky limestone beds, deeply karsted, brecciated in its upper part, white to pale grey in color, and is marked by the occurrence of abundant gastropod shells and locally ostracods (Fig. 3.33). In the El-Ga'ah well, three horizons of this facies have been observed alternating with argillaceous sand sediments facies. Interpretation: The formation of this facies requires the presence of water bodies, but probably different than those required to form the palustrine facies of the En Nahud -El Khowei area. The scarcity of sandy component and the low density ascribed to the presence of abundant diatoms shells suggest a deeper depositional depth. This facies is, therefore, interpreted as deposited in lacustrine environments. The deeply karsted and brecciated upper surfaces of the limestone beds indicate subaerial exposure and is common in lacustrine environment [START_REF] Freytet | Lacustrine and palustrine carbonate petrography: an overview[END_REF][START_REF] Alonso-Zarza | Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record[END_REF]). An idealized sketch of the distribution of continental carbonate facies in Uruguay proposed by Alonso-Zarza et al. (2011) shows that the lacustrine facies are formed under a thicker water column than the palustrine facies.

Lakes are dynamic systems sensitive to subtle changes in climate, such as fluctuations in precipitation and run-off. Lacustrine environments, therefore, are unstable, and as a result, lacustrine facies patterns are vertically and laterally complex [START_REF] Tucker | Carbonate sedimentology[END_REF]. This variability leads to alternating sedimentation features in these lacustrine facies.

This alternating feature and the existence of karsted and brecciated features in the upper surface of each carbonate level, indicates that the lake has filled up and evaporated more than one time.

After each filling up stage, argillaceous sand sediments are overlain by lacustrine deposits, and after each evaporative stage, desiccation and brecciated features took place on the upper surface of the lacustrine limestone. Alonzo-Zarza et al. ( 2011) reported that lacustrine facies may alternate with palustrine facies. In continental series, lacustrine limestone is often associated with other deposits including shales or varved clays, sometimes rich in organic matter, and fine, medium or coarse sand [START_REF] Freytet | Petrography and paleo-environment of continental carbonate deposits with particular reference to the Upper Cretaceous and Lower Eocene of Languedoc (Southern France)[END_REF].

Argillaceous sandstone facies

Description: It consists of fine sand grains in a friable argillaceous matrix, locally containing irregular lacustrine limestone clasts. Its color is mainly grey, especially around El-Ga'ah and Sodari, and is yellowish in north Bara (Fig. 3.34). In the El-Ga'ah section it is intercalated with the lacustrine limestone facies. Distribution: This facies is restricted to the northern parts of the area, around El-Ga'ah and Sodari, and locally in north Bara.

Interpretation: The alternation of this facies with the lacustrine facies suggests alternating lacustrine and alluvial depositional environments, resulting in the deposition of alternating lacustrine limestone and argillaceous sand sediments, respectively.

The existence of lacustrine limestone fragments in this argillaceous facies indicates that the underlying lacustrine limestones have been reworked and redeposited with the argillaceous sand sediments.

Clinoform bearing, coarse clastic facies

Description: This facies comprises coarse sand and gravel beds, the strata of which exhibit a marked inclination. The color varies between red and pale yellow; the red color dominates in the upper part, while the pale yellow is restricted to the lower beds. It overlies the lacustrine limestone facies of the El Ga'ah paleo-lake. The thickness of these sediments exceeds 5 meters in the excavated section and decreases toward the lake, as a result of the clinoform nature of the beds (Fig. 3.35). The beds are inclined toward the East or Southeast (toward the lake). Moreover, some fine-grained beds exhibit well preserved gastropod shells and thick root molds (Fig. 3.36). Interpretation: The coarse material of this facies indicates the proximity of a stream that supplied the clastic material. The well-marked clinoforms and the fact that these coarse-grained clastic deposits overly lacustrine sediments led us to interpret them as delta sediments prograding into the El Ga'ah paleolake. This indicates that this area, and probably part of the northern study area, locally experienced a strong fluvial activity during the late phase of lake forming. The presence of shells of gastropods mostly living at the shoreline, suggests an intermittent regime of the river feeding the delta. The thick root molds indicates vegetation growing close to the shoreline.

Grain size analysis

Introduction

The grain size of a total of 48 samples, collected from excavated sections in four sites, were analyzed. All sections are formed of two facies: the mottled facies in their lower part and the red sand facies in their upper part (Fig. 3.37). Field observation showed that these facies display variations in terms of grain size. Therefore, the grain size analysis have been carried out not only to understand how important these variations are, but also to specify the origin of the sands and the transport and deposition processes. The studied sections are North Bara3, North Bara1, West El-Obeid (Ayara) and East Abu-Zabad (Dabkar). They are aligned along a 280 km long, NNE-SSW trending transect (Fig. 3.38). The topographic profile shows a NNE-dipping slope with average gradient of 0.85 m/km, and a short, steep SSW-dipping slope to the South (Fig. 3.39).

Results

Cumulative curves

The 48 cumulative curves representing the 48 samples from four sections have been plotted in semi-log distribution graph, and compiled on one graph gathering data from each section (Fig.

3.40).

The curves are distributed in size range between 2.6 and 0.012 mm. The maximum and minimum grain sizes are recorded in the North Bara3 and East Abu Zabad sections, respectively. On the other hand, the size range is different from one section to another. While in north Bara3, it ranges between 2.6 and 0.029 mm, it is comprised between 1.1 mm and 0.015 mm in North Bara1, between 1.0 to 0.015 mm in west El Obeid, and between 0.8 and 0.012 mm in East Abu Zabad.

The measures in phi units for Φ 5, Φ 16, Φ 25, Φ 50, Φ 75, Φ 84, and Φ 95 have been obtained from each curve, in order to calculate the graphical statistics of the mean, standard deviation (sorting), skewness and kurtosis (Table 3.4). Frequency curves have been built from the cumulative curves following [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF]. However, the frequency curves are not applicable to calculate graphical statistics [START_REF] Folk | Longitudinal dunes of the northwestern edge of the Simpson Desert, northern Territory, Australia, 1. Geomorphology and grain size relationships[END_REF]; therefore, some representative frequency curves presented in fig. 3.41 only show the normal grain size distribution.

Abbreviations (table 3.5) have been used to characterize the graphical statistic descriptions presented in Table 3.6 and Figures 3.42 to 3.45.

North Bara1 section

For this section, the mean grain size of samples ranges between 0.22 and 0.29 mm (2.28 -1.94 Φ) (average = 0.25 mm (2.15 Φ)). 53% of the samples fall within the medium sand range, while 47% are within the fine sand. The standard deviation values range between 0.72 and 0.97 Φ (average = 0.8 Φ). All standard deviation values described samples as moderately sorted. The skewness values range between -0.312 and +0.086 Φ (-0.098 Φ in average). 60% are considered symmetrical, 33% are coarse skewed and only 7% (one sample) is very coarse skewed. The kurtosis values vary from 0.99 to 1.31 Φ (average = 1.17 Φ). 77% of the samples are leptokurtic and 33% are mesokurtic (Fig. 3.43).

2.1.4. East Abu Zabad section

Samples from this section show mean grain size values ranging from 0.17

Log -Probability curves

The log-probability curve is believed to be meaningful with regard to depositional processes and to the modes of transport, which include: surface creep or traction, saltation and suspension. Each transportation mode represents an individual sub-population separated by truncation points [START_REF] Visher | Grain size distributions and depositional processes[END_REF]. [START_REF] Bagnold | Grain size distributions and depositional processes[END_REF][START_REF] Visher | Grain size distributions and depositional processes[END_REF] found that these populations of sediment transport may be mixed in the same sample. However, the log-probability curve normally exhibits two or three straight line segments, and in some cases, four segments could be found [START_REF] Visher | Grain size distributions and depositional processes[END_REF].

Forty-eight log-probability curves have been plotted in probability scale graphs. The plots were then categorized, on one hand to analyze the lower part of the mottled facies, the upper part of the same facies mottled and the red sand facies, and on the other hand, to show the horizontal variation and/or similarities (Fig. 3.46). In addition, log-probability curves of each section have been illustrated in one common graph to show the vertical variation and/or similarity of the curves with time (Fig. 3.47). Most of the curves show three straight line segments, considered as three populations of grains transported by distinct processes (traction, saltation, suspension). Only five curves from North Bara3 and two curves from East Abu Zabad show four straight line segments; four populations are thus recognized, representing traction, suspension and two kinds of saltation transport processes (Fig. 3.47). The curves show remarkable variations when comparing the same facies or horizon from different sites. When comparing the curves of a same site, the North Bara3 section shows remarkable variations in the population, while the other sites show clear similarities in the shape of the curves and their positions in the plot. The most similar curves was that obtained from the West El Obeid section (Fig. 3.47). The truncation points of all curves have been determined and summarized in table 3.7 to present only the maximum (max) and minimum (min) values for : the coarse traction (the coarsest end in the curve), traction-saltation, and saltationsuspension truncation points (TP). However, the north Bara3 section shows the greatest values for the coarse traction (max and min) and also for the maximum traction-saltation truncation points, while it shows lower values for the minimum traction-saltation and saltation-suspension (max and min) truncation points. For the other three sites, the truncation points for each population are close to each other with some variations. Generally, according to these illustrated curves, it appears that the dominant population in terms of percentage is the saltation population.

Interpretations

Cumulative curves

According to the cumulative curves of grain size distribution and the grain size ranges (Fig. 3.40), the North Bara3 section is marked by the coarsest grain size, followed in decreasing grain size order, by the North Bara1, West El Obeid and East Abu Zabad section, thus expressing a southward fining trend. This strongly suggests a southward decreasing transportation energy. Figure 3.48 illustrates the grain size distribution for all sections. Moreover, the width of the grain size distribution areas of each section shows a remarkable variability. Curves from North Bara3 show a wide grain size distribution reflecting a strong variability in transport energy through time. In North Bara1, distribution curves are moderately wide, reflecting also energy variability along section. Curves from the other sites are narrow, indicating low variability of transport energy through time for West El Obeid, and energy stability for East Abu-Zabad.

Grain size distribution

Grain size cumulative plots have been constructed for each sections (Fig. 3.49) and were correlated between the studied sections (Fig. 3.38), in order to obtain a stratigraphic and geographical image of the transportation processes and energy.

3.5.3.2.A. Vertical grain size distribution

Table 3.8 summarizes the grain size percentages along each section. The distribution indicates domination of the coarser sizes in the northern sections (north Bara 3 and north Bara 1), while the southern sections are dominated by fine grain sizes. This distribution indicates a general southward fining trend, which in turn reflects the southward decreasing energy. The distribution in the north Bara3 section shows mixed grain size percentages close to each other in the most of the size, except the fine gravel and the silt portions that show very low percentages, which indicate a poor sorting degree. The relatively high percentages of the very coarse and coarse sand indicate high energy along the north Bara3 section with some variation in this high energy (Fig. 3.49 A). Moreover, the uniform grain size distribution with time suggests uniform transportation energy and therefore, uniform of aeolian energy.

In the north Bara1 section, sediment distribution shows the domination of medium and fine sand, a considerable amount of very fine sand, rare silt and very coarse sand, very rare or absence of very coarse sand, and the lack of fine gravel. Therefore, this indicates generally moderate energy through time, although with some variation (Fig. 3.49 B). Moreover, the uniform grain size distribution through time suggests uniform transportation energy and therefore, uniform aeolian processes.

The sediment distribution in the west El Obeid section shows the predominance of fine sand, a considerable amount of medium and very fine sand, rare silt and very coarse sand, and the lack of very coarse sand and fine gravel. This distribution evidences a low energy through time, with very sporadic variations (Fig. 3.49 C). The uniform grain size distribution suggests uniform transportation energy and rather constant aeolian processes.

In the east Abu Zabad section, grain size distribution shows the domination of fine sand, a considerable amount of medium and very fine sand, a remarkable amount of silt, rare coarse sand, and the lack of very coarse sand and fine gravel. The predominance of fine sand and abundance of very fine sand and silt indicates very low energy through time with no variation (Fig. 3.49 D). This uniform grain size distribution suggests constant transportation energy and uniform aeolian processes.

3.5.3.2.B. Geographical grain size distribution

Based on the above mentioned data, I constructed horizontal profiles of grain size distribution for the three lithological units: upper red aeolian unit, upper part of the mottled unit, and lower part of the mottled unit (Fig. 3.50).

The grain size percentages for the red sand sediments are presented in Table 3.9. These percentages show (1) the lack of fine gravel and of very coarse sand, except in North Bara3, (2) the southward decrease of medium to coarse sand and overall southward increase of fine grained sand, and (3) a relative stability of very fine sand and silt all along the profile. This pattern indicates a decreasing transport energy from north to south (Fig. 3.50 A), and thus, strongly suggests that the wind was blowing from North or NNE to South or SSW, and that its intensity was decreasing southward. The grain size percentages for the upper part of the mottled facies are presented in table 3.10. The fine gravel and very coarse sand are only present in North Bara3. The amount of coarse and medium sand decreases southward (except in North Bara), while the percentage of fine sand, very fine sand and silt increases southward. This distribution indicates that high energy transport dominated in the northern parts, and was decreasing southward (Fig. 3.50 B). As for the sand sediments facies, we conclude that the wind was blowing from North to South, and its energy was decreasing in the same direction. The grain size percentages for the lower part of mottled facies are presented in Table 3.11. There is a total absence of fine gravel along the profile. The amount of very coarse, coarse and medium sand decreases southward, while that of fine sand and silt increases in the same direction. The percentage of very fine sand remains constant all along the profile. This distribution indicates that during deposition of this part of the mottled unit, the wind was blowing from North to South and its transportation energy was decreasing southward (Fig.

3.50C).

Graphical statistic parameters

It is difficult to recognize the lateral evolution of the graphical statistic parameters through their individual plots (see the result section, Figs. 3.42 to 3.45). Therefore, a single plot of graphical statistic parameters of all studied sections through time have been built (Fig. 3.51). Other diagrams have been drawn according to the graphical statistic relations. This graphical relations include mean grain size-sorting (Fig. 3.52), mean grain size-skewness (Fig. 3.53) and skewnesssorting, relations (Fig. 3.54).

3.5.3.3.A. Mean grain size

The diagram showing the mean grain size for each site shows a very organized distribution (Fig. 3.51) that strongly supports the gradual decrease in wind energy from North to South. The very similar mean size along the East Abu Zabad, similar mean size along the West El Obeid, and semi similar mean size along the North Bara1 section indicates very stable, stable, and semi stable energy in the East Abu Zabad, West El Obeid, and North Bara1 respectively. On the other hand, this vertical similarity in the mean size for each section indicates uniformity of the sediment origin, uniformity of sedimentation processes, and energy stability through time for both the red aeolian and mottled facies. Dispersion of mean grain size value in North Bara3 indicates variable energy through time.

3.5.3.3.B. Kurtosis

As the sections have a North-South arrangement, the kurtosis shows a systematic lateral variations : the curves evolve from North to South, from platykurtic to leptokurtic and very leptokurtic. This indicates a wide range of grain size and energy variability in the North, and a narrow range of grain size and the stability of transportation energy in the South.

3.5.3.3.C. Skewness

The skewness shows strong variability in all sections, except in East Abu Zabad, which shows almost constant values.

The skewness is only a function of the available size and it is difficult thus to use it as direct indicator for the energy strength, but it can be used as indicator for the energy fluctuation through time. According to the skewness values shown in fig. 3.51, these are variable in the northern sections (North Bara 3 and 1), semi-homogenous in the west El Obeid section, and very homogeneous for the East Abu Zabad section (southern end of the studied sections). This indicates a more variable energy in North Bara3, less variations in North Bara1, stable to semistable energy in West El Obeid, and a very stable energy in the East Abu Zabad section.

3.5.3.3.D. Standard deviation (sorting)

The sorting plot shows very little vertical variation in each individual section, but geographical variations of the sorting degree are well visible.

Except in East Abu Zabad, the sorting is poor in the North and better in the southern areas. The moderate sorting in East Abu Zabad is discussed below (cf. intercorrelations section).

Intercorrelations

3.5.3.4.A. Mean grain size vs sorting

The mean grain sizesorting plot (Fig. 3.52) shows very good linear relation between the different sites, and a clear trend from North to South. This relation indicates a gradual decreases in the energy from the North Bara3 to the West El Obeid section. The southern end of the studied profile (East Abu Zabad section) is marked by the finest mean grain size, but a less good sorting than the West El Obeid site (Fig. 3.52). This peculiarity can be ascribed to a sudden lowering down of the wind energy, due to the geographical location of East Abu Zbad on the lee side of a topographic crest, which shelters this place from high energy dominant winds (see topographic profile, Fig. 3.39). This lowering down of energy would lead to the rapid deposition of the transported load with less efficient sorting. This interpretation is supported by some morphological features: [START_REF]Hyphaene thebaica, 3: Bracharia ramose, 4[END_REF] what is called the Kordofan sand zone extends about 40 km south of East Abu Zabad, and (2) the higher vegetation cover in the more humid southern areas may contribute to the decrease of wind energy.

3.5.3.4.B. Mean grain size vs skewness

The mean grain size vs skewness relation does not show a linear relationship, but rather a "V" shape (Fig. 3.53). Energy is a positive function of the mean grain size (the greater the size, the greater the energy), while the same skewness value can be found in both high and low energy. The turning point of the "V" indicates in general the medium energy (whatever the strength is), while the arms of the "V" indicate high energy and low energy trends : the arm toward the coarser mean size indicates high energy, and the other arm indicates low energy.

3.5.3.4.C. Sorting vs skewness

The sorting (standard deviation) vs skewness relationship provides linear relation from North Bara3 to West El Obeid. However, the East Abu Zabad samples do not fall in a logical position according to the wind direction, since East Abu Zabad sands are less sorted than the West El Obeid samples and part of those of North Bara1. However, this diagram supports a decreasing transport energy from North to South (Fig. 3.54).

Fig. 3.56: Evolution of transport processes in the north Bara1 section. (A) expressed in percentages;

(A) expressed in grain size.

3.5.3.5.1.C. West El Obeid section

Grains transported by traction in West El Obeid represent a low percentage, with average value of ≈ 2 % in the mottled facies, and ≈ 4 % in the red sand sediments facies (Fig. 3.57A). The grain size range of this population is generally narrow and stable through time, ranging from ≈ 1 to ≈ 0.5 mm in the mottled facies, and 1 to 0.35 mm in the red sand sediments facies. The saltation population is quite predominant all along the section with an average percentage of ≈ 87 % in the mottled facies, and ≈ 92 % for the red sand sediments facies. When expressed in grain size, the saltation population is very constant through time with minimum size ≈ 0.109 mm. The suspension population represents ≈ 11 % in the mottled facies and ≈ 4 % in the red sand sediments facies, and its grain size is generally less than 0.109 mm.

This analysis shows that ( 1) the depositional processes did not change through time, (2) the high amount of saltation population in both facies suggests active dune formation, and (3) wind energy remained stable through time, at least during deposition of the studied units.

3.5.3.5.1.D. East Abu Zabad section

In East Abu Zabad, the traction population shows very low percentage (average ≈ 2 %) (Fig. 3.58 A). Its grain size range is narrow and constant, varying between ≈ 0.812 and ≈ 0.5 mm. The saltation population is highly dominant all along the section, with an average of ≈ 82 % in the mottled facies and ≈ 89 % in the red sand sediments facies. Expressed in grain size, the saltation population is finer and less well sorted in the red sand sediments facies than in the mottled facies. The minimum grain size of this population is ≈ 0.117 mm in the mottled facies and ≈ 0.102 mm in the red aeolian deposits. The suspension population have an average value of ≈ 16 % in the mottled facies and ≈ 9 % in the red sand facies. Grain size of this population is usually less than 0.109 mm.

These data indicate that (1) the aeolian depositional processes did not change between the mottled and red sand sediments facies, (2) sand dune likely formed as suggested by the high percentage of the saltation population in both facies, (3) wind energy remained stable through time, and (4) the relatively small maximum size of the traction population indicates the domination of a low aeolian energy regime.

Geographical interpretation

Three horizontal plots of the mode of transport population illustrate its variation across the studied area. These populations are shown in percentage and grain size, for the lower part and upper part of the mottled facies, and for the red sand sediments facies (Figs. 3.59 and 3.60).

3.5.3.5.2.A. Variation in the mottled facies

Both parts of the mottled facies show an absolute domination of the saltation population through time, which favors sand dune building during deposition of this facies. The suspension population in the mottled facies remarkably increases southward, while the grain size of the traction population decreases southward in a comparable amount, thus indicating a southward decrease of the energy regime (Fig. 3.59).

3.5.3.5.2.B. Variation in the red sand sediments facies

The saltation population highly dominates during deposition of the red sand sediments facies, except in North Bara3, where the traction population is significant. Therefore, sand dune building should be rare to the North, and increased to the south. The suspension population shows low and stable percentages in the area. Grain size of the traction population is relatively fine and slightly decreases southward. This suggests a relatively low and stable energy regime, both in time and space (Fig. 3.60).

Sedimentary and paleogeographic evolution

According to the local variations in sediments from one site to another, the study area has been divided into five areas : the Dilling, Abu Zabad-El Fula, El Obeid-South El Obeid, En Nahud-El Khowei and North Bara-Sodari areas. The sedimentary evolution has been described for each area, and based on these areas, a summary of the paleogeograhic evolution for the study area is presented. To understand the paleogeographic and environmental evolution through time, six paleogeographic maps have been constructed for the following periods : 13000 -10000, 10000 -8000, 8000 -6000, 6000 -3000, 3000 -1000, and 1000 cal. yr. BP to present.

Sedimentary evolution

Sedimentary evolution of the Dilling area

The Dilling area is dominated by fluviatile channel deposition at the base of the succession. These deposits are overlain by distal flood plain deposits, in which pedogenetic activity is evidenced by abundant root molds and development of calcareous nodules. This kind of pedogenesis indicates the development of a significant vegetation cover, the existence of a relatively shallow water table and therefore, a humid climate. Fluviatile channel deposits overly the flood plain deposits. These coarse fluviatile deposits are made of conglomerate containing numerous reworked calcareous nodules. As evidenced by the current features (trough and planar bedding), these conglomerates were deposited by powerful streams, which supports a humid climate.

The upper and thickest deposits around Dilling are dominated by either proximal fluviatile, or distal flood plain sediments, as well as by local aeolian deposition. However, these local aeolian deposits are highly pedogenetized as shown by the intensive development of calcareous nodules and root traces, indicating a highly vegetated landscape and a humid climate (Fig. 3.63).

Sedimentary evolution of the Abu Zabad -El Fula area

Aeolian deposition is the dominant depositional mechanism in this area in addition to local channel deposition (Fig. 3.64). According to the existence or absence of the pedogenetic processes, these aeolian deposits are thought to have been submitted to two different environments.

The subsequently pedogenetized aeolian deposits, which dominate most of the succession (First and possibly Second stratigraphic unit), are assumed to have experienced wet vegetated environment. Moreover, this wet environment was coeval with strong fluviatile activity leading to the deposition of thick reworked conglomerate between Abu Zabad and El Fula, and of some lenses of conglomerate East of Abu Zabad.

Non pedogenetized fine aeolian sand mark top of the sedimentological succession (Fourth unit).

Because of the lack of subsequent pedogenesis, these upper aeolian deposits suggest deposition under arid climate.

Sedimentary evolution of the El Obeid -South El Obeid area

In the El Obeid-South El Obeid area, the lower part of the succession is dominated by mottled, fine sand of aeolian origin. These aeolian deposits recorded a rather dense vegetation, evidenced by abundant calcareous nodules and root molds and traces (Fig. 3.65).

Sedimentary evolution of the En Nahud -El Khowei area

The succession in this area is divided into three unis. The lower or First unit is dominated by fine sands deposited by aeolian processes. However, the present-day mottled facies indicates subsequent pedogenesis processes (calcareous nodules and root molds), indicative of a rather dense vegetated cover, as mentioned for the previous areas.

The middle part of the succession (Second unit) is dominated by limestone and sandy limestone beds, marked by gastropod shells, root molds and karstification. These features indicate a palustrine environment (Fig. 3.66).

The upper part of the succession (Fourth unit) is dominated by non pedogenetized, red fine sands, deposited through aeolian mechanism, and marked by the occurrence of well-preserved gastropod shells. The gastropod shells provide good arguments for a seasonally dry and wet or semi-wet climate, depending on the nature of the gastropods, which require some seasonal water to build their shells. The lack or scarcity of root marks suggests a semi-arid to arid environment.

Sedimentary evolution of the Sodari -North Bara area

In this area, the succession comprises three parts. The lower part of the succession is formed by pedogenetized, medium to coarse sand ascribed to the First unit. As indicating by the grain size analysis, they were deposited by aeolian processes, while pedogenesis indicates a further evolution under humid climate, and a vegetated environment.

The middle part of the succession is formed of an alternation of argillaceous sand and limestone beds, marked by karstification, gastropod shells, ostracods, root molds, wood and leave remnants (Second unit). The limestone and its mentioned features indicate a lacustrine environment, while the interbedded argillaceous sand sediments indicate periods of low lake levels, or the activity of stream flowing into the lake (Fig. 3.67).

The upper part of the succession (Fourth unit) is dominated by non pedogenetized, medium to coarse, red sand, deposited through aeolian processes, mostly during a dry climate period.

Sedimentation and paleogeography for the ≈ 13 000 -10 000 yr BP period

This period is marked by mottled facies in most of the study area and by erosion around, and south of, the Dilling area, since no sedimentation is recorded for this period in the latter areas (Fig. 3.68). According to the grain size analysis (section 3.2.2), the mottled facies is an aeolian deposit brought by Southward blowing winds, since the grain size of these sediments decreases southward. Once deposited, pedogenesis occurred (calcareous nodules and root molds), indicating that the area was covered by vegetation and experienced a relatively humid climate, probably after deposition.

Around Dilling, the humid climate combined with a rough topography favored erosion and sediment transport by rivers, thus explaining the observed sedimentary hiatus. This contrast between the northern and southern part of the studied area illustrates a marked climatic gradient between the northern areas submitted to deposition of aeolian sands proceeding from the Sahara located farther North, rapidly colonized by vegetation responsible for mottling, and the southern area submitted to heavier rainfalls responsible for erosion and removing of sediments by rivers.

Sedimentation and paleogeography for the ≈ 10 000 -6000 yr BP period

During the first 2000 yr of this period, the Dilling area may have experienced erosion or nondeposition, hence maximum recorded age for sediments overlying the Basement rocks is about 7600 yr BP in fluviatile conglomerates of reworking basement rocks and calcareous nodules. This channel facies is also locally present in East Abu Zabad, between Abu Zabad and El Fula. In the northern parts of the region, beside the local deposition of lacustrine, palustrine and argillaceous sand sediments facies, the mottled facies experienced pedogenetic processes represented by mottling and the development of calcareous nodules (Fig. 3.69).

The topography and geology of the northern areas played an important role in the distribution of facies. Limestone and sandy limestone deposited in the En Nahud-El Khowei area, are known only on Nubian sandstone outcrops, which form a slightly undulating plateau, 600 to 700 m above sea level (asl) (Fig. 1.6). Interbedded limestone and argillaceous sand sediments are only known in the North, where they are restricted to small depressions located along dry rivers dammed downstream by large rectilinear dunes (El Ga'ah, Sodari). They are located at 450 to 550 m asl.

The depositional environment during this period was generally wet and vegetated as indicated by the deposition of limestone and sandy limestone in the En Nahud-El Khowei area that indicate palustrine environment in this area. Development of calcareous nodules and root molds in the mottled facies is assumed to occur during this humid period. In the northern part, the limestone level is thicker suggesting deposition under thicker water column, and suggests a lacustrine environment with local strong fluvial activities in the northern parts as evidenced by the progradation of the El Ga'a delta sediments, which indicates strong freshwater runoff and sediment influx (Fig. 3.70). This lacustrine environment was interrupted by periods of low lake level or of fluviatile activity, resulting in the deposition of interbedded argillaceous sand sediments.

Sedimentation and paleogeography for the ≈ 6000 -3000 BP period

At this time the northern parts of the study area and the northern part of the Kordofan region in general did not record deposits, while in the central and southern parts (El Obeid, Abu Zabad, El Fula areas), aeolian deposition took place, with local flood plain deposition in the southern parts around the Dilling area and further south (Fig. 3.71). These flood plain deposits are marked by root molds and the development of calcareous nodules in the Dilling area, and by mottling of previously deposited aeolian sands in the south El Obeid area. These mottled deposits are rich in organic matter.

Although the En Nahud-El Khowei area (which extends to the south close to Abu Zabad) is located at the central part of the study area, it did not receive any sediments during the period 6 -3 ky BP. This may be due to its high topographic elevation which may have provoked a stronger exposition to wind activity. As matter of fact, aeolian erosion is evidenced by deflation surfaces in the northern part of the studied region, locally marked by iron gravel horizon observed north of Bara and representing deflation lags (Glennie, 1987) (Fig. 3.72). Such deflation surfaces indicate that the northern part of the studied area experienced aeolian erosion probably after 6 kyr BP. In addition, deep erosion is clearly evidenced in the En Nahud-El Khowei area by the undulated erosional surface on top of the palustrine carbonates (Fig. 3.73). Therefore, the apparent 6 -1 ky BP hiatus observed there, may be due to the aeolian erosion of sediments deposited after 6 ky BP.

Sedimentation and paleogeography for the ≈ 3000 -1000 yr BP period

This period is marked by an erosional hiatus evidenced by the eroded surface capping the mottled facies in southeast El Obeid, and the flood plain facies in the Dilling area (Fig. 3.74). This period was thus marked by aeolian erosion and/or non-deposition in most of the study area, although fluviatile activity cannot be ruled out in the southern part of the Kordofan region, where dates are scarce.

Sedimentation and paleogeography between ≈ 1000 yr BP and Present

After a prolonged period of erosion which continued from ≈ 3 to ≈ 1 kyr BP in the Kordofan region, deposition resumed ≈ 1000 year ago in the whole region with aeolian deposits (Fig. 3.76). The southward decreasing grain size of the iron-rich aeolian sand sediments indicate that the sand proceed from the northern Saharan areas. The absence of the pedogenetic processes of the aeolian deposits in the northern part of the Kordofan region, indicates that depositional environment was mainly dry and windy. The aeolian deposits are locally interbedded with flood plain deposits south of El Obeid and in the Abu Zabad-El Fula area. Around Dilling, deposition resumed with channel deposits, and went on with flood plains deposits. Therefore, the southern part of the region recorded a more humid climate than the northern part during this last 1000 year, as evidenced by the current features, the root molds, some plant remnants (leave and wood fragments), and the local development of calcareous nodules, observed in the upper deposits.

IV. PALEOCLIMATIC EVOLUTION

Introduction

Several proxies have been used in this investigation in order to reconstruct the climate evolution of the Kordofan region during the latest Pleistocene-Holocene period. Beside the initially presented sedimentological proxy in the previous chapter, this chapter includes the results and interpretation of a variety of studies : paleontology and paleobiology of the gastropod sub-fossil shells in five sections, palynology in three sites, oxygen and carbon stable isotopes of gastropod shells and calcareous nodules in five sections, major elements geochemistry in two sections, clay mineralogy in one section, and paleohydrology of the area.

Paleontology and paleobiology

Paleontological investigation includes the study of the gastropod sub-fossil shells and of the palynological assemblages.

Gastropod paleontology and paleobiology

Gastropods shells are observed in abundance in the central and northern parts of the study area (Fig. 4.1). Twelve species of gastropod shells have been identified with the help of Dr Dirk Vandamme (Ghent University, Belgium). The identified species include: Biomphalaria sudanica, Biomphalaria pfeifferi, Gabbiella senaariensis, Bulinus truncatus, Melanoides tuberculata, Radix natalensis, Pila wernei, Lanistes carinatus, Caracolus sp., Limicolaria flammea, Limicolaria caillaudi and Zootecus insularis (plate 4.1).

Geographic range and habitat of the identified gastropods

Nowadays, it is rare to find living snails in the study area. To use these shells as clues to understand the ecological and climatic conditions during their life, it is important to obtain information about the habitat of these species from literature. The identified species included aquatic, semi-aquatic and land snail species. Seven of them are aquatic freshwater snails : Biomphalaria sudanica, Biomphalaria pfeifferi, Gabbiella senaariensis, Bulinus truncatus, Melanoides tuberculata and Radix natalensis. The semi-aquatic species are : Pila wernei and Lanistes carinatus. The land snail species are : Limicolaria flammea, Limicolaria caillaudi and Caracolus sp.

Table 4.1 shows the relative abundances of these gastropod shells in studied sites. Among the twelve species, only five were found in En Nahud, the most abundant being Pila wernei. Eight species were found in the East En Nahud section, among which Radix natalensis was the most abundant, and Limicolaria caillaudi the less abundant. In the east El Khowei site, eleven species were recognized (the only missing species is Zootecus insularis), Radix natalensis was the most abundant, and Caracolus sp. was the less abundant species and has been only observed in this site. Eight species were found in the El Ga'ah area, Melanoides tuberculata is the most abundant, and Limicolaria flammea is the less abundant. In the Sodari area, seven species were recognized, Melanoides tuberculata was the most abundant, Gabbiella senaariensis was the less abundant, and Zootecus insularis has been only observed in the Sodari area. Six species were identified in the north Bara site, among which Biomphalaria sudanica and B. pfeifferi dominate, while Limicolaria caillaudi is the scarcest. The distribution and abundance of gastropods differ considerably according to the species (Table 4.1). Four species are consistently abundant and widespread : Biomphalaria sudanica, B. pfeifferi, Melanoides tuberculata and Radix natalensis; four species are common (Pila wernei, Lanistes carinatus, Limicolaria flammea, Gabbiella senaariensis), and three species are uncommon (Limicolaria caillaudi, Bulinus truncatus, Zootecus insularis). The only rare genus was Caracolus sp., since only two shells were observed in the East El Khowei section.

4.2.1.1.A. Aquatic snails

Biomphalaria pfeifferi (Krauss 1948)

Geographic range: most of Africa from the Sudan southwards; isolated populations living in the Sahara are recorded (Brown, et al 1984). In Sudan it is distributed in a variety of areas of the Darfur province and southwards from the Nile/Atbara confluence, with scattered localities in the Sudd (Southern Sudan); it is common in the eastern Gezira region (Williams and[START_REF] Williams | Aquatic snails of the Jonglei region, southern Sudan, and transmission of trematode parasites[END_REF].

(marginal, floating and emergent vegetation; [START_REF] Ndifon | Ecology of freshwater snails in south-western Nigeria. I: Distribution and habitat preferences[END_REF]. It has been recorded in comparatively permanent sites in rivers, khors and lagoons, where some plant species as Echinochloa, Eichhornia, Vossia and Typha are present in some localities. [START_REF] Williams | Aquatic snails of the Jonglei region, southern Sudan, and transmission of trematode parasites[END_REF] noted that B. pfeifferi was more common where the vegetation was dominated by grass (Brown et al., 1984). In Congo, B. pfeifferi has been collected from irrigation channels with mainly clayey substratum [START_REF] Schultheiß | Freshwater molluscs of the Eastern Congo: notes on taxonomy, biogeography and conservation[END_REF]. Recent studies on alive B. pfeifferi distribution and habitat carried out in South Africa, shows that this species was present in a wide variety of water bodies: dams, rivers and streams, although it prefers the perennial, standing, clear and fresh water bodies. The majority of samples came from sites where mean annual temperature ranges from 15 to 25°C. B. pfeifferi is sensitive to low temperatures. In addition, the highest percentages of samples were recovered from sites that fell within the two intervals of mean annual rainfall ranging from 300 to 600 mm and 600 to 900 mm (Kock et al., 2003).

Biomphalaria sudanica (Martens, 1870)

Geographic range: The distribution of Biomphalaria sudanica is Sudan, Southern Ethiopia to Lake Chad to the West; and southwards through Uganda and the basins of the lakes Victoria and Tanganyika. Brown et al. (1984) reported that it was common in the White Nile and in various parts of the Sudd region.

Habitat: B. sudanica is found in a variety of habitats such as flooded area near the main Nile river, rainwater lakes and swamps, in the White Nile where papyrus islands were grounded Habitat: P. wernei prefers highly shaded habitat [START_REF] Ndifon | Ecology of freshwater snails in south-western Nigeria. I: Distribution and habitat preferences[END_REF] in various standing waterbodies, both permanent and seasonal freshwater systems (IUCN, 2010) like seasonal pools (Brown et al., 1984), lakes such as Lake Chad, and rivers, especially the White Nile (Gardner, 1932;Van Damme, 1984;Pachur and Kröpelin, 1987;[START_REF] Peters | The faunal remains from Shaqadud[END_REF]Brown, 1994 in Fuller andSmith, 1998). However, it requires at least semi-permanent water (Fuller and Smith, 1998). In Jonglei (Southern Sudan), its habitat is characterized by the full presence of some plant like Echinochloa, Ipomoea and water lilies (Brown et al., 1984).

Lanistes carinatus (Olivier, 1804) Geographic range: The Nile and canals of Egypt; Sudan, southeast Ethiopia, Somalia, Kenya and Uganda. Its distribution in Sudan was formerly widespread, according to the occurrence of subfossil shells (Brown et al., 1984).

Habitat: Ditches and pools flooded by rain and the Nile, where Echinochloa and Eichhornia were present (Brown, et al 1984).

4.2.1.1.C. Land snails

Limicolaria flammea (Müller, 1774) Geographic range: Limicolaria flammea has been known in western Africa for a long time, but it has been found recently in tropical Asia, in particular Singapore [START_REF] Tan | Limicolaria flammea (Müller, 1774), another potentially invasive African land snail in tropical Asia[END_REF].

Habitat: Few information is gathered about the habitat of these species. It is very common during the wet season (Amusan et al., 2002;[START_REF] Idohou | Soil factors affecting density of three giant land snail species in different habitats of Dassa-Zoume` district (Central Benin)[END_REF]. However, Limicolaria species are known to occur in abundance in forest edges and modified habitats such as plantations and farms [START_REF] Raut | An update on the West African Limicolaria flammea (Müller, 1774) in Singapore, and its distinction from the confamilial Achatina fulica Bowdich, 1822 (Mollusca: Grastropoda: Achatinidae)[END_REF][START_REF] Raut | An update on the West African Limicolaria flammea (Müller, 1774) in Singapore, and its distinction from the confamilial Achatina fulica Bowdich, 1822 (Mollusca: Grastropoda: Achatinidae)[END_REF]Low, 2011). It has been observed more active in long grass at forest edges [START_REF] Tan | Limicolaria flammea (Müller, 1774), another potentially invasive African land snail in tropical Asia[END_REF].

Limicolaria caillaudi (Pfeiffer, 1852)

Geographic range: Its present-day distribution extends from eastern Sudan to Ethiopia, and southward to Tanzaniya (Crowley andPain, 1970, in Peters, 1991).

Habitat: The exact requirements of this species are not well known but it seems that L. caillaudi has a considerable tolerance with respect to mean annual precipitation [START_REF] Peters | The faunal remains from Shaqadud[END_REF]. Williams and Adamson (1980) describe it as typical of the acacia tall grass plains, bounded by the 500-800 mm isohyets. According to Tothill (1948b), L. fiammata (= L. caillaudi, cf. Crowley and Pain, 1970) occurs in well-drained areas with tall grass, and in clay-pans, characterized by a mean annual precipitation between 400 and 800 mm [START_REF] Peters | The faunal remains from Shaqadud[END_REF].

Zootecus insularis (Ehrenberg 1831)

Geographic range: The Zootecus genus in general is widely spread all over the world including Pakistan, India, Afghanistan, Africa and Arabian countries [START_REF] Qamar | Identification of the species of genus Zootecus on the basis of morphology[END_REF]. Zootecus insularis nowadays occurs in the semi-arid regions of India, Arabia, Eritrea, Egypt, Sudan, Senegal and the Cape Verde Islands. In Africa, the distribution in prehistoric times of this species includes, for the moment, Nubia (Martin 1968), the Western Desert (Gautier 1980), the Acacus Massif (Gautier and Van Neer 1982) and central Sudan [START_REF] Peters | The faunal remains from Shaqadud[END_REF].

Habitat: Living species of Zootecus insularis in Pakistan indicate that it occurs in the grass land habitat.

Caracolus sp.

The genus Caracolus in general is a land snail [START_REF] Bishop | A new species of Caracolus (Pulmonata: Camaenidae) from the Oligocene of Nebraska and the biotic history of the American camaenid land snails[END_REF]. [START_REF] Van Der Schalie | A new species of Caracolus (Pulmonata: Camaenidae) from the Oligocene of Nebraska and the biotic history of the American camaenid land snails[END_REF] found Caracolus most commonly on trees of wet tropical forest at El Yunque, while along the dry, haystack knolls of the northern coast, it was usually buried or moving (in wet weather) among the decayed leaves and stones on the slopes. Also in Puerto Rico, Baker (1961) noted that the species is a very good climber on tree trunks, often seen 5-7 m above ground [START_REF] Bishop | A new species of Caracolus (Pulmonata: Camaenidae) from the Oligocene of Nebraska and the biotic history of the American camaenid land snails[END_REF].

Quantitative analysis of the stratigraphic distribution of gastropods

The analysis of the stratigraphic distribution of gastropod shells was carried out in the sections of En Nahud and East El Khowei. In both sections, the gastropod shells are concentrated in the sandy carbonate facies and the upper red sand facies (Second and Fourth units). Only the in situ, well-preserved gastropod shells were counted. 175 shells of four different species were counted in the En-Nahud section (Fig. 4.2), and 739 shells of eleven species were counted in the east El Khowei section (Fig. 4.3). In En Nahud, the fauna of the sandy limestone is dominated by the aquatic species Radix natalensis and the semi-aquatic species Pila wernei. The contact between the sandy limestone and the overlying red sand sediments is dominated by the semi-aquatic species Pila wernei, while the land snail species Limicolaria flammea dominates in the upper part of red sand sediments faciest.

In the east El Khowei section, the sandy carbonate is dominated by the aquatic species Gabbiella sennariensis, Biomphalaria sp., Melanoides turberculata, Bulinus truncatus and Radix natalensis; the contact between the sandy carbonate and the upper red sands are dominated by the aquatic species Gabbiella sennariensis and in a lesser extent, by the semiaquatic species Lanistes carinatus, while the land snail species Limicolaria flammea dominates in the red sand facies.

Interpretation

As mentioned previously, the identified gastropod species belong to aquatic, semi-aquatic and terrestrial environments. Their stratigraphic distribution in the En Nahud and East El Khowei sections, have been plotted in term of cumulative percentages for each group (Figs. 4.4 and 4.5). These diagrams clearly indicate an absolute domination of the aquatic and semi-aquatic species in the sandy carbonate facies, reflecting humid climatic conditions between ≈ 10 and ≈ 6 ky BP. The lower part of the red sand facies is mostly dominated by the land snails, and "locally" by semi-aquatic and aquatic groups. This distribution in the lower part of the red sand unit indicates wet to dry conditions environment (i.e. the area had experienced sub-humid to semi-arid climate) between ≈ 1000 and ≈ 700 yr BP. However, aeolian erosion took place between ≈ 6 and 1 ky BP, or at least ≈ 3 and 1 ky BP, and the local abundance of the semi-aquatic and aquatic groups at the base of the red sand may indicate that, either these shells were reworked from the lower palustrine limestone, or the early 300 years from this 1000 years period were wetter than the rest of the time-span, or climate was seasonally wet.

In both sections, the upper part of the red sand unit is totally dominated by land snail species, which indicates that the area experienced arid climatic conditions during the last ≈ 1000 yr. This vertical evolution, changing from typical aquatic to terrestrial land snail species, occurred as a normal adaptation of the gastropod communities to the gradual climatic change from mostly humid to mostly arid conditions, of course passing through sub-humid and semi-arid conditions. On the other hand, the presence of the aquatic species Gabbiella senaariensis, Bulinus truncatus, Melanoides tuberculata and the semi-aquatic species Lanistes carinatus around El-Khowei and their absence around En-Nahud, reflect paleoenvironmental differentiation between the two sites. This suggests that the El-Khowei area was more vegetated between ≈ 10 and 6 ky BP than the En-Nahud area, the former being characterized by swampy environments, while the latter was marked by isolated ponds or pools, in spite of being submitted to the same climate.

Palynology

Twenty-nine samples from six sites were analyzed for palynological studies. The investigated sites are: North Dilling, South El Obeid 3 (Khour Es Sonjokaya), South El Obeid 1 (Khour Es Sikairan), East El Obeid (Khour Taggat), West El Obeid (Ayara), En Nahud, and El Ga'ah (Fig. 4.6). The last three sites were barren of pollens, and have been excluded from the results. The twenty-two analyzed samples from the first three sites yielded a total of 1408 pollen and spores. The South El Obeid 3 site yielded 831 palynomorphs, 399 were counted in samples from North Dilling, and only 178 palynomorphs were counted in samples from South El Obeid 1.

All palynomorphs have been identified, they produced 90 different types table 4.2. 1213 palynomorphs out of the total counted specimens have been classified into four main groups in order to help in the paleoclimate interpretation. The identified groups are : aquatic, tropical, savanna, and arid indicators. This classification is based on the understanding of the related habitat and ecological information, which have been gathered from various studies.

The aquatic group includes freshwater algae and other aquatic flora. This group includes Chomotriletes (Latrubesse et al., 2010;Akyuz et al., 2016), Ovoidites (Scafati et al., 2009;[START_REF] Worobiec | Spore-pollen and phytoplankton analysis of the Upper Miocene deposits from Józefina (Kraków-Silesia Upland, Poland)[END_REF], Schizosporis (Scafati et al., 2009;Latrubesse et al., 2010;Akyuz et al., 2016), Lecaniella forma [START_REF] Worobiec | Fossil zygospores of Zygnemataceae and other microremains of freshwater algae from two Miocene palaeosinkholes in the Opole region, SW Poland[END_REF], Spirogyra (zygospore) (Chambers et al., 2014), Chlorosarcina superba [START_REF] Lukavský | Algae, Cyanobacteria and Chytridiales of Èerné Lake inthe Bohemian Forest (Šumava, Czech Republic)[END_REF], Sigmopollis laevigatoides [START_REF] Worobiec | Spore-pollen and phytoplankton analysis of the Upper Miocene deposits from Józefina (Kraków-Silesia Upland, Poland)[END_REF], Alternanthera nodiflora [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Alternanthera sessilis (El-Amier, 2015), Lemna gibba (El-Amier, 2015; [START_REF] Perveen | A Palynological Survey of Aquatic Flora of Karachi-Pakistan[END_REF], Inaperturopolenites, Potamogeton pectinatus (El-Amier, 2015), Limnophyton obtusifolium [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF] and Echinochloa crusgalli (El-Amier, 2015). Plates 4.2 and 4.3 show some representative pollens of this group.

The tropical climate indicators group includes Proxapertites (Monga et al., 2015), Retipollenites, Longapertites, Laevigatosporites (Paul et al., 2015), Araucariaceae [START_REF] Lu | Determination of the molecular signature of fossil conifers by experimental palaeochemotaxonomy -Part 1: The Araucariaceae family[END_REF][START_REF] Stukins | Middle Jurassic vegetation dynamics from allochthonous palynological assemblages: An example from a marginal marine depositional setting; Lajas Formation, Neuquén Basin, Argentina[END_REF], Aneilema johnstonii [START_REF] Gosling | Atlas of the tropical West African pollen flora[END_REF], Tiliacora funifera [START_REF] Gosling | Atlas of the tropical West African pollen flora[END_REF], Palmidites plicatus (Monga et al., 2015), Lygodiumsporites (Monga et al., 2015), Borassus aethiopum [START_REF] Gosling | Atlas of the tropical West African pollen flora[END_REF] and Plantago major (Clapham et al., 1989). Plate 4.4 shows some of the representative pollens of this group.

The savanna group indicators include Baissea multiflora, Graminidites [START_REF] Worobiec | Spore-pollen and phytoplankton analysis of the Upper Miocene deposits from Józefina (Kraków-Silesia Upland, Poland)[END_REF], Silene burchellii [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Cymbopogon schoenanthus [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF] and Monolites alveolatus. Plate 4.5 shows some representative specimens of this group.

The arid indicator group includes Hyphaene thebaica [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Hyparrhenia hirta [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Classopollis spp. (Volkheimer et al., 2008;[START_REF] Riding | Isotopic and palynological evidence for a new Early Jurassic environmental perturbation[END_REF][START_REF] Stukins | Middle Jurassic vegetation dynamics from allochthonous palynological assemblages: An example from a marginal marine depositional setting; Lajas Formation, Neuquén Basin, Argentina[END_REF], Evolvulus alsinoides [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Desmodium dichotonum [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Bracharia ramosa [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Gynandropsis gynandra [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Farsetia stenoptera [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF], Farsetia hamihonni [START_REF] El Ghazali | A study of pollen flora of Sudan[END_REF]. Gelasinospora cf. (Chambers et al., 2014), Leoisphaeridia sp. [START_REF] Onoduku | Palynology, Palynostratigraphy and Paleoenvironmental Analysis of Maiganga Coal Mine, Gombe Formation, Nigeria[END_REF], Leiotriletes adriennis [START_REF] Uzodimma | Palynostratigraphy, Age Determination and Depositional Environments of the Imo Shale Exposures at the Okigwe/Port Harcourt Express Road Junction Okigwe, Southeastern Nigeria[END_REF], and Erica arbora [START_REF] Lézine | A late Holocene pollen and climate record from Lake Yoa, northern Chad[END_REF]. Plate 4.6 illsutrates some representative pollens of this group.

North Dilling site

Eight samples were analyzed from this site, with a total thickness of 380 cm. Among the aquatic flora, the Chomotriletes freshwater algae are the most abundant, with secondary abundance of Sigmopollis laevigatoides and Inaperturopolenites.

South El Obeid 3 site

Nine samples were analyzed samples from this site along a 440 cm thick section. The freshwater algae Chomotriletes and Lecaniella forma dominate the aquatic flora, with secondary abundance of the freshwater algae Schizosporis. The tropical group is dominated by Araucariaceae sp., Aneilema johnstonii and Tiliacora funifera. The savanna group is dominated by Baissea multiflora, Silene burchelli and Graminidites, while Farsetia stenoptera, Farsetia hamihonni, Classopollis spp., Hyparrhenia hirta and Bracharia ramosa dominate the arid group (table 4.2).

In the lower part of the section (Fig. 4.8), the aquatic and savanna groups are dominant with percentages up to 47 % each, the tropical indicator group is very rare (≈ 6%) and species of the arid group are recorded. The upper part of the section is dominated by arid indicators with an average percentage of about 70 % (increasing upward), while the tropical group records ≈ 17 % and the aquatic group ≈ 11 % in average; specimens of the savanna group are very rare in this part of the section (≈ 2 %). The middle part is more balanced, with ≈ 45 % of aquatic, ≈ 15 % of tropical, ≈ 20 % of savanna and ≈ 20 % of arid indicator group.

South El Obeid 1 site

We analyzed five samples from this 380 cm thick section. According to the stratigraphic distribution (Fig. 4.9), no pollens of the arid group have been found in the lower part of the section, which is dominated by the aquatic group (up to 82 %), followed by tropical indicators (≈ 14 %), the savanna group being very rare (≈ 3 % in average).

The middle part (depth 125 -240 cm) contains ≈ 76 % of aquatic pollens, ≈ 8 % in average of tropical, ≈ 12 % of savanna, and ≈ 4 % of arid, indicators. The upper part of the section is dominated by the arid indicator group (average of 55 %), with subordinate species of the savanna (≈ 30 %) and aquatic (≈ 15 %) groups, and is devoid of palynomorphs of the tropical group.

The interpretation includes both the stratigraphic and geographical distribution. The stratigraphic interpretations are presented in figs 4.10 to 4.12. The geographical interpretation have been carried out through the correlations between the three studied sites and includes correlation for three different periods: ≈ 3500, ≈ 1000, and ≈ 500 y BP (Figs. 4.13 to 15).

Interpretation of the palynological results

Stratigraphic interpretation

In the lower parts for the North Dilling and South El Obeid 1 sections (≈ 4.8 to ≈ 3 ka BP), the domination of the aquatic and tropical groups, indicates that the region experienced a wet climate, probably sub-humid to semi-arid. Moreover, the marked predominance of freshwater algae in the aquatic group indicates that many swamps were present in the area. This climate, however, experienced some local fluctuations as shown in the South El Obeid 3 site, the lower part of which is dominated by the aquatic and savanna groups. These conditions suggest that the region was probably located within the tropical rainy belt of the African continent. The appearance of the arid group in the middle parts of the three sites (≈ 1000 to ≈ 800 yr BP) indicates environmental and climatic shifts toward less swampy environment and less wet climate than in the lower part.

The arid group increases then rapidly upward (younger than 1000 yr BP) and the upper parts of the sections show relatively few percentages of the other groups. This rapid increase in the arid group percentages reflects an abrupt environmental and climatic change to dry environment and arid climate. Therefore, this indicate that the studied region was probably located north of the rainy tropical belt of the African continent.

Geographical interpretation

Around 3500 yr BP the southern part of the studied area was dominated by the aquatic group, although in south El Obeid 3, both the savanna and aquatic groups prevailed (Fig. 4.13). This indicates that this region experienced wet conditions, which reflect sub-humid climate and swampy environment. Thus, between ≈ 5 and 3 ka BP, this area was located within the rainy belt of the tropical Africa.

Around 1000 yr BP, the Kordofan region recorded the appearance of an arid vegetation cover, although the aquatic flora and freshwater algae were still the prevailing groups (Fig. 4.14). The tropical climate indicators are still abundant and the savanna group is locally abundant. These data indicate that, by ≈ 1000 yr BP, the environment and climate was drier than at 3 ka BP. This climatic shift may have already started during the hiatus, ≈ 3000 yr ago. Around 1000 yr BP, the climate between El Obeid and Dilling was semi-arid, the environment was swampy along the rivers and dry elsewhere, and the investigated sites were probably located at the northern end of the tropical rainy belt.

From ≈ 500 yr BP up to the present, the investigated sites are dominated by the arid group, with locally high percentages of the savanna group, while the aquatic and tropical groups do not exceed together 30 % (Fig. 4.15). The domination of the arid group indicates that the studied area experiences mostly dry conditions, with only seasonal swamps. This arid climate suggests that the region was located north of the African tropical rainy belt. The sedimentological analysis of the red sand Unit showed that the latter is of aeolian origin, which is inconsistent with the interpretation of the isotopic data. However, gastropods from the lower part of the red sand facies are Pila wernei, a semi-aquatic species, and their shell was filled and covered with the carbonate mud of the underlying palustrine limestone. This strongly suggests that these snails were actually leaving during deposition of the palustrine carbonate, and were reworked during the aeolian erosional period (≈ 3 to 1 kyr BP). Thus, the negative δ 18 O values may correspond to the time the underlying limestone were deposited (≈ 10 to 6 ky BP), which corresponds to the African Humid Period (AHP), as supported by the aquatic and semi-aquatic way of life of the gastropods. In contrast, in the upper part of the En-Nahud 1 and 2 sections, the shells of terrestrial land snails are enriched in 18 O and depleted in 16 O. Since evaporation has a major influence on the isotope composition of standing water bodies (Leng and Marshall, 2004) and 16 O is easier to evaporate than 18 O, the isotopic enrichment in 18 O is interpreted as indicating an arid climate [START_REF] Abell | Oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in Afar region of Ethiopia[END_REF]. Moreover, the water body may have played an important role in the variation of δ 18 O between the lower aquatic shells and the upper land snail shells. As a matter of fact, if the water body remains for a long time, more evaporation is expected to occur, and the light oxygen ( 16 O) must be reduced through this evaporation, which makes the water body less depleted in 18 O than the isotopic composition of the initial rainfall. [START_REF] Goodfriend | Stable isotope composition of land snail body water and its relation to environmental waters and shell carbonate[END_REF] reported that the isotopic composition of snail in a water body is related to atmospheric water vapor and the isotopic composition of the water body, while the land snail shell carbonate 18 O should provide a reliable indication of rainfall 18 O. This assumption or interpretation indicates that the oxygen incorporated in the aquatic shells is isotopically less depleted than the real depletion of meteoric water at time of precipitation. Hence the enriched composition of the land snails of the upper part reflects a real 18 O enrichment of the meteoric water.

East El-Khowei site

Results

samples from the Upper red sand Unit, one shows oxygen isotope values highly depleted in 18 O (δ 18 O = -8.0). An acceptable explanation is that the shell may has been reworked from the palustrine limestone, eroded in the surrounding area. The vertical distribution of δ 18 O values shows two distinct parts in the Lower Unit. These two parts correspond to the grey mottled, and pale-yellow mottled horizons, respectively. The grey mottled horizon (520 -230 cm) shows δ 18 O values mostly ranging between -7 and -6.3 ‰ (except one sample > -6.3 ‰), while the pale-yellow mottled horizon (230 -120 cm) shows δ 18 O values comprised between -8.3 and -9.8 ‰. On the other hand, the δ 13 C values decrease upward in the mottled facies; the lower part of the succession shows little negative δ 13 C values, compared to the upper part that is depleted in 13 C.

Interpretation

The West El-Obeid site shows generally negative δ 18 O values, with narrow range indicating that wet conditions prevailed during the formation of the calcareous nodules. As a matter of fact, the development of calcareous nodules, occurred after deposition of the enclosing matrix. Moreover, the upper part of the curve (above depth 240 cm) shows more negative δ 18 O values, than those of the lower part (below depth 240 cm). This may indicate that the calcareous nodules of the upper part developed under wetter conditions that those developed in the lower part (Fig. 4.23). The variation in δ 18 O below and above depth 220 cm, maybe due to groundwater effects, which may reduce the lighter oxygen ( 16 O) through the previous continued evapotranspiration from the lower part, while calcareous nodules above the depth 220 cm, represent more recent water of the AHP and therefore, more depleted than the lower part.

Regarding the interpretation of carbon isotopes, the variation from less negative δ 13 C values in the lower part, to more negative δ 13 C values in the upper part, may reflect a difference in vegetation type and/or density, differentiation in the biological activities, or variation in the 13 C/ 12 C ratio in the dissolved CO2. All analyzed samples from North Dilling site are calcareous nodules formed within silty sand or reworked in conglomerate beds. For these nodules, δ 18 O varies from -8.2 ‰ to -5.9 ‰ (with narrow range of only up to 2.3 ‰ difference) and δ 13 C from -4.7 ‰ to 0.4 ‰ (with relatively narrow range of up to 5.1 ‰ difference). Generally, samples from the lower part of the section show less negative δ 18 O values than samples from the upper part (Fig. 4.24).

Interpretation

The negative values of δ 18 O and their narrow range indicate that the nodules developed under relatively wet conditions, with little or no climatic variation, although a mild shift toward less depleted values upward is visible. The relatively more positive δ 13 C values with their relatively narrow range indicate little environmental variability with some stability in vegetation type and/or density, little variation in the biological activity, and low variability of 13 C ratios in the dissolved CO2 (Fig. 4.25).

Geochemistry of major elements

XRF analyses of major elements were carried out for twenty-two samples collected from two sites : North Bara 3 (Jabrat Esh Sheikh) and West El-Obeid (Ayara). The results are presented in table 4.3 and figures 4.17 and 4.18. The calculated values for the chemical index of alteration (CIA) show high values ranging from 82.9 to 91.4 (average 89.4) in the North Bara 3 section, and from 87.5 to 90.3 (average 88.8) in the West El Obeid section. The CIA values are very homogeneous in both sections with very little variations (Figs. 4.26 and 4.27). In the North Bara 3 section, the mottled unit is marked by slightly higher values than the Upper unit of red sand sediments (Fig. 4.27), while the relation is reversed in the West El Obeid section.

Results

SiO2

The ratio of SiO2 to Al2O3 is moderately high in both sections with little variability (Table. 4.3). It ranges between 6.4 and 5.0 in the West El Obeid section with a slight increasing upward trend toward the red sand of the Upper unit (Fig. 4.27). In the North Bara3 section the silicaaluminium ratio ranges between 6.0 and 3.8 with some variability along the section, in both the mottled sand facies of the Lower unit, and the red sand facies of the Upper unit (Fig. 4.27).

The ratio of CaO to SiO2 is generally low in both sections (Table. 4.3). It ranges between 0.01 and 0.25 in the West El Obeid section with a remarkable upward decrease, so that the calciumsilica ratio of the mottled sands (Lower unit) is higher than the red sands of the Upper unit (Fig. 4.26). In the North Bara3 section, the CaO to SiO2 ratio ranges between 0.01 and 0.36 with remarkably low ratios in the Upper unit and the upper part of the Lower unit (Fig. 4.27).

Interpretation of XRF analyses

The high content in SiO2 is due to the sandy nature of the analyzed samples. Al2O3 generally accumulates in form of clay minerals [START_REF] Sheldon | Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols[END_REF]. The relatively high concentration in Al2O3 indicates significant amounts of clay minerals formed in the source area as a product of chemical weathering. The moderate concentration in iron oxide Fe2O3 may indicate an oxidation that mostly occurred in the source area, below the same conditions that led to the formation of the clay minerals. These concentrations in aluminum and iron oxide indicate that the source area probably experienced alternating seasonal wet and dry conditions, as aluminum tends to develop mostly in wet condition. If the climate is wet all over the year, the chance of oxidation will be reduced, and therefore, the iron oxides indicate an alternation of wet and dry environment.

CIA is a measurement of the hydration and weathering of feldspar minerals to form clay minerals. As clay content increases, Al should also increase, whereas Ca, K and Na contents should decrease, leading to higher CIA values [START_REF] Sheldon | Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols[END_REF]. High CIA values (76 to 100) in the sedimentary rocks suggest intense chemical weathering in the source region [START_REF] Fadipe | Provenance, diagenesis and reservoir quality of the lower Cretaceous sandstone of the Orange basin, South Africa[END_REF]Srivastava et al., 2013;[START_REF] Madhavaraju | Paleoclimate, paleoweathering and paleoredox conditions of Lower Cretaceous shales from the Mural Limestone, Tuape section, northern Sonora, Mexico: Constraints from clay mineralogy and geochemistry[END_REF]. The SiO2 content and SiO2/Al2O3 ratio are the most commonly used geochemical criteria to determine the abundance of quartz, feldspar and clay, and for differentiating mature and immature sediments [START_REF] Potter | Petrology and chemistry of modern big river sands[END_REF][START_REF] Madukwe | Geochemistry, Classification and maturity of the Sandstone facies of the Abeokuta Formation, south west Nigeria[END_REF]. High ratios indicate mineralogically mature (quartzose, rounded) sediments, while low values of SiO2/Al2O3 ratio indicate high degree of clayness [START_REF] Madukwe | Geochemistry, Classification and maturity of the Sandstone facies of the Abeokuta Formation, south west Nigeria[END_REF]. In our case study, the relatively high SiO2/Al2O3 ratio indicates predominating silicate minerals and low degree of clayness. Since the formation of most of the clay minerals requires humid condition, the low degree of clayness in the studied sections may indicate that the Lower and Upper unit were deposited under dry environmental conditions (Figs. 4.29 and 4.30).

Low CaO oxide suggests a chemical destruction under oxidizing conditions during weathering [START_REF] Oni | Depositional environments signatures, maturity and source weathering of Niger ddelta sediments from an oil well in southeastern Delta State, Nigeria[END_REF]. Therefore, the very low CaO/SiO2 ratio for the Upper unit of the red sand deposits in both sections may indicate that oxidation rate was greater than the pedogenetic processes, resulting in the predominance of the silicate minerals (mainly SiO2) which may deposited under dry conditions (Figs. 4.29 and 4.30). On the other hand the CaO/SiO2 ratio in both sections increases in the mottled sand of the Lower unit, which may indicate that the Lower unit experienced remarkable pedogenetic processes. Moreover, since these pedogenetic processes decrease upward in the Lower unit and the CaO/SiO2 ratio may provide an indicator of pedogenesis degree. If so, the fact that the upper part of the Lower unit shows CaO/SiO2 ratios geater in the northern West El Obeid section than in the southern North Bara3 section, suggests that pedogenesis processes were more intense in the South than in the North, and therefore, that the southern part of the study area was wetter than the northern part dat that time.

Clay mineralogy

Clay mineralogy analyses has been carried out in eight samples from the Lower, Third, and Fourth Units of the East El Obeid section.

Results

The major clay mineral assemblages in the analyzed samples include smectite, kaolinite, illite in addition to interstratified layers illite-smectite, chlorite-smectite and kaolinite-smectite (Table 4.4). The smectite and illite represent the greater percentages among the interstratified minerals, and consequently these stratified layers are considered either smectite or illite according to the dominant species. The vertical distribution shows a remarkable predominance of smectite and kaolinite along the section, while illite is less present. The relation between kaolinite and smectite is reversed along the section, since smectite decreases upward, while kaolinite increases upward and becomes dominant in the upper unit (Fig. 4.31).

Interpretation

Kaolinite and smectite form during chemical weathering in warm and wet climate, while illite and chlorite form during physical weathering [START_REF] Madhavaraju | Clay mineralogy of the Late Cretaceous and early Tertiary successions of the Cauvery Basin (southeastern India): implications for sediment source and palaeoclimates at the K/T boundary[END_REF]. Smectite with mixed layered clays is indicative of seasonally warm climatic conditions [START_REF] Madhavaraju | Clay mineralogy of the Late Cretaceous and early Tertiary successions of the Cauvery Basin (southeastern India): implications for sediment source and palaeoclimates at the K/T boundary[END_REF].

Prevailing smectite and kaolinite in the section indicate that the source area of the investigated units experienced chemical weathering rather than physical weathering before transport of the sediment. Moreover, the slight upward increase of kaolinite suggests a slight increase of chemical weathering in the source area of the Upper unit.

Paleohydrogy

Past hydrological records are important to understand the variability of paleoclimate, since they provide information about the geometry of the water table.

Results

Paleohydrological information is given by some paleolakes and ponds or pools, which had never been mentioned before. These paleolakes are scattered in the northern part of the study area around El Ga'ah and Sodari, and in the central part, around El Khowei and En Nahud. In the northern part, the most important paleolakes are the El Ga'a Lake and the El Firja depression lake, north of Sodari (Fig. 4.32), beside many other small paleolakes or pools. In the northern part, the distribution of the paleolakes is controlled by former sand dunes, which form North-South trending dams or barriers across valleys or depressions. The El Ga'ah paleoake covers an approximately 490 km 2 wide area, is orientated north-south, with a maximum length of 38 km and a maximum width of 22 km, and the elevation of its deepest part is ≈ 450 m asl. The elevation of four shoreline points was measured: three at the eastern side of the lake and another point on its western side, which was identified by the concentration of gastropod shells, archeological and bone remnants. A topographic profile has been drawn from a digital elevation model (DEM) joining the western shoreline point to one of the eastern points (Fig. 4.33). This profile shows that the maximum lake level was ≈ 472 m asl (maximum depth of 22 m) and the average thickness of the water column was about 17 m. Therefore, the water volume of the El Ga'a Lake was ≈ 8.33 km 3 (8,330,000 m 3 ). According to similar potteries from dated sites in northern and central Sudan, the fragments of dotted wavy line potteries collected on the western shoreline of the paleolake indicate that the maximum lake level was achieved between ≈ 7000 and 6000 yr BP and gave way to human settlements.

The El Firja depression Lake (north of Sodari) covers an area of about 290 km 2 and ranges between 500 and 540 m asl. It has a very irregular shape due to sand dune tongues, and is orientated north-south with a maximum length of 32 km, and a maximum width of 17 km. Its shoreline is marked by concentrations gastropod shells and by archeological and bone remnants.

In the central part of the study area, the El Khowei and En Nahud swamps/pools are important hydrological features (Fig. 4.34). The El Khowei swamp seems to have been formed along what is today a semi-fossil river. These swamps are located between 590 and 620 m asl and are very irregular in shape, probably controlled by the drainage system. A topographic profile (Fig. 4.35) shows that the swamp level was around ≈ 605 m asl (i.e. maximum depth of 15 m).

Interpretation

The sand dunes that controlled the development of the northern lakes indicate that these lakes were closed lakes. Thus, the maximum lake levels were relatively stable for a long time (possibly thousands of years). The 22 m maximum depth of the El Ga'ah paleolake indicates that precipitations were abundant enough to allow the presence of a permanent lake, even during the dry season, and that the mean annual evaporation was lower than the mean annual rainfall.

The geomorphology of the central area indicates that the El Khowei swamp developed along the wadies. These wadies were truly runny, slowly running or standing. The deposition of palustrine limestone and the preservation gastropod shells in the palustrine limestone provide evidence that water of these wadies were semi-permanent standing water. On the other hand, deposition of sandy limestone away from the main channel of the wadi, may indicate that the water level of the swamp or wadi was not stable, and fluctuated between two levels; a temporary high level depositing sandy limestone, and a permanent low level where limestone were laid down.

During high water level, the sandy limestone was deposited as a result of the reworking of the pre-existing sediments, or of mixed, aeolian and palustrine deposits. This temporary water level may have lowered down rapidly due to the southward surficial runoff, and to direct infiltration to recharge the groundwater aquifer in the underlying Cretaceous-Paleogene "Nubian sandstone". Since the El Khowei-En Nahud area is a high plateau without feeding river, the groundwater table that controlled the development of swamps was close to the land surface.

The present day groundwater level in the En Nahud-El Khowei aquifer ranges between 550 m asl to the North and 470 m asl to the South [START_REF] Elmansour | Assessment of the groundwater of En Nuhud basin, West Kordofan State-Sudan[END_REF], which indicate a southward general flow direction, similar to the surficial flow.

V. DISCUSSION

Stratigraphy, sedimentology and paleoclimate

The sedimentological and stratigraphic study in the Kordofan region enabled us to reconstruct the following sedimentological and climatic evolution during the terminal Pleistocene-Holocene period, which correlates with the other regional sedimentological and climatic records for the same interval.

Period prior to ≈ 10 ka BP

Aeolian deposition took place prior to ≈ 10 ka BP and covered most of the studied area except the southern part. This deposits represent the lower stratigraphic unit in the studied succession.

During deposition of this unit, climate was probably dry with intensive aeolian deposition in the central and northern parts of the study area. Due to non-deposition in the southern part, it can be assumed that either the depositional environment was dry associated with aeolian erosion, or it was wet compared to the northern part, the aeolian sediments being removed out by rivers. We correlate this dry, windy period with the 20-12 ka BP time-span, which is known as a period of dune building in the central Sahara, Eastern Sahara, the Nile basin and East Africa [START_REF] Nicholson | African Environmental and Climatic changes and the general atmospheric circulation in Late Pleistocene and Holocene[END_REF][START_REF] Woodward | The Nile: Evolution, Quaternary River Environments and Material Fluxes[END_REF]Tierney et al., 2011b;[START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF][START_REF] Williams | Climate Change in Deserts (past, present and future)[END_REF]. Globally, this arid period coincides with the Last Glacial Maximum (LGM) [START_REF] Woodward | The Nile: Evolution, Quaternary River Environments and Material Fluxes[END_REF]. Some hydrological proxies suggest that during most of the LGM, moisture transport from the Indian Ocean was enhanced to easternmost Africa and that wet conditions prevailed in the Challa region (Tierney et al., 2011b). But elsewhere on the African continent, the LGM was typically dry [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Gasse | Climatic patterns in equatorial and southern Africa from 30,000 to 10,000 years ago reconstructed from terrestrial and nearshore proxy data[END_REF]. The maximum aridity was probably achieved between 18,000 and 14,000 B.P. when the Sahara invaded the Sahel and moved 400-800 km southward [START_REF] Bakker | Late Quaternary Palaeoenvironments of the Sahara Region[END_REF]Pachur and Kröpelin, 1987;[START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. This aeolian activity formed various dune types in Eastern Sahara. Accordingly, barchans and linear dunes are recognized in the Western Desert of Egypt [START_REF] Hamdan | Source of the aeolian dune sand of Toshka area, southeastern Western Desert, Egypt[END_REF], and longitudinal ridges and transverse dunes are known in the Kordofan region and the Libyan Desert [START_REF] Edmond | The Distribution of the Kordofan Sands "Anglo-Egyptian Sudan[END_REF]. These longitudinal or linear dunes reflect strong wind energy resulting in widespread dune fields in northern Sudan, extending from latitude 10° N to the South to the Egyptian border the North [START_REF] Abdu | The Crescentic dunes of northern Sudan[END_REF]. During this dry period, the Sahara extended southward over time before most of these dunes were stabilized by vegetation (Warren, 1970).

Period between ≈ 10 and ≈ 6 ka BP

Lacustrine, palustrine and fluviatile deposits were recognized in most parts of the study area, dated between ≈ 10 and ≈ 6 ka BP in the northern and central part, and from ≈ 8 to ≈ 3 ka BP in the southern part. During this period, the environment was wet, vegetated across the study area, and locally swampy. This kind of depositional environment have already been reported in several sites across the African continent during the well-known "African Humid Period" (AHP) [START_REF] Demenocal | Green Sahara: African Humid Periods paced by Earth's orbital changes[END_REF] or "lacustrine period" [START_REF] Williams | Climate Change in Deserts (past, present and future)[END_REF]. Although much of the references indicate that the AHP lasted from ≈ 12 to 6 ka BP, [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF] reported that the real start of this wet conditions began almost everywhere at about 14-15 ka BP, although maximum humidity was not achieved until the early Holocene because of a return to more arid conditions during the so-called "Younger Dryas" event, which is well recorded in Eastern Africa between 12.8 -11.6 ka cal BP (Tierney et al., 2011b;[START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF].

According to previous works, the age of the wet phase varies from one site to another. It has been dated between 11 and 5 ka in North and East Africa (Tierney et al., 2011a), between 9.5 and 4.5 ka BP in NW Sudan (Pachur and Kropelin, 1987;[START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF][START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF], between 9 and 6 ka BP across North Africa (DeMenocal and [START_REF] Demenocal | Green Sahara: African Humid Periods paced by Earth's orbital changes[END_REF], and between 6.5 and 4.5 ka BP as a second lacustrine phase in the Nile Valley and part of Eastern Sahara (Nicholson and Flohan, 1980). During this period, the present-day hyperarid Saharan desert was vegetated and covered with numerous scattered lakes [START_REF] Cole | Contrasting compositions of Saharan dust in the eastern Atlantic Ocean during the last deglaciation and African Humid Period[END_REF] and many development stages of these lakes are generally of the same age [START_REF] Bakker | Late Quaternary Palaeoenvironments of the Sahara Region[END_REF]. Most of the lacustrine lakes in northern and northwestern Sudan were dated as early as mid-Holocene: Dry Selima Lake deposits range from 8 820 ± 80 to 5 570 ± 60 yr BP [START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF], West Nubian Palaeolake sediments range from 9 400 to at least 3 800 yr BP [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF], and paleolakes formed along the Wadi Howar are dated between 9 500 and 4 500 yr BP (Kröpelin, 1993, in Abell and[START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF].

According to our data, this wet phase lasted in Kordofan at least from ≈ 10 to 6 ka BP in the North and up to 3 ka BP in the South of the studied region. However, the dated samples may not represent the base and top of the palustrine/lacustrine carbonate. In El Ga'ah, the well caved near the village may not have reached the base of the lacustrine deposits, and in the En Nahud area, the erosional surface that caps the palustrine carbonate suggests that the upper part of this limestone has been eroded. Therefore, the ≈ 10 ka BP and ≈ 6 ka BP dates obtained in palustrine limestones near En Nahud do not accurately represent the base and top of the palustrine limestone and therefore, the wet period may have started earlier than ≈ 10 ka BP and may have come to an end after 6 ka BP. Moreover, the lacustrine deposits dated north of the study area at Dry Selima, West Nubian Palaeolake, and Wadi Howar lake, not only support continuity of the wet phase in the central and northern parts of the study area after 6 ka BP, but also according to the starting dates, they provide other evidence that this wet period had started earlier in the Kordofan region than in north-central, northwestern, and northern Sudan. According to the indicated ages, the wet period started in the Kordofan region ≈ 500 year earlier than in Wadi Howar (north-central Sudan), ≈ 600 year earlier than in the West Nubia Palaeolake (NW Sudan), and ≈ 1200 year earlier than in the Dry Selima Paleolake. This information suggests that the AHP did not occur abruptly, neither across the continent nor across the Eastern Sahara, but it occurred gradually through a gradual northward shift of the Inter Tropical Convergence Zone (ITCZ).

Taking in consideration that the lakes of northern Kordofan and those previously studied (Wadi Howar, West Nubia, Selima) are latitudinally arranged. The northern end of the Kordofan lakes is observed up to latitude 14°30ʹN, Wadi Howar flows from the west to the east between latitudes 16°30ʹN and 18°30ʹN, but its greatest part "which contains the lakes" fall at latitude 17°30ʹN. The center of the West Nubian Paleolake is located approximately at 18°30ʹN. The dry Selima paleolake is located at 21°23ʹN. The latitudinal position of these lakes and the known starting time of the wet phase in these lakes will help in the determination of four approximate positions of the northern limit of the northern end of the ITCZ (Fig. 5.1).

The first position of the ITCZ was located between the northern end of the Kordofan lakes and Wadi Howar lakes. The wet phase started in Kordofan at ≈ 10 000 yr BP, and started ≈ 500 yr later in Wadi Howar. That means that at about 9 750 yr BP, the ITCZ was fluctuating around latitude 16°N.

The second position of the ITCZ was between Wadi Howar and the West Nubian Paleolake, since the wet phase occurred at ≈ 9 500 and ≈ 9 400 year BP, respectively, at these two sites. This suggests that at about 9450 yr BP the ITCZ was fluctuating around latitude 17°N.

The third position of the ITCZ was between the West Nubia and the Selima paleolake. The wet phase began at ≈ 9 400 and ≈ 8 800 year BP, respectively, at these two sites. This suggests that at about 9 100 yr BP, the ITCZ was fluctuating around latitude 20°N (Fig. 5.1). Fluviatile sediments recorded in abundance in the southern part of the study area, and locally as either conglomeratic lenses or as very local delta sediments (El Ga'ah) in other areas, can be correlated with similar deposits or flood events reported throughout the central Sahara during the early Holocene until 6 500 yr BP [START_REF] Bakker | Late Quaternary Palaeoenvironments of the Sahara Region[END_REF]. Fluviatile sediments in the Western Nubian Paleolake are formed of coarse sand to fine-grained gravels mostly of aeolian origin and redeposited by local fluvial transport ≈ 9 000 year BP ago [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF]. Fluvial sediment input in the northern Red Sea suggests substantially higher rainfall and freshwater runoff in this area between 9.25 and 7.25 ka BP [START_REF] Arz | Mediterranean Moisture Source for an Early-Holocene Humid Period in the Northern Red Sea[END_REF].

They can be correlated with a series of flood events well documented in the White and Blue Nile Rivers, either as flood, or high water levels during the early to mid-Holocene, which recorded high White Nile levels around 14.7-13.1 ka, 9.7-9.0 ka, 7.9-7.6 ka, 6.3 ka and 3.2-2.8 ka [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF]. Additionally, peak floods are evident toward 9 500 yr BP and again at 8 500-7 000 yr BP, with less extensive flooding at about 7 000, 5 500, 3 000, 2 700, and 2000-1500 yr BP (Williams and Adamson, 1980).

Since the White Nile is a river originating from lakes Lakes Victoria and Albert, the flood events or high level along the White Nile reflect intensive rainfall around the equator, which leads to an overflow at Lakes Victoria and Albert. Such overflows of Lakes Victoria and Albert are recorded soon after 12 500 BP and brought dramatic and enduring changes to the lower White Nile valley. By 11 500-11 000 BP, the White Nile was up to 5 m higher and 20-40 km wider than today [START_REF] Williams | Late Pleistocene desiccation along the White Nile[END_REF][START_REF] Klappa | Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance[END_REF]. During the flood events in the White Nile, the previously mobile desert dunes became vegetated, and many dunes became wholly or partly buried beneath the fluvio-lacustrine clays [START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF]. In the Blue Nile, high flood levels are recorded around 13.9-13.2 ka, 8.6 ka, 7.7 ka and 6.3 ka [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF]. As a result of overflow between 12 000 and 8 000 yr BP, the White Nile remained high and formed an extensive lake up to 30 km wide or more in addition to other small lakes west of the White Nile (20 km northwest of Jebel Aulia, south of Khartoum) [START_REF] Williams | Late Pleistocene desiccation along the White Nile[END_REF]. This lake dammed by a former sand dune developed during the LGM between 20 and 18 ka ago [START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF]. However, such lakes formed close to the White Nile as a result of flooding will not provide the real climate of their sites in Sudan, but instead, they can provide climatic proxies for the equatorial Africa in particular Lakes Victoria and Albert. In contrast the lakes of Northern Kordofan and flood events in Southern Kordofan provide the climatic proxies for Kordofan region itself, due to the local origin of these and of the flooding wadies or seasonal rivers.

North of the Kordofan region, fluviatile and/or alluvial sediments distributed along the lower part of Wadi Howar exhibit gravelly sand, silty fine sand and alluvial sand in addition to local lacustrine marl and chalk (Kröpelin and Soulié-Märsche, 1991). These deposits were laid down between about 9 500 and 4 500 years ago, when the lower Wadi Howar flowed through an environment characterized by numerous ground water outlets and freshwater lakes (Pachur and Kröpelin, 1987). Wadi Howar constituted the largest tributary to the Nile from the Sahara between the Mediterranean Sea and the Atbara River, and it drained a 400 km wide area with present rainfall of 25 mm/year (Pachur and Kröpelin, 1987). Nowadays, Wadi Howar is defunct and its course is only marked by linear tree vegetation, sustained by a ground water table some 6 to 10 m below the surface (Pachur and Kröpelin, 1987).

In some sites in lower Wadi Howar (jebel Rahib), the former river bed is blocked by a 15 m high and 5 km wide dune barrier. Pachur and Kröpelin (1987) reported some flooding events between ≈ 5600 and 7000 yr BP in the eastern part of lower Wadi Howar. Such wadi floods or overflows also have been reported in some wadies in West Nubia: Wadi Fesh-Fesh in Western Nubia was surrounded by numerous scattered swamps [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF].

In the Kordofan region the paleowadi activity, represented by either fluviatile, or palustrine depositional environments, has been recorded in several sites mostly coeval to Wadi Howar and Wadi Fesh-Fesh. This similarity of depositional environment between Kordofan, northern Sudan and Eastern Sahara in general, suggests that the same wet climatic condition was prevailing at the same time, although according to the carbon dating, this wet phase started earlier in the South (older than 10 ka BP.), while in Eastern Sahara this wet phase started around 9.5 ka BP (Pachur and Kröpelin, 1987).

Period after ≈ 6 ka BP

Although incomplete, the sedimentary record in the study area, allows to distinguish three main peridos between 6 ky BP and Present times (see Figs. 3.18 and 6.1): a transitional, aridification period (6-3 ky BP), a hiatus corresponding to non deposition or erosion due to strong aeolian activity (3-1 ky BP), and a dry period dominated by aeolian deposits (1-0 ky BP). As a whole, the prolonged hiatus (≈ 6 to ≈ 1 ka BP in the North, ≈ 3 to ≈ 1 ka BP in the South) and the generally dry, windy environment recorded after 1000 yr BP result from the aridification of the climate in the northern part of the study area, the southern parts remaining wetter.

The erosional surface at the top of the palustrine limestone in the En Nahud-El Khowei area suggests that sedimentation did not come to an end at ≈ 6 ka BP. Instead, the apparently 5000 yr lasting hiatus has been probably amplified by significant wind erosion that removed part, or all, sediments deposited between 6 and 3 ka BP in this area. As a matter of fact, considering the relatively narrow latitudinal extension of the study area (about 3.5 latitudinal degrees), it is difficult to consider that in the northern/central part of the study area, the wet phase ended 3000 year age earlier than in the southern parts. Therefore, we shall consider that the trasitional, aridification period occurred between 6 and 3 ky BP in the study area.

This aridification period is correlated with the well-documented aridity that already considerably advanced farther North, in Eastern Sahara by 6 ka BP [START_REF] Manning | The demographic response to Holocene climate change in the Sahara[END_REF]Tierney and deMenocal, 2013). Such aridification is evidenced by ( 1) the desertification and aeolian deflation during the Middle and Late Holocene (≈ 5.4 ka BP) recorded in Egypt and northern Sudan [START_REF] Nicoll | Recent environmental change and prehistoric human activity in Egypt and Northern Sudan[END_REF]; (2) deflation and erosional surfaces recorded in the Western Nubia Paleolake at ≈ 4 ka BP [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF]; (3) aridification that began in NE Africa at ≈ 6 kyr BP [START_REF] Jung | Stepwise Holocene aridifcation in NE Africa deduced from dust-borne radiogenic isotope records[END_REF], and (4) a shift from an earlier predominantly monsoonal climate regime, to one dominated by northern Mediterranean fluxes recorded after 4000 cal yr BP at Lake Yao in Chad [START_REF] Lézine | A late Holocene pollen and climate record from Lake Yoa, northern Chad[END_REF]. Nevertheless, the timing and amplitude of the transition between the "green Sahara" and the present day hyper-arid desert at the end of the AHP is still a matter of debate [START_REF] Lézine | A late Holocene pollen and climate record from Lake Yoa, northern Chad[END_REF]. The drying events stated elsewhere in the Sahara may be explained by fading rainfall at a specific latitudinal position at a certain moment, or by local dropping of groundwater (Kuper and Kropelin, 2006). According to [START_REF] Lézine | A late Holocene pollen and climate record from Lake Yoa, northern Chad[END_REF], the shift from the wet monsoonal climate to the dry one occurred gradually through transitional climatic conditions that lasted from ≈ 5.5 to 2.7 ka BP.

Therefore, the aridification period appears to have occurred earlier at northern latitudes than in southern ones, since it is recorded at ≈ 5.4 ka BP in southern Egypt and northern Sudan, ≈ 4 ka BP at the West Nubia Paleolake, and ≈ 3 k BP in central-southern Kordofan. Taking in consideration that aeolian erosion removed part of the sediments deposited during the wet or transition periods in Kordofan and West Nubia, this indicates that aridity in these areas was achieved later than these dates. However, this remarkable southward shift of aridity suggests a southward migration of ITCZ, that was moving northward during the AHP. This southward shift of the ITCZ resulted in the narrowing of the tropical rainy belt. The available dates of the aridification period can help in the determination of an approximate positon of the ITCZ (or northern end of the equatorial rainy belt) during the transitional period. The ITCZ was approximately fluctuating around latitude 21°N at ≈ 5.4 ka BP, around latitudes 18 -19°N (west Nubia) at ≈ 4 ka BP, and around latitudes 13 -13.5°N at ≈ 3 ka BP.

A period of strong aeolian activity is well recorded in NE Africa between ≈ 5.4 and 2.7 ky BP (see above). In the study area, the beginning of aeolian related hiatus likely did not occur exactly at ≈ 3 ka BP, since part of the former deposits may have been removed. Therefore, the wind erosion may have begun after 3 ka BP. In any case, the wind erosion was stronger in the North than in the South of the study area (Figs. 3.18 and 6.1). Several causes may be invoked to explain this feature : (1) the northern areas are closer to the Saharan wind storms, and thus more exposed to aeolian erosion; (2) assuming that winds were blowing from the North, as indicating by the grain size analysis (section 3.5), the southern areas (South of El Obeid-En Nahud) are located leeward, while the northern areas are located windward (see Fig. 3.39) and are, therefore, more exposed to the wind action; and (3) the southern areas being wetter than the northern ones, the vegetal cover may have limited the erosion.

After the recorded wind-related hiatus (≈ 3-1 ka BP), the climate seems to have been comparable to that of today.

Discussion of the isotopic results

The isotopic composition of the analyzed samples can be correlated with some previous isotopic investigations carried out in Sudan and surrounding areas.

Oxygen isotope values for gastropod shells from the Esh Shawal area (Central Sudan between latitudes 13.5 and 14°N, western bank of the White Nile) range from -0.927‰ to +6.537‰ (PDB) [START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF]. These shells were collected at 100-110 cm below the surface and accumulated during the peak of the White Nile palaeolake transgression, at 11 500-11 000 BP [START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF]. The isotope results of Esh Shawal gastropods generally tend toward positive values when compared to our isotopic values. The positive isotopic values from the Esh Shawal site suggest that the AHP did not occur in central Sudan before 11 ka BP and consequently, that the arid period known as the period of dune building, was still dominating the climate of Central Sudan west of the White Nile around latitude 13.5°N. This provides more evidences that the wet lacustrine period have started in northern Kordofan between the proposed 10 ka BP (age of our palustrine and lacustrine carbonates) and 11 ka BP (according to oxygen isotope of the Esh Shawal gastropods). However, the entire White Nile flood plain may reflect the climatic conditions of the source area rather than the local climate, therefore, the enriched δ 18 O of Esh Shawal area reflect drier condition at the source area (Lake Victoria and Albert) taking in consideration the evaporation effect along the course of the White Nile, which reduces the lighter oxygen ( 16 O).

On the other hand, oxygen stable isotope analysis of 8500 to 7000 yr old gastropods from shallow claypans near Wadi Mansurab in north-central Sudan reveals that the shell carbonate is highly depleted in δ 18 O (-5.3‰ to -12.7‰ PDB), with extreme variability up to 6-7‰ PDB [START_REF] Ayliffe | Stable carbon and oxygen isotopic composition of early-Holocene gastropods from Wadi Mansurab, north-central Sudan[END_REF]. These variability is interpreted as due to seasonal rainfall regime, characterized by a high degree of inter-annual variability.

Isotope values from shells of the bivalve Etheria elliptica (dated 6653 ± 110 yr BP) from Wadi Howar yielded highly depleted δ 18 O values (-10‰ to -12‰ PDB) with some variations [START_REF] Rodrigues | Seasonality in the early Holocene climate of Northwest Sudan: interpretation of Etheria elliptica shell isotopic data[END_REF]. This high depletion and variations in δ 18 O are interpreted as due to high seasonality of monsoonal rains [START_REF] Rodrigues | Seasonality in the early Holocene climate of Northwest Sudan: interpretation of Etheria elliptica shell isotopic data[END_REF]. Such depleted gastropod shells oxygen isotope were also reported by [START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF] in Selima, North Sudan, and in some paleolakes along Wadi Howar. Moreover, these depleted δ 18 O values suggest that north-central and northwestern Sudan (Wadi Mansurab and Wadi Howar) were located within the tropical rain belt, but the variability in these depleted δ 18 O values evidences that the seasonal fluctuation of the ITCZ was more erratic, as this period corresponds the late phase of the AHP.

In our study, coeval gastropod shells from En Nahud and El Khowei recorded depleted but less variable δ 18 O values, compared to those recorded from Wadi Mansurab and from Wadi Howar bivalve shells. This may indicate that the Kordofan region was affected by the ITCZ seasonal fluctuations later than the Wadi Mansurab and Wadi Howar areas, located farther north.

In the Afar region in Ethiopia, freshwater gastropods of early to mid-Holocene age (11 070 ± 160 to 6 670 ± 110 yr BP) recorded δ 18 O between -4 to -6 ‰, while modern shells of the same area yielded δ 18 O values between -1.5 and + 6 ‰ along the spiral growth (from the aperture to the apex), with most values around -1.5‰ [START_REF] Abell | Oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in Afar region of Ethiopia[END_REF]. The depleted δ 18 O values of the early to mid-Holocene are interpreted as due to episodes of high rainfall in the Ethiopian highlands, while arid spells produced enriched δ 18 O values in the modern shells [START_REF] Abell | Oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in Afar region of Ethiopia[END_REF]. Compared to isotopic values of the Ethiopian shells of early to mid-Holocene age, our results suggest that the meteoric water of the Kordofan region was more depleted in δ 18 O than that of the Afar region in East Africa. This variation suggests that during the early to mid-Holocene period, either the Kordofan region was wetter than eastern Africa, or, more probably, the latter region received rainfall from a different source. As a matter of fact, the East African rainfalls are mostly influenced by Indian Ocean sea surface temperature variations, while those of West and Central Africa [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF] are mostly of Atlantic origin with probably some influences of the Congo Air Basin. These complex rainfall sources may explain isotopic variations at the same time in different regions, even though the regional climate is comparable.

Studying the isotopic composition of modern African freshwater gastropods, [START_REF] Abell | Oxygen isotope ratios in modern African gastropod shells: A data base for paleoclimatology[END_REF] evidenced variable oxygen isotopic ratios across the continent. Among these samples, those from the surrounding regions of Sudan and the Sahara can help in this discussion. According to [START_REF] Abell | Oxygen isotope ratios in modern African gastropod shells: A data base for paleoclimatology[END_REF], the Congo samples recorded δ 18 O values between +2.77 and +2.62‰, Uganda from +4.0 to +1.73‰, and Kenya +3.67 and -3.57‰. To the East, in samples from Ethiopia, δ 18 O ranges between -3.58 and -4.4 ‰. One sample from Sudan recorded δ 18 O of -3.54 ‰. In North Africa samples from northeast Egypt range between -6.39 and +2.15‰, around -9.87‰ in Libya, and about -8.38‰ in Algeria. To the west, samples of Nigeria recorded δ 18 O between -4.00 and +4.07 ‰. Although samples of Ethiopia, Sudan and Nigeria are approximately from the same latitudinal zone, those from Sudan and Ethiopia are more negative than those of Nigeria. This variation may be due to the variation of the water vapor source, since the Indian Ocean represent the main source of the rainfall in the Ethiopian highland, while in Sudan and Nigeria the rainfall source is from the Atlantic Ocean. However, assuming that the Sudanese and Nigerian rainfall come from the same source, why δ 18 O values are negative in Sudan and positive in Nigeria and in other Sahelian countries of West Africa ? These variations may be due the continental or latitude effect, which decreases δ 18 O values with increasing distance from the source. Northern Nigeria is about 1500 km from the Atlantic Ocean, while Sudan is more than 3500 km apart from the same ocean. Therefore, the water vapor form the Atlantic Ocean will travel along large distances to reach Sudan, will be precipitated and evaporated repeatedly crossing the Congo Basin, thus depleting the rainfall in the heavier 18 O isotope. For instance, the meteoric water in Uganda and Southern Sudan has a present-day δ 18 O value of about -2‰, while the corresponding value for present-day gastropods in Lake Victoria is +1.5‰ [START_REF] Williams | Late Quaternary environments in the White Nile region, Sudan[END_REF]. On the other hand, the highly depleted North African shells are due to different climate and water rain source, as North Africa is dominated by the Mediterranean Sea climate.

In our case, the variation in δ 18 O between the highly depleted values in the shells from the carbonate horizon (Second unit), and the less depleted values of the shells from the upper unit of red sand sediments, may be due the combination of evaporative and temperature effects.

Since the gastropod shells analyzed in this study are from the same latitudinal zone, the vertical variation in δ 18 O for the same site is only due to climatic changes (evaporation and temperature) through time. [START_REF] Abell | Oxygen isotope ratios in modern African gastropod shells: A data base for paleoclimatology[END_REF] summarized some problems in the use of stable isotope ratios of gastropod shells as environmental indicators: (1) the shells of most freshwater molluscs are generally aragonite.

The aragonite crystal structure is thermodynamically unstable compared to calcite under most ambient conditions (Folk, 1965). Therefore, recrystallization may occur with concomitant isotopic reequilibration with the new environment.

(2) Modern gastropods can usually be collected alive, so there is no question of their age, but ancient gastropod shells are often quite robust and can be preserved for millions of years. Reworking of the sediments in which the shells were embedded can produce a mixture of shells of different ages and environments. It is essential to use gastropod shells that give every evidence of being in situ [START_REF] Abell | Oxygen isotope ratios in modern African gastropod shells: A data base for paleoclimatology[END_REF].

(3) The isotopic content of a body of water do not reflect necessarily accurately the local environment as a whole. For example, some species of gastropods can aestivate successfully, and their isotope ratios can reflect highly evaporative conditions such as in temporary irrigation ditches or ephemeral lakes and streams. Such gastropods will have isotope ratios significantly different from gastropods growing in the same locality but in permanent rivers and lakes. In the present study, some anomalies in gastropod shells oxygen isotope may be due to some of these problems. Very negative δ 18 O values have been recorded in the East El Khowei section, among less depleted values from the upper unit (aeolian red sand sediments). An acceptable explanation is that the analyzed shell may have been reworked from underlying palustrine unit, which is marked by such depleted value.

Concerning the isotopic composition of the calcareous nodules, our interpretation proposed that the calcareous nodules of the lower unit of the mottled sand sediments and those formed within the second unit in the southern part, were mostly developed during the AHP. Although these nodules formed during this wet phase, their δ 18 O values are less depleted than gastropod shells of the same period. Ground water composition has probably a major influence in the development of these calcareous nodules. If so, the isotopic composition of the nodules results from a combination of different isotopic sources that include the initial groundwater, the recharged water during the wet phase, and the evaporative condition during formation of the nodules, which may modify the isotopic composition of the newly recharged water. According to this, the subsurface isotopic composition will differ from the surficial one, in spite of similar climatic condition.

Paleohydrology

The most important and already investigated hydrological features are paleolakes of the Mega Chad Lake [START_REF] Bouchette | Hydrodynamics in Holocene Lake Mega-Chad[END_REF], Western Nubia in NW Sudan [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF][START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF], Selima in north Sudan [START_REF] Abell | Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs[END_REF], some small lakes along Wadi Howar in NW Sudan (Pachur and Kröpelin, 1987) and the 400 km long Wadi Howar paleohydrological network in northwestern Sudan, which linked the mountains of Western Sudan to the Nile (Pachur and Kröpelin, 1987). [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF] concluded that the hydrological history of Africa appears to have been a complex alternation of wet and dry episodes with abrupt transitions and sometimes these fluctuations recorded short fluctuation events rapidly followed by a return to initial conditions [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. Such fluctuations of the water level have been evidenced through structural characteristics of the sediment in the West Nubian Paleolake; temporarily the lake dried up and desiccation cracks formed in the calcareous muds are filled with fragments of lake chalk and carbonate mud [START_REF] Pachur | Paleoclimatic Implications of Late Quaternary Lacustrine Sediments in Western Nubia, Sudan[END_REF]. In the El Ga'ah Lake (this study), water level fluctuations have been evidenced through two observations: (1) the alternating lacustrine limestone and argillaceous sand sediments beds, which indicate environmental changes (Fig.

3.6);

(2) the desiccation features on top of each lacustrine limestone horizon, which indicate drying up of the lake and subaerial exposure of the deposited carbonate mud. Because of dating scarcity, it is difficult to determine correctly the periods of El Ga'ah Lake low level, but two drying up event prior the last drying phase have been observed through the alternation of these sediments.

Although the paleolakes of the Northern Kordofan and the Central Kordofan swamps, and those previously investigated (West Nubian paleolake, Wadi Howar lakes, Maegalake Chad), are all approximately of early-Holocene age, they are marked by some hydrologic variations. The most important variation represent the type or the mechanisms of the precipitation and the kind of the infilling water of these lakes. The West Nubian, Wadi Howar and Megalake Chad are filled up by meteoric water precipitated over mountainous areas of Jebl Marra, Ennedi Mountains and Tibesti Mountain, and their catchment areas are marked by extensive drainage areas, gathering many tributaries. In contrast, the source water of the Kordofan lakes or swamps is only from local precipitation over the surrounding catchment areas of normal reliefs (no mountainous areas). Mountains usually attract clouds and rains (topographic effect), which is only very partially the case of the North Kordofan lakes. Due to this topographic effect, the high level periods of the West Nubian, Wadi Howar and Chad lakes may have started before, and ended after, the Kordofan lakes. For instance, in spite of subsequent aeolian erosion in Kordofan and West Nubia, the youngest recorded ages of the lakes are ≈ 6 ka BP in Kordofan (14°30'N) and ≈ 3800 yr BP in West Nubia (18-19°N).

Much of the groundwater of the West Nubian paleolake is fossil water and was accumulated during different geological times much wetter than the present-day hyper-arid climate of Eastern Sahara. This basin has an area of ≈ 13 160 km 2 and a 1.6 km thick Paleozoic and Mesozoic sedimentary section capable of holding ≈ 1 120 km 3 of groundwater if the sediments are completely saturated [START_REF] Elsheikh | Geology and geophysics of the West Nubian Paleolake and the Northern Darfur Megalake (WNPL-NDML): Implication for groundwater resources in Darfur, northwestern Sudan[END_REF]. Nowadays, the water table in West Nubia is estimated to be ≈ 50 m deep [START_REF] Gossel | A very large scale GIS-based groundwater flow model for the Nubian sandstone aquifer in Eastern Sahara (Egypt, northern Sudan and eastern Libya)[END_REF], while in Wadi Howar the groundwater table lays 6 to 10 m below the surface (Pachur and Kröpelin, 1987). [START_REF] Gossel | A very large scale GIS-based groundwater flow model for the Nubian sandstone aquifer in Eastern Sahara (Egypt, northern Sudan and eastern Libya)[END_REF] proposed that groundwater of the Eastern Sahara aquifers was infiltrated during the wet periods 20 000 and 5 000 yr BP ago.

The existence of megalakes in Chad, West Nubia and Selima, of small lakes in Wadi Howar and of the recently discovered lakes in the Kordofan region, all of early to mid-Holocene age, reflect that the hydrological situation was comparable across this region in spite of the previously mentioned topographic effect. The lacustrine-palustrine carbonates deposited in these lakes provide evidences for mostly permanent surficial water bodies and provide an indicator that groundwater table was very shallow (few meters below the surface) in areas surrounding the lakes, and was in direct contact with the water bodies. Since some evidences indicate that the lake levels was fluctuating, the groundwater table was fluctuating too. These fluctuations probably constitute a normal response to the ITCZ fluctuations. Such fluctuations have already been documented in the West Nubian paleolake between 9400 and 3800 14 C yr BP [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF]. Later on, [START_REF] Hoelzmann | Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate[END_REF] recognized that between 10.9 and 8.9 ka BP, the lake was shallow with fluctuating salinity. After about 8.7 ka BP, the lake level rose, water became fresh to oligo-saline, and the lake had a minimum depth of at least 8 m. This record shows that lakes in Eastern Sahara registered short-term climatic events, as long as they were isolated from large-scale artesian groundwater systems [START_REF] Hoelzmann | Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate[END_REF].

The deepest depression of megalake Chad is less than 200 m in elevation [START_REF] Bouchette | Hydrodynamics in Holocene Lake Mega-Chad[END_REF]. Four distinct paleo-lake levels have been identified in megalake Chad at 290 m, 300 m, 320 m and 330 m. The water depth, derived from present-day topography of Lake Chad, exceeded 150 m at its deepest part in the central northern sub-basin [START_REF] Bouchette | Hydrodynamics in Holocene Lake Mega-Chad[END_REF]. The maximum water depth estimated in the West Nubian paleolake between 9400 and 7500 14 C yr BP is 15 m [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF], while the present groundwater in West Nubia is ≈ 50 m deep [START_REF] Gossel | A very large scale GIS-based groundwater flow model for the Nubian sandstone aquifer in Eastern Sahara (Egypt, northern Sudan and eastern Libya)[END_REF][START_REF] Elsheikh | Geology and geophysics of the West Nubian Paleolake and the Northern Darfur Megalake (WNPL-NDML): Implication for groundwater resources in Darfur, northwestern Sudan[END_REF] and its central part is relatively flat with a mean elevation of ≈ 550 m [START_REF] Elsheikh | Geology and geophysics of the West Nubian Paleolake and the Northern Darfur Megalake (WNPL-NDML): Implication for groundwater resources in Darfur, northwestern Sudan[END_REF]. In the western part of the lower Wadi Howar, Pachur and Kröpelin (1987) mention a paleolake ("2" in their Fig. 1a, 17°N) about 40 km to the south of the channel of Wadi Howar and of its tributaries, indicating that the water table was there at the surface (545 m asl). The elevation of the deepest part in El Ga'ah Lake is ≈ 450 m asl. Since the only measured maximum shoreline is ≈ 472 m asl, the maximum water depth estimated is about 22 m, and according to a hand-dug well drilled by the local citizens in El Ga'ah, the groundwater table in weathered basement aquifer is about 80 m deep (i.e. 400 m asl). In the El Khowei-En Nahud swamps, the deepest depression is ≈ 590 m asl and the shoreline is estimated at ≈ 610 m asl. According to [START_REF] Elmansour | Assessment of the groundwater of En Nuhud basin, West Kordofan State-Sudan[END_REF], the present elevation of the groundwater table in En Nahud-El Khowei basin ranges between 550 and 500 m asl. For the latter area, it is noteworthy that they are located on a plateau that dominates surrounding areas and do not receive any significant river. Therefore, the presence, there, of swamps indicates that, at least during the wet season, rainfall were sufficient to allow semi-permanent water bodies to occur.  The correlation of this results with some similar climatic studies in the Eastern Sahara indicates that these climatic changes resulted in gradual northward movement of the northern limit of the tropical rainy belt during the AHP.  During the aridification period that followed the AHP, the climatic change resulted in the gradual southward shift of the rainy belt and of the ITCZ.

Perspectives

The reconstruction of the sedimentary and paleoclimatic evolution of the Kordofan region in late Pleistocene-Holocene times is the first attempt in this area. Despite the fact that the final output of this study provided very valuable information about the sedimentary and climatic evolution in this region in particular and Eastern Sahara in general, there is great potential for further work to be conducted in this region and its surroundings for a better understanding of this evolution. The future work that may be done include the following points:

 The actual dates and duration of the African Humid period should be specified, in order to reconstruct the movements of the ITCZ. This requires more dating of the base and top of the lacustrine/palustrine facies, and more survey of the overlying deposits, in order to specify the age of the wind-related hiatus that followed the AHP.  In spite of the lack of organic matter in the red sand facies, dating the base of this facies would help specifying the age of the end of the preceding hiatus. An alternating dating method, such as Luminescence dating will provide some clues to fix the dating problems of such kind of oxidized sediments.

 Since this study mainly dealt with pollens of the southern areas, additional palynological investigation might be carried out in other parts of the Kordofan region to build a wider palynological record and, combined with chronological dates, to refine the paleoenvironmental and paleoclimatic reconstructions.

 Paleobiology of gastropods combined with the Chemical Index of Alteration (CIA), seems to be a promising and effective tool to assess the mean annual rainfalls experienced by a region, and thus to constrain the paleohydrological regime of the area.  Systematic analysis of oxygen isotope should be done on individual gastropod shells along the spiral growth line (from the apex to aperture) to determine whether the wet and dry episodes, are due to mere seasonality or to ITCZ fluctuations.  Intensive archeological investigations are very important to be achieved across the region, particularly around the shorelines of the northern lakes of El Ga'ah and Sodari and around the En Nahud -El Khowei swamps. Such investigations will improve the understanding of the climate-human settlement relations.  More regionally, climatic studies in the eastern Sudan will help to determine whether the Eastern monsoon of the Indian Ocean hade any influences on the precipitation pattern in this part of Sudan, or if the Ethiopian highlands prevent moisture to cross further West, the Kordofan region remaining dominated by the southwesterly moisture from the Atlantic Ocean and the Congo Basin.

Fig. 1

 1 Fig. 1.1 Location map of the study area 2 Fig. 1.2 Regional climatic zones around Sudan 3 Fig. 1.3 Mean annual rainfall across Kordofan Region 3 Fig. 1.4 Geomorphological map of the study area 5 Fig. 1.5 Digital elevation model of the Kordofan Region 6 Fig. 1.6 Detailed digital elevation model of the study area 6 Fig. 1.7Relief of the study area 7 Fig.1.8 Distribution of pre-Quaternary gelogical units in the Kordofan Region

Fig

  Fig. 2.1 Locations of the sampling Fig. 2.2 The decay curve of radiocarbon Fig. 2.3 OxCal calibration curve of one probability range -sample from East Abu Zabad Fig. 2.4 OxCal calibration curve shows two probability ranges -sample from Dilling Fig. 3.1 Location of the studied sections Fig. 3.2 Stratigraphic units of the Dilling area Fig. 3.3 Stratigraphic units of the Abu Zabad -El Fula area Fig. 3.4 Stratigraphic units of the El Obeid -South El Obeid area Fig. 3.5 Stratigraphic units of the En Nahud -El Khowei area Fig. 3.6 Stratigraphic units of the North Bara -Sodari area Fig. 3.7 Location of the potteries and bones collected from the El Ga'ah paleolake shoreline Fig. 3.8 Position of dated samples in the North Dilling 2 section Fig. 3.9 Position of dated sample in the North Dilling 1 section Fig. 3.10 Position of dated samples in the East Abu Zabad section Fig. 3.11 Position of dated samples in the East El Khowei section Fig. 3.12 Section near En Nahud Fig. 3.13 Position of dated samples in the East El Obeid section Fig. 3.14 Position of dated samples in the south El Obeid1 section Fig. 3.15 Position of dated samples in the El Ga'ah well Fig. 3.16 Time-correlation transect between El Fula and Dilling Fig. 3.17 Time-correlation transect between En Nahud and El Obeid Fig. 3.18 Time-correlation transect between El Ga'ah and Dilling Fig. 3.19 Mottled facies examples Fig. 3.20 Nodular calcrete or calcareous nodules

Fig

  Fig. 3.21 Nodular calcrete including iron concretions Fig. 3.22 Tubular calcrete Fig. 3.23 Field examples of conglomerates Fig. 3.24 Topographic profile from southeast En Nahud to east El Fula Fig. 3.25 Moderate regime channel facies Fig. 3.26 Lens-shaped, medium-grained sandstone bed Fig. 3.27 Flood plains Facies Fig. 3.28 Cyclic sedimentation Fig. 3.29 Fining upward successions Fig. 3.30 Red clayey sandstone facies Fig. 3.31 Palustrine facies Fig. 3.32 channel deposits of Palustrine facies Fig. 3.33 Carbonate facies Fig. 3.34 Argillaceous sandstone facies Fig. 3.35 View of the El Ga'ah delta facies Fig. 3.36 Details of the delta facies Fig. 3.37 Location map of the sections studied for grain size analysis Fig. 3.38 Lithology of the studied sections and location of the samples analyzed for grain size analysis

Fig. 3 FigFig

 3 Fig. 3.51 Curves of pre-and post-hiatus graphical statistic plots for all sections Fig. 3.52 Plot of mean grain size vs. sorting of the studied samples indicating the wind energy Fig. 3.53 Plot of Mean grain size vs. Skewness of the studied samples indicating the transportation energy Fig. 3.54 Plot of Skewness vs. Sorting of the studied samples indicating the transportation energy Fig. 3.55 Evolution of transport processes in the North Bara3 section Fig. 3.56 Evolution of transport processes in the north Bara1 section Fig. 3.57 Evolution of transport processes of the West El Obeid section

Fig. 1 . 1 :

 11 Fig. 1.1: Location map of the study area.

Fig. 1 .

 1 Fig. 1.4: Geomorphological map of the study area.

Fig. 1 .

 1 Fig. 1.5. Digital elevation model of the Kordofan Region.

Fig. 1 .

 1 Fig. 1.7. Relief of the study area.

Fig. 1 .

 1 Fig. 1.10: Current climatic zones of Africa (after Ngaira, 2007).

Fig. 1 .

 1 Fig. 1.11: Sketches of the general patterns of winds and pressure over Africa. Dotted lines indicate the Intertropical Convergence Zone (ITCZ), dashed lines indicate the Congo Air Boundary (afterNicholson, 1996[START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF].

Fig. 2 . 1 .

 21 Fig. 2.1. Sample location.

  002004 …………………………….. (2.5)

  Rspl = 18 O/ 16 O ratio of the sample; and Rstd = 18 O/ 16 O ratio of the standard Δ 18 O is the difference between the R value of the sample and the standard expressed in terms of permille relative to the R value of the standard.

Fig. 3 . 1 .

 31 Fig. 3.1. Location of the studied sections.

  Four stratigraphic units have been distinguished across the study area. The first unit is found in all areas, except in the Dilling sector. The second unit is found in the Dilling, El Khowei-En Nahud and North Bara-Sodari areas. The third unit crops out in the El Obeid-South El Obeid and Dilling sectors. The upper or fourth unit is detected in the whole area.3.1.1. Stratigraphy of the Dilling areaThree sections have been studied in the Dilling area. These are the Dilling section in the main Khour Abu-Habil seasonal river bank, and two sections North of the Dilling town (North Dilling 1 and 2), along another branch of Khour Abu-Habil.

Fig. 3

 3 Fig. 3.2: Stratigraphic units of the Dilling area.

Fig. 3

 3 Fig. 3.3: Stratigraphic units of the Abu Zabad -El Fula area. Legend in figure 3.2.

Fig. 3

 3 Fig. 3.4: Stratigraphic units of the El Obeid -South El Obeid area. For the legend refer to figure 3.2.

Fig. 3

 3 Fig. 3.5: Stratigraphic units of the En Nahud -El Khowei area. Legend in figure 3.2.

Fig. 3

 3 Fig. 3.6: Stratigraphic units of the North Bara -Sodari area. Legend in figure 3.2.

  The dotted wavy line has a wide distribution west of the Nile across the Sahara-Sahel Belt(Mohammed-Ali, 2003) where it is observed in Wadi Howar(Jesse, 2000). Some sites of the Neolithic dotted wavy line ceramics have been dated as Middle Holocene. Radiocarbon dates obtained from Neolithic level in the Shaqadud Neolithic site provided ages of 6880 ± 180 BP (55-62 cm below datum) and 6430 ± 180 BP (85-90 cm below datum;[START_REF] Marks | The Wavy Line and the Dotted Wavy Line Pottery in the Prehistory of the Central Nile and the Sahara-Sahel Belt[END_REF] Mohammed-Ali, 1991, in Mohammed-Ali, 2003). Radiocarbon dates, taken from bone or snails, place the Early Khartoum type ceramics in Wadi Howar in the 6 th millennium BP (Jesse,

Fig. 3

 3 Fig. 3.7: Location of the potteries and bones collected from the El Ga'ah paleolake shoreline.

Fig. 3

 3 Fig. 3.8: Position of dated samples in the North Dilling 2 section. Mean sedimentation rate for the lower unit is ≈ 0.52 mm/yr.

Fig. 3

 3 Fig. 3.9: Position of dated sample in the North Dilling 1 section.

Fig. 3 .

 3 Fig. 3.10: Position of dated samples in the East Abu Zabad section.

Fig. 3

 3 Fig. 3.12: Section near En Nahud. Two 14 C ages for the Second unit were obtained from other two sites in En Nahud, and north En Nahud (Awlad Zibare village). They yielded ages of 6419 and 9788 cal yr BP, respectively.

Fig. 3 .

 3 Fig. 3.13: Position of dated samples in the East El Obeid section.

Fig. 3

 3 Fig. 3.14: Position of dated samples in the south El Obeid1 section. The average accumulation rate of sediments for the Upper unit is 2.76 mm/yr (5.4 mm/yr between ≈ 1100 and 900 yr ago).

Fig. 3 .

 3 Fig. 3.15: Position of dated samples in the El Ga'a well. The accumulation rate for lower part of the section is ≈ 2.4 mm/yr, and ≈ 0.47 mm/yr for the upper part.

Fig. 3 .

 3 Fig. 3.16: Time-correlation transect between El Fula and Dilling.

Fig. 3 .

 3

Fig. 3 .

 3 Fig. 3.19: Mottled facies examples. (A) Mottled horizon from east El Obeid section: the lower part is greenish to grey, rich in organic matter, while in the reddish brown upper part, organic matter is scarcer or oxydized. (B) South El Obeid3 section (Khour Es-Sonjokaya): the mottled facies crops out below the white line, the dark lower part is richer in preserved organic matter than the upper part. (C) El-Fula area: pale grey mottled facies rich in calcareous nodules. (D) East Abu Zabad section: thick unit of mottled sand sediments (below the solid line). The dashed line separates a lower part containing calcareous nodules, from an upper part devoid of calcareous nodules.

Fig. 3 .

 3 Fig. 3.20: Nodular calcrete or calcareous nodules. (A) Numerous nodules formed in flood plain facies -Dilling area; (B) Rare nodules formed in mottled, fine sand sediments facies -East Abu Zabad; (C) Numerous calcareous nodules formed in mottled fine sand sediments -El Fula; (D) Moderately abundant nodules in mottled, medium to coarse sand sediments -North Bara.

Fig. 3 .

 3 Fig. 3.22: Tubular calcrete. A and B : East El Khowei; C : North Bara.

Fig. 3 .

 3 Fig. 3.23: Examples of conglomerates. (A) Conglomerate exclusively made of iron concretion, East of El Fula. (B) Conglomerate made of iron concretion with subordinate reworked calcareous nodules and poorly sorted lithoclasts, East of Abu-Zabad. (C) Conglomerate with abundant quartz and calcareous nodules, Dilling area. (D) Conglomerate with iron concretions and minor quartz clasts, El Khowei.

Fig. 3 .

 3 Fig. 3.24: Topographic profile from southeast En Nahud to east El Fula. The topographic profile passes across an iron-rich outcrop of Nubian sandstone in the North to the iron-rich conglomerate of the El Fula section to the South.

Fig. 3 .

 3 Fig. 3.25: Moderate regime channel facies. (A) Planar cross bedded, poorly sorted sand with some lithoclasts mostly concentrated in the lower part, Dilling area. (B) Coarse, planar cross-bedded sand in the upper part, laminated medium sand in the middle part, medium sand in the lower part, and few lithoclasts in the upper part, Dilling area. (C) Alternating coarse-and medium-grained, trough crossbedded sand beds, East Abu-Zabad. The coarse sand is poorly sorted and contains gravels (lithoclasts).

( D )

 D Fig. 3.26: Lens-shaped bed, medium-grained sand sediments.

Fig. 3 .

 3 Fig. 3.27: Flood plains Facies. (A) South El-Obeid3 (Es-Sonjokaya): alternating silt and rippled fine sand. (B) Dilling: sandy shale, rippled in the upper part and laminated in the lower part. (C) South El-Obeid2: laminated fine sand and shale. (D) South El-Obeid1 (Khour Kazgail): well sorted, medium rippled sand, planar laminae and planar cross bedding in the upper part; well sorted coarse sand sediments with low degree of planar cross bedding in the lower part.

Fig. 3 .

 3 Fig. 3.30: Red clayey sand facies. (A) Red clayey fine sand facies (above the white line, South El-Obeid2 (Kazgail)). (B) Red clayey fine sand sediments facies, the upper half part is laminated (El Khowei). (C) Red clayey fine sand sediments facies (above the white line), eroded by the overlying laminated fine sand (El-Fula). (D) Medium to fine, red clayey sand sediments (above the white line, North Bara).

Fig. 3 .

 3 Fig. 3.32: Palustrine facies reworked in channel deposits.

Fig. 3 .

 3 Fig. 3.33: Carbonate facies. (A) Outcrop of diatomite limestone formed in a small lake (Sodari). (B) Detail of photograph A, showing deep karstifications infilled with argillaceous sand sediments.

Fig. 3 .

 3 Fig. 3.34: Argillaceous sand sediments facies. (A) Yellow argillaceous sand (below the dashed line) (North Bara). (B) Grey argillaceous sand (below the dashed line), marked by well-preserved gastropod shells in the upper part (Sodari).

Fig. 3 .

 3 Fig. 3.35: View of the clinoform bearing coarse clastic facies in El Ga'ah.

Fig. 3 .

 3 Fig. 3.36: Details of the delta facies. (A) Gastropod shells. (B) Gastropod shells horizon between the two yellow lines, very coarse sand above the upper yellow line, and gravelly bed at the lower part of the image. (C) Root molds concentration zone.

Fig. 3 .

 3 Fig. 3.37: Lithology of the studied sections and location of the samples analyzed for grain size analysis.

Fig. 3 .

 3 Fig. 3.39: NE-SW topographic profile showing the gradient in the study area (NW is on the left).

Fig. 3 .

 3 Fig. 3.40: Cumulative grain size distribution zones of all sample's curves.

Fig. 3 .

 3 Fig. 3.41: Representative frequency curves of the studied sections. A, B: Symmetrical curves, curve B can be classified as near-symmetrical. C, D: Fine skewed curves. E, F: Coarse skewed curves.

Fig. 3 .

 3 Fig. 3.42: Graphical statistic plots against depths for the north Bara3 section.

Fig. 3 .

 3 Fig. 3.43: Graphical statistic plots against depths for the north Bara1 section.

Fig. 3 .

 3 Fig. 3.44: Graphical statistic plots against depths for the west El Obeid section.

  to 0.19 mm (2.65 -2.54 Φ), with average value of 0.18 mm (2.58 Φ). All samples are characterized by very fine sand mean value. The standard deviation ranges between 0.80 and 0.87 Φ (average = 0.85 Φ), and all samples are moderately well sorted. The skewness values range from 0.09 and 0.2 Φ (average = 0.16 Φ) and all samples are classified as fine skewed. The kurtosis values range from 1.57 to 1.89 Φ (average = 1.72 Φ) and all samples fall within the very leptokurtic zone (Fig. 3.45).

Fig. 3 .

 3 Fig. 3.45: Graphical statistic plots with depth for the east Abu Zabad section.

Fig. 3 .

 3 Fig. 3.46: Representative Cumulative Probability Curves for the whole area. (A) Curves for the lower part of the mottled facies; (B) Curves for the upper part of the mottled facies; (C) Curves for the red sand sediments facies.

Fig. 3 .

 3 Fig. 3.48: Grain-size distribution diagram for all samples from all sections.

Fig. 3 .

 3 Fig. 3.49: Vertical evolution of grain size percentages and their interpretation. (A) North Bara3 section; (B) North Bara1 section; (C) West El Obeid section; (D) East Abu Zabad section.

Fig. 3 .

 3 Fig.3.51: Curves of pre-and post-hiatus graphical statistic plots for all sections. The curves for both parts have been homogenized according to the thickest sections (North Bara 1 for the post-discontinuity, East Abu Zabad for the pre-discontinuity). For more details of each section refer toFigures 3.42 to 3.45.

Fig. 3 .

 3 Fig. 3.52: Plot of mean grain size vs sorting of the studied samples indicating the wind energy.

Fig. 3 .

 3 Fig. 3.53: Plot of mean grain size vs skewness of the studied samples indicating the transportation energy.

Fig. 3 .

 3 Fig. 3.54: Plot of Skewness vs Sorting of the studied samples indicating the transportation energy.

Fig. 3 . 55 :

 355 Fig. 3.55: Evolution of transport processes in the North Bara3 section. (A) expressed in percentages; (B) expressed in grain size.

Fig. 3 .

 3 Fig. 3.57: Evolution of transport processes of the West El Obeid section. Expressed (A) in percentages; (B) in terms of grain size.

Fig. 3 .

 3 Fig. 3.58: Evolution of transport processes in the East Abu Zabad section. Expressed (A) in percentages; (B) in terms of grain size.

Fig. 3 .

 3 Fig. 3.59: North-South evolution of the transport processes for the mottled facies. Expressed (A) in percentages; (B) in terms of grain size.

Fig. 3 .

 3 Fig. 3.60: North-South evolution of the transport processes red sand sediments facies. Expressed (A) in percentages; (B) in terms of grain size.

Fig. 3 .

 3 Fig. 3.61: Mean grain-size distance relation, illustrating the variation in transport energy.

Fig. 3 .

 3 Fig. 3.63: Sedimentary evolution of the Dilling area.

Fig. 3 . 64 :

 364 Fig. 3.64: Sedimentary evolution of the Abu Zabad -El Fula area. Same caption as Fig. 3.63.The upper part of the succession (Fourth stratigraphic unit) is dominated, either by aeolian, non pedogenetized, red fine sand sediments, or by fluviatile alternation of fine to medium sand, as indicated by current ripples and cross beddings. This upper part of the succession is interpreted as deposited by aeolian activity suggesting a dry climate, and was locally reworked by rivers indicating the occurrence of significant, although possibly rare, rainfalls, and thus suggesting a semi-arid climate.

Fig. 3 .

 3 Fig. 3.65: Sedimentary evolution of the El Obeid -South El Obeid area. Same caption as Fig. 3.63.

Fig. 3 . 66 :

 366 Fig. 3.66: Sedimentary evolution of the En Nahud -El Khowei area. Same caption as Fig. 3.63.

Fig. 3 .

 3 Fig. 3.67: Sedimentary evolution of the Sodari -North Bara area. Same caption as Fig. 3.63.

Fig. 3 . 68 :

 368 Fig. 3.68: Paleogeographic map for the period 13000 -10000 yr BP.

Fig. 3 .

 3 Fig. 3.69: Paleogeographic map for the 10000 -6000 yr BP period

Fig. 3 .

 3 Fig. 3.70: Progradation of deltaic sediments in the El Ga'a lake. (A and B) Location of the lakes and delta sediments. (C) Distal photograph showing the clinoforms of the delta sediments toward the lake. (D) Detail of the delta progradation (yellow lines). This delta overlies lake sediments (white color).

Fig. 3 .

 3 Fig. 3.71: Paleogeographic map for the 6000 -3000 yr. BP period

Fig. 3 .

 3 Fig. 3.73: Erosional surface separating the palustrine facies (Second unit) from the upper aeolian facies (Fourth unit) in En-Nahud (A) and El Khowei (B).

Fig. 3 . 74 :

 374 Fig. 3.74: Erosional surfaces. (A) Dilling area. (B) South El Obeid area (Khour Es Sonjokaya).

Fig. 3 . 76 :

 376 Fig. 3.76: Paleogeographic map for the period 1000 yr BP to Present

Fig. 4 . 1 :

 41 Fig. 4.1: Geographic distribution of the studied gastropod shells

Fig. 4

 4 Fig. 4.2: Quantitative analysis of the gastropod shells in the En Nahud section.

Fig. 4

 4 Fig. 4.3: Quantitative analysis of the gastropod shells in the East El Khowei section

Fig. 4

 4 Fig. 4.4: Evolution of the gastropod assemblages in the En Nahud section.

Fig. 4

 4 Fig. 4.5: Evolution of the gastropod assemblages in the East El Khowei section.

Fig. 4

 4 Fig. 4.6: Location map of the sites investigated for palynology.

1 :

 1 Plate 4.2: Representative pollens of freshwater algae, Aquatic Group indicator. Scale bar = 20 µm. Chomotriletes rubinus, 2: Chomotriletes circulus, 3: Chomotriletes minor, 4: Chlorosarcina superba, 5, 6 and 7: Ovoidites sp., 8: Ovoidites vangeelii, 9: Schizosporis reticulatus, 10: Lecaniella forma, 11: Spirogyra (zygospore), 12: Sigmopollis laevigatoides.

  Plate 4.3: Representative pollens of the aquatic flora -Aquatic Group indicator. Scale bar = 20 µm. 1&2: Alternanthera nodiflora 3: Alternanthera sessilis 4: Potamogeton pectinatus 5: Lemna gibba 6: Inaperturopolenites dubius 7: Inaperturopollenites hiatus 8: Limnophyton obtusifolium 9: Echinochloa crus-galli.

1 :

 1 Plate 4.4: Representative pollens of the Tropical Group indicator. Scale bar = 20 µm. Proxapertites cursus, 2: Proxapertites operculatus, 3: Laevigatosporites haardtii, 4: Laevigatosporites ovatus, 5: Lygodiumsporites, 6: Longapertites marginatus, 7: Araucariacites australis, 8 and 9: Araucariacites sp., 10: Aneilema johnstonii, 11: Plantago major, 12: Tiliacora funifera.

Fig. 4

 4 Fig. 4.7: Proportions of the pollen groups in the North Dilling section.

Fig. 4

 4 Fig. 4.8: Proportion of pollen groups in the South El Obeid 3 section.

Fig. 4 .

 4 Fig. 4.10: Interpretation of the pollen groups for the North Dilling site.

Fig. 4 .

 4 Fig. 4.11: Interpretation of the pollen groups for South El Obeid 3 site.

Fig. 4 .

 4 Fig. 4.12: Interpretation of the pollen groups for the South El Obeid 1 site.

Fig. 4 .

 4 Fig. 4.13: Percentages of the pollen groups at ≈ y BP.

Fig. 4 .

 4 Fig. 4.17: Evolution of the oxygen and carbon isotopes in the En Nahud 2 section.

Fig. 4 .

 4 Fig. 4.18: Interpretation of the oxygen and carbon isotopes -section En Nahud1.

Fig. 4 .

 4 Fig. 4.19: Interpretation of the oxygen and carbon isotopes in the En Nahud 2 section.

Fig. 4 .

 4 Fig. 4.21: Evolution of the oxygen and carbon isotopes in the East El Khowei section.

Fig. 4 .

 4 Fig. 4.23: Evolution of the oxygen and carbon isotopes in the West El Obeid section.

Fig. 4 .

 4 Fig. 4.23: Interpretation of the oxygen and carbon isotopes -West El Obeid section.

Fig. 4 .

 4 Fig. 4.24: Evolution of the oxygen and carbon isotopes in the North Dilling section.

Fig. 4 .

 4 Fig. 4.25: Interpretation of the oxygen and carbon isotopes -North Dilling section.

Fig. 4 .

 4 Fig. 4.26: Plot of the climofunctions in the West El Obeid section.

Fig. 4 .

 4 Fig. 4.27: Plot of the climofunctions in the North Bara3 section.CIA values ranging from 95 to 100 indicate intensive chemical weathering, whereas CIA values lower than 50 indicate the near absence of chemical weathering and suggest cool and arid conditions(Fedo et al., 1995). Depending on the parent material chemistry, CIA changes during weathering may be quite large or relatively small. Parent materials that have already been cycled as sediments, or which are clay-rich in the first place, may start out with CIA values of 60-70, with weathering leading to even higher values[START_REF] Sheldon | Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols[END_REF]. Therefore,

Fig. 4 .

 4 Fig. 4.29: Interpretation of the climofunctions in the West El Obeid section.

Fig. 4 .

 4 Fig. 4.30: Interpretation of the climofunctions in the North Bara3 section.

Fig. 4 .

 4 Fig. 4.31: Clay mineralogy in the East El Obeid section: results and interpretation.

Fig. 4 .

 4 Fig. 4.32: Location of the paleolakes in the northern area (white in this Google image). Note the N-S trending dunes, which dam the course of rivers or delineate small depressions.

Fig. 4 .

 4 Fig. 4.33: Profile showing the maximum level of the El Ga'ah paleolake.

Fig. 4 .

 4 Fig. 4.34: Location of swamps in the central area.

Fig. 5. 1 :

 1 Fig. 5.1: The northern limit of the rainy belt for different intervals during the African humid period.The positions of the northern limit of the rainy bilt are based on ages of lacustrine lake sediments of El Ga'ah, Wadi Howar, West Nubia, and Selima Lake. Remark the gradual northward enlargement of the rainy belt. The lower and upper numbers in each lake represent the beginning and the end of AHP, respectively. The estimated mean annual precipitation (MAP) and mean annual temperature (MAT) for the AHP are based on Climofunctions of major elements geochemistry.

Fig. 5 . 2 :

 52 Fig. 5.2: Profile of Eastern Sahara, showing the reconstructed hydrological situation during the AHP and the present water table.

Fig. 5

 5 Fig. 5.3: Mean grain size and SiO 2 /Al 2 O 3 plots for the North Bara3 section.

Fig. 5

 5 Fig. 5.4: Mean grain size and SiO 2 /Al 2 O 3 plots for the West El Obeid section.

Figure 6 . 1 :

 61 Figure 6.1: Synthesis of the sedimentary and climatic evolution of the study area since 14 ky BP.

Table 3

 3

	.1	Geographical information of the studied sections
	Table 3.2	Carbon dating results
	Table 3.3	Summary of the sedimentary facies
	Table 3.4	Graphic measure of phi unit from curves of the analyzed samples
	Table 3.5	Abbreviation of the graphical statistic descriptions
	Table 3.6	Calculated graphical statistics and their descriptions for all
		samples
	Table 3.7	Brief summary of truncation points
	Table 3.8

Table 3 .

 3 11 horizontal grain size distributional percentage for the lower part of the mottled facies

	Table 4.1	Relative abundances of gastropod sub-fossil shells in the study	116
		area
	Table 4.2	Counting of the identified palynomorphs	126
	Table 4.3	XRF analysis results	150
	Table 4.4	Clay mineralogy results	154

  Among the rocks underlying Kordofan, these authors distinguished the basement complex of Precambrian age, the Nawa Series of Late Paleozoic age, the Nubian Series of Mesozoic age (recently renamed Cretaceous sediments), laterite of Early to Middle Paleogene/Neogene age, the Um Ruwaba Series of Pliocene to Pleistocene age, and surficial deposits of Quaternary age.

.1.2. Procedures of AMS 14 C dating used in the Poznań Radiocarbon Laboratory

	at the Poznań Radiocarbon Laboratory,
	Poland, by Dr. Tomasz Goslar, and (2) at the Laboratoire des Sciences du Climat et de
	l'Environnement (LSCE), in Gif sur Yvette, France, by Dr. Jean Pascal Dumoulin, through the
	Institut de Recherche pour le Développement (IRD).
	2.4Procedure of 14 C dating with the AMS technique, consists of a few stages: chemical pre-
	treatment, production of CO2 and graphitisation, AMS 14 C measurement, calculation of 14 C age
	and calibration of 14 C age.
	2.4

.1.2.1. Chemical pre-treatment

	humic acids dissolved in NaOH).
	Methods of chemical pre-treatment generally follow those used in the Oxford Radiocarbon
	Accelerator Unit, as described by Brock et al. (2010, Radiocarbon, 52, 102-112).
	-Samples of charcoal, wood or other plant remains (after mechanical removal of macroscopic
	contamination visible under binocular) are treated with 1M HCl (80°C, 20+ min), 0.025-0.2M
	NaOH (room temperature for fragile plant remains, 80°C for wood) and then 0.25M HCl (80°C,
	1 hour). After treatment with each reagent, the samples are rinsed with deionised water
	(Millipore) until pH = 7. For the first HCl treatment, longer time (20+) is applied, if emanation
	of gas bubbles from sample is still visible. The step of NaOH treatment is repeated a few times,
	generally until no more coloration of the NaOH solution appears (coloration of solution is
	caused by humic acids dissolved in NaOH), but the NaOH treatment is interrupted if there is a
	danger of complete dissolution of the sample.
	-Samples of sediments are usually treated with 1M HCl (room temperature overnight and then
	80°C, 1+ hour), 0.2M NaOH (80°C, 10 min) and then 0.25M HCl (80°C, 1 hour). After
	treatment with each reagent, the samples are rinsed with deionised water (Millipore) until
	pH = 7. For the first HCl treatment, longer time (1+) is applied, if emanation of gas bubbles
	from a sample is still visible. The step of NaOH treatment is repeated a few times, generally
	until no more coloration of the NaOH solution appears (coloration of solution is caused by

291. 2.4.1.2.3. Accelerator mass spectrometry (AMS) 14 C measurement Measurements

  described in this point, are performed in the AMS 14 C Laboratory of the A. Mickiewicz University in Poznań. Cooperation between the Poznań Radiocarbon Laboratory and the AMS 14 C Laboratory is regulated by the Agreement between Foundation of the A. Mickiewicz University and the A. Mickiewicz University.

1.2.4. Calculation of 14 C age

	Conventional 14 C age is calculated using correction for isotopic fractionation (according to
	Stuiver, Polach 1977, Radiocarbon 19, 355), basing on ratio 13 C/ 12 C measured in the AMS
	spectrometer simultaneously with the ratio 14 C/ 12 C (note: the measured values of  13 C depend
	on isotopic fractionation during CO2 reduction and isotopic fractionation inside the AMS
	spectrometer, and as such, they cannot be compared with values of  13 C determined with
	conventional mass spectrometers on gas samples). Uncertainty of calculated 14 C age is
	determined using uncertainty implied from counting statistics, and also spread (standard
	deviation) of partial 14 C/ 12 C results, whichever is bigger. Uncertainties of 14 C/ 12 C ratios
	measured on standard samples are additionally taken into account. The 1-sigma uncertainty of
	conventional 14 C age given in our reports, is the best estimate of the total uncertainty of
	measurement.
	2.4

.1.3. Procedures of AMS 14 C dating used in the Laboratoire des Sciences du Climat et de l'Environnement -Plateforme Nationale LMC14 2.4.1.3.1. Chemistry of organic matter

	Samples marked A are treated in an excess of 0.5N hydrochloric acid for several hours at 80°C
	to remove carbonates and then rinsed with ultrapure water to neutral pH.

12942 yr BP).

	Fig. 2.4. OxCal calibration curve showing two probability ranges -sample from Dilling. The sample
	code is Del 2-1.

Fig. 2.3. OxCal calibration curve showing one probability range -sample from East Abu Zabd. The sample code is Dab 1. 14 C date is 10960 ± 100 yr BP. The curve indicates only one probability (95.4%) with calendar age ranging between 11093 and 10756 yr BC. The average of this range is 10925 yr BC (equivalent to 14 C date is 595 ± 30 yr BP. The curve shows two probabilities (69.7 % and 25.7 %) with calendar age ranges of 1298 -1371 and 1379 -1410 yr AD. The average of these two ranges is calculated from the small value of the first probability (1298) and the big value of the second probability (1410) it is 1354 yr AD (equivalent to

663 yr BP).

	Various methods and programs have been developed for the calibration of radiocarbon dates,
	of which the most important are: the Dendrochronological calibration method, INTCAL
	calibration program, and OxCal calibration program.
	The online OxCal calibration program has been used in this study. The program is always
	developing through time, the present version of this program is 4.3 and works with IntCal13.
	Both, version 4.3 and the old one 4.2 have been used during this study. A variety of different

chronological models can be included in the analysis. The information for such analysis can be entered using the user interface or in the form of text command files. The calibration output is usually given in term of probability. The accuracy of this program have maximum probability of 95.4% which may be represented in one probability or more. Each probability expresses the calibrated 14 C age in term of calendar age range either "anno Domini" AD or "before Christ" BC then the average of this range will consider the age of the sample. In turn, this average age can be converted into BP "before present" calendar age. Figures 2.3 and 2.4 show two calibrated ages for two samples among the analyzed samples for this study.

Table 3 . 1 :

 31 Geographical information on the studied sections.

	Section	Longitude /	Latitude /	Elevation / m asl
		deg.min.sec.	deg.min.sec.
	Dilling	29 39 22.5	12 02 23
	North Dilling1	29 39 26.2	12 05 0.9
	North Dilling2	29 39 46.5	12 05 3.96
	El Fula	28 22 55	11 43 58
	East El Fula	28 22 48.7	11 44 1.7
	West El Fula	28 17 52.3	11 41 06
	East Abu Zabad1	29 28 16	12 20 49
	East Abu Zabad2	29 24 16	12 21 11
	East Abu Zabad3	29 28 47	12 20 59
	East El Obeid1	30 17 43.4	13 12 55.8
	East El Obeid2	30 18 21.4	13 12 06.1
	West El Obeid	29 59 59.7	13 10 3.7
	South El Obeid1	30 08 31.2	12 56 44.4
	South El Obeid2	30 04 47.7	12 48 13.3
	South El Obeid3	29 45 45.4	12 19 24.4
	East El Khowei	29 42 10.7	13 06 35.8
	El Khowei	29 12 53.2	13 03 20.8
	West El Khowei	28 54 15.9	12 55 34
	East En Nahud	28 43 25.1	12 49 49
	North Bara1	30 25 55.3	13 51 58.5
	North Bara2	30 28 38.7	14 04 36.1
	North Bara3	30 45 13.1	14 30 48.3
	El Ga'ah well	29 59 12.6	14 14 34

Table 3 .2: Carbon dating results

 3

	Lab Cod	Laboratory Sample name	Section	Depth cm	material analyzed	BP Age 14 C	±BP Error	(AD or BC) Age cal yr	yr BP Age cal	Remark
	49432		NDel 16-17	North Dilling2	375	paleosols Organic	4055	30	2853 BC
	49433	LSCE	NDel 16-18	North Dilling2	337	paleosols Organic	3720	30	2116 BC
	49434		NDel 16-20	North Dilling2	135	paleosols Organic	740	30	1258 AD
	50046		NDel 16-2	North Dilling1	250	Charcoal	6750	110	5665 BC
	Poz-79741 Poznan	Del 1-2	Dilling	40	Charcoal	165	30	1771 AD
	Poz-79743		Del 2-1	Dilling	185	Charcoal	595	30	1354 AD
	50047		Ga 6-1	El-Ga'ah	360	Paleosol Organic	9195	45	8521 BC
	49435	LSCE	Ga 6-3	El-Ga'ah	290	Paleosol Organic	9040	40	8226 BC
	49436		Ga 6-7	El-Ga'ah	85	Paleosol Organic	5095	30	3924 BC
	Poz-79737		Kh 1-1	South El Obeid1 185	Charcoal	910	30	1112 AD
	Poz-79739 Poz-81611	Poznan	Kh 1-2 En Nahud	South El Obeid1 280 En Nahud *	Charcoal Limestone	1120 5565	30 35	929 AD 4402 BC
	Poz-81229		AWZ1-4	En Nahud	*	Limestone	8680	80	7771 BC
	49437	LSCE	Sonj 9	Khour Es Sonjokaya	340	paleosol Organic	3140	30	1421 BC
	Poz-79736		KT 1-1	East Elobeid1	220	paleosol Organic	3050	35	1314 BC
	Poz-0	Poznan	KT 4-2	East El Obeid	65	bone	>0				not suitable (<0,1%N 2.5%C)
	Poz-79740		Ka 1-1	South El Obeid3 30	Charcoal	132.87				Modern (0.35 pMC)
	50044	LSCE	Dod 1	East El Khowei 440	paleosol Organic	6960	50	5951 BC

Table 3 .3: Summary of the sedimentary facies.

 3

Table 3 .4: Graphic measure of phi unit from curves of the analyzed samples.

 3

	Sample	Φ 5	Φ 16 Φ 25 Φ 50	Φ 75	Φ 84	Φ 95
	N.Bara 3-6	-0,58	0,03	0,42	1,43	2,47	2,74	3,32
	N.Bara 3-7	-0,38	0,12	0,42	1,45	2,47	2,74	3,25
	N.Bara 3-8	-0,38	0,07	0,36	1,00	1,79	2,00	2,93
	N.Bara 3-9	-0,49	-0,14	0,12	1,00	2,12	2,64	3,18
	N.Bara 3-10	-0,38	0,01	0,25	1,29	2,40	2,72	3,18
	N.Bara 3-11	-0,38	-0,14	0,23	1,56	2,51	2,74	3,32
	N.Bara 3-12	-0,32	0,06	0,47	1,84	2,74	3,00	3,18
	N.Bara 3-13	-0,32	0,06	0,45	1,67	2,51	2,84	3,64
	N.Bara 3-14	-0,26	0,34	0,89	2,00	2,74	2,95	3,64
	N.Bara 3-16	0,29	0,97	1,43	2,22	2,71	2,89	3,44
	N.Bara 3-17	0,25	0,84	1,36	2,18	2,64	2,90	3,40
	N.Bara 3-18	0,20	0,79	1,25	1,89	2,66	2,86	3,46
	N.Bara 3-19	0,15	0,59	0,97	1,84	2,64	2,84	3,56
	N.Bara 3-20	0,10	0,56	0,94	1,89	2,64	2,89	3,57
	N.Bara 3-21	0,07	0,49	0,89	1,72	2,69	2,94	3,61
	N.Bara 1-2	0,74	1,15	1,38	2,05	2,47	2,74	3,40
	N.Bara 1-3	0,68	1,15	1,45	2,01	2,32	2,65	3,32
	N.Bara 1-4	0,81	1,36	1,69	2,25	2,66	2,82	3,35
	N.Bara 1-5	0,94	1,60	1,84	2,36	2,72	2,88	3,56
	N.Bara 1-6	0,64	1,36	1,74	2,27	2,64	2,76	3,18
	N.Bara 1-7	0,64	1,36	1,74	2,28	2,60	2,74	3,18
	N.Bara 1-8	0,67	1,40	1,76	2,28	2,64	2,78	3,18
	N.Bara 1-9	0,62	1,34	1,69	2,25	2,64	2,93	3,40
	N.Bara 1-10	0,97	1,43	1,74	2,25	2,69	2,84	3,44
	N.Bara 1-11	0,89	1,40	1,64	2,32	2,70	2,94	3,80
	N.Bara 1-13	0,86	1,32	1,60	2,22	2,74	3,01	4,51
	N.Bara 1-14	0,84	1,43	1,69	2,25	2,78	2,89	3,99
	N.Bara 1-15	1,00	1,47	1,81	2,32	2,77	3,01	3,90
	N.Bara 1-16	0,97	1,47	1,84	2,25	2,80	3,02	3,94
	N.Bara 1-17	0,69	1,15	1,43	2,01	2,56	2,92	3,74
	Ay 1	1,27	1,74	2,00	2,47	2,84	3,06	4,32
	Ay 2	1,15	1,69	1,94	2,32	2,76	2,99	4,21
	Ay 3	1,15	1,64	1,84	2,22	2,58	2,77	3,06
	Ay 4	1,22	1,74	2,00	2,32	2,75	2,94	3,84
	Ay 5	1,40	1,89	2,06	2,40	2,78	2,98	3,84
	Ay 6	1,36	1,89	2,08	2,47	2,84	3,00	4,06
	Ay 7	1,29	1,74	2,00	2,32	2,73	2,87	3,64
	Ay 8	1,15	1,71	1,89	2,29	2,65	2,81	3,32
	Ay 9	1,22	1,74	2,02	2,40	2,75	2,92	3,59
	Dab 1	1,32	1,84	2,18	2,51	2,94	3,40	4,47
	Dab 2	1,36	1,84	2,12	2,60	2,88	3,37	4,41
	Dab 3	1,43	1,89	2,25	2,56	2,89	3,32	4,35

Table 3 .5: Abbreviation of the graphical statistic descriptions.

 3

	Standard deviation		Skewness	Kurtosis
	Description		Abbreviation	Description	Abbreviation Description	Abbreviation
	Very well sorted	VWs	Very	fine	Vfsk	Very	Vpk
				skewed			platykurtic
	Well sorted		Ws	Fine skewed	Fsk	Platykurtic	Pk
	Moderately sorted	Ms	Symmetrical	Sy	Mesokurtic	Mk
	Poorly sorted	Ps	Coarse skewed	Csk	Leptokurtic	Lk
	Very	poorly	VPs	Very	coarse	Vcsk	Very	Vlk
	sorted			skewed			leptokurtic
	Extremely poorly	ExPs				Extremely	Exlk
	sorted						leptokurtic

Table 3 .6: Calculated graphical statistics and their descriptions for all samples.

 3

	Sample	Mean	Standard de deviation	Skewness	Kurtosis
			Value / phi	Description Value / phi	Description Value / phi	Description
	N.Bara 3-14	1,76	1,25	Ps	-0,21	Csk	0,87	Pk
	N.Bara 3-13	1,52	1,30	Ps	-0,08	Sy	0,79	Pk
	N.Bara 3-12	1,63	1,27	Ps	-0,22	Csk	0,64	Vpk
	N.Bara 3-11	1,39	1,28	Ps	-0,11	Csk	0,67	Pk
	N.Bara 3-10	1,34	1,22	Ps	0,06	Sy	0,68	Pk
	N.Bara 3-9	1,17	1,25	Ps	0,19	Fsk	0,75	Pk
	N.Bara 3-8	1,02	0,98	Ms	0,10	Sy	0,95	Pk
	N.Bara 3-7	1,44	1,20	Ps	-0,01	Sy	0,72	Pk
	N.Bara 3-6	1,40	1,27	Ps	-0,04	Sy	0,72	Pk
	N.Bara 3-21	1,72	1,15	Ps	0,04	Sy	0,80	Pk
	N.Bara 3-20	1,78	1,11	Ps	-0,09	Sy	0,84	Pk
	N.Bara 3-19	1,75	1,08	Ps	-0,05	Sy	0,83	Pk
	N.Bara 3-18	1,84	1,01	Ps	-0,05	Sy	0,95	Mk
	N.Bara 3-17	1,97	0,99	Ms	-0,27	Csk	1,00	Mk
	N.Bara 3-16	2,03	0,96	Ms	-0,26	Csk	1,02	Mk
	N.Bara 1-11	2,22	0,83	Ms	-0,057	Sy	0,99	Lk
	N.Bara 1-10	2,17	0,72	Ms	-0,071	Sy	1,24	Mk
	N.Bara 1-9	2,17	0,82	Ms	-0,180	Csk	1,07	Lk
	N.Bara 1-8	2,15	0,73	Ms	-0,135	Csk	1,22	Lk
	N.Bara 1-7	2,12	0,73	Ms	-0,287	Vcsk	1,15	Lk
	N.Bara 1-6	2,13	0,73	Ms	-0,312	Csk	1,21	Lk
	N.Bara 1-5	2,28	0,72	Ms	-0,281	Csk	1,17	Lk
	N.Bara 1-4	2,14	0,75	Ms	-0,160	Csk	1,19	Mk

Table 3 .7: Brief summary of truncation points.

 3

				Truncation points in mm
	Site	Coarse traction TP	Traction-saltation TP	Saltation-suspension TP
		max	min	max	min	max	min
	North Bara3	2.378	1.149	1.366	0.406	0.117	0.058
	North Bara1	1.516	0.707	0.574	0.42	0.131	0.095
	West El Obeid	0.93	0.841	0.536	0.354	0.125	0.108
	East Abu Zabad	1.072	0.707	0.582	0.435	0.121	0.096

Table 3 .8. Summary of the grain-size percentage for all sections.

 3

	Grain size		Location or section
		North Bara3	North Bara1 West El Obeid East Abu Zabad
	Fine gravel%	1 -2	-	-	-
	very coarse sand %	4 -20	1 -3	-	-
	Coarse sand %	11 -37	2.5 -7	1.5 -3	2 -4
	Medium sand%	20 -30	31 -50	17 -27	14 -23
	Fine sand %	15 -45	26 -56	56 -60	51 -65
	Very fine sand %	3 -17	8 -17	12 -16	17 -20
	Silt %	0.5 -4	1 -6	1.5 -5	5 -7

Table 3 .9: Grain size percentage for the red sand sediments facies.

 3

			Location or section
	Grain size	North Bara3	North Bara1 West El Obeid	East Abu
					Zabad
	Fine gravel %	-	-	-	-
	very coarse sand %	14	-	-	-
	Coarse sand %	20	6	4	3
	Medium sand %	18	35	26	26
	Fine sand %	28	46	56	50
	Very fine sand %	17	11	12.5	15
	Silt %	3	6	1.5	6

Table 3 .10: Horizontal grain size distributional percentage for the upper part of the mottled facies.

 3

			Location or section
	Grain size	North Bara3	North Bara1 West El Obeid	East Abu
					Zabad
	Fine gravel %	1.5	-	-	-
	very coarse sand %	9.5	-	-	-
	Coarse sand %	40	6	4	3
	Medium sand %	19	35	16	17
	Fine sand %	20	42	60	59
	Very fine sand %	9	14	16.5	16
	Silt %	1	3	3.5	5

Table 3 .11: Horizontal grain size distributional percentage for the lower part of the mottled facies.

 3

			Location or section
	Grain size	North Bara3	North Bara1 West El Obeid	East Abu
					Zabad
	Fine gravel%	-	-	-	-
	Very coarse sand %	7	3	1	-
	Coarse sand %	13	4	3	2
	Medium sand%	30	34	21	18
	Fine sand %	35	42	55	59
	Very fine sand %	13	12.5	14.5	14
	Silt %	2	4.5	5.5	7

Fig. 3.50: NNE-SSW distribution of the grain size percentages and interpretations. (A) Red sand sediments facies; (B) upper part of the mottled facies; (C) lower part of the mottled facies.

Table 4 .

 4 1: Relative abundances of gastropod sub-fossil shells in the study area.

126

  Table 4.2: Counting of the identified palynomorphs.

	Continue (Table 4.2): Continue (Table 4.2): Continue (Table 4.2): Aquatic Chomotriletes minor Chomotriletes circulus Chomotriletes rubinus Ovoidites sp. Ovoidites vangeelii Schizosporis reticulatus Lecaniella forma Spirogyra (zygospore) Sigmopollis laevigatoides Chlorosarcina superba Alternanthera nodiflora Alternanthera sessilis Lemna gibba Inaperturopolenites sp. Inaperturopolenites dubius Laevigatosporites ovatus Araucariacites sp. Araucariacites australis Aneilema johnstonii Tiliacora funifera Borassus aethiopum Palmidites plicatus Lygodiumsporites Plantago major Savanna group Baissea multiflora Graminidites boivinii Farsetia hamihonni Gelasinospora cf. Mimosa oudica Leoisphaeridia sp. Xylia avansii Spinomonosulcites achinatus Leiotriletes adriennis Erica arbora Cephalomappa Acacia sp. Not classified Cola hispida Cicatricosisporites dorogensis Lasiosphon kraussianus Abutilon ramosum Pycnanthus dinklagei Pinuspollenites Pinuspollenites Mauritidites crassibaculatus Polygalaceae cf Commelina africana Chlorophytum floribundum Graminidites sp. Silene burchellii Cymbopogon schoenanthus Monolites alveolatus Maerua oblongifolia Polygala abyssinica Typha domingensis Leoisphaeridia sp. Suaeda sp Kapurdipollenites gemmatus Borreria densiflora Limnobiophyllum sp. Chenopodiaceae	North Dilling D 8 D 7 D 5 D 3 1 7 10 1 2 1 D 11 D 9 4 6 3 4 1 1 5 4 3 2 5 1 5 3 1 5 North Dilling D 11 D 9 D 8 D 7 D 5 D 3 2 2 20 2 6 6 6 5 4 6 7 North Dilling D 11 D 9 D 8 D 7 D 5 D 3 North Dilling D D D D D D 11 9 8 7 5 3 3 6 9 4 2 3 3 5 2	D 2 2 2 1 D 1 12 D 2 D 1 2 D 2 D 1 D D 5 2 1 2 3 3 4 4 1 6 16 3	South El-Obeid 1 Kh 2 Kh Kh 4 Kh 5 4 8 2 3 7 7 1 4 5 3 1 2 3 2 4 2 3 2 3 2 1 1 3 3 7 South El-Obeid Kh 1 Kh 2 Kh 4 Kh 5 1 5 2 South El-Obeid Kh 1 Kh 2 Kh 4 Kh 5 South El-Obeid Kh Kh Kh 1 Kh 2 4 5 1 11 3 3 1 2 1 1 1 19 1 1	Kh 6 1 Kh 6 Kh 6 Kh 6 3 2 3	Sonj 14 15 19 10 12 9 40 Sonj 14 8 5 19 Sonj 14 Sonj 14 5 3 5 16 2	Sonj 12 2 4 1 5 1 9 Sonj 12 3 2 3 6 Sonj 12 Sonj 12 1 5 1	Sonjokaya (El-Obeid Dilling road) Sonj 10 Sonj 8 Sonj 6 Sonj 5 Sonj 4 4 4 1 7 5 2 3 2 2 7 1 1 2 3 6 4 4 10 2 2 31 Sonjokaya (El-Obeid Dilling road) Sonj 10 Sonj 8 Sonj 6 Sonj 5 Sonj 4 2 22 16 4 13 1 6 10 Sonjokaya (El-Obeid Dilling road) Sonj 10 Sonj 8 Sonj 6 Sonj 5 Sonj 4 Sonjokaya (El-Obeid Dilling road) Sonj Sonj Sonj Sonj 10 Sonj 8 6 5 4 9 2 1 3 5 3 6 1 1 2	Sonj 2 5 5 Sonj 2 11 14 11 Sonj 2 Sonj 2 5 9 5 8 12 4 Sonj 1 1 6 3 1 Sonj 1 3 1 Sonj Sonj 1 1 37 7 3 9 3 12
	Potamogeton pectinatus Arid group Pycnanthus dinklagei											4		1	1 2
	Inaperturopollenites hiatus Hyphaene thebaica Piptostigma mayumbense	5				3				2		3		1	5	7	3
	Limnophyton obtusifolium Hyparrhenia hirta Rhynchelytrum repens			6 4 9 2 7				14	1				11	28	8
	Echinochloa crus-galli Classopollis sp. Iodes ovalis Tropical climate indicators Proxapertites assamicus Proxapertites cursus Proxapertites operculatus Retipollenites enigmata Longapertites marginatus Laevigatosporites haardtii Classopollis meyerianus Evolvulus alsinoides Aeschynomene indica Desmodium dichotonum Dandotiaspora plicata Bracharia ramosa Baculatisporites sp. Gynandropsis gynandra Nevesisporites vallatus Farsetia stenoptera Chlorosarcina superba Hibiscus sp.	3 5 3 4	3	3 7	2 6 2 8 22 1 17 10 5 4 1	1 3	1 2 2	1 1	1		7			26 13	3 1 2 1	15 7 7	13 2 35
									127 128 129
					130

  The Araucariaceae sp. recorded the highest occurrence among the tropical group, followed by Proxapertites. The savanna group is dominated by Silene burchellii, Graminidites bombusoidis and Baissea multiflora. The arid group is dominated by Gynandropsis gynandra, Hyparrhenia hirta and Evolvulus alsinoides, with secondary abundance of Desmodium dichotonum and Classopollis spp. (table 4.2).

  The freshwater algae Chomotriletes and Ovoidites dominate the aquatic flora. The tropical group is generally rare and its most abundant representant is Araucariaceae sp. (7 pollens). The savanna group is also rare, except Cymbopogon schoenanthus with 22 counted pollens, while 9 pollens are classified in four different savanna species. The arid group is dominated by Hyparrhenia hirta and Gelasinospora cf. (table 4.2).

  is the most dominant element in both sites, with concentrations between74.42 and 54.31 % in North Bara 3 and between 57.03 and 58.4 % in west El Obeid. Al2O3 is the second most abundant element with values of 15.39 to 12.75 % in North Bara 3, and 15.22 to 13.93 % in west El Obeid. The third element in abundance is Fe2O3 with values of 7.95 to 5.32 % in North Bara 3, and 7.52 to 5.1 % in west El Obeid. CaO follows with concentration of 0.95 to 4.75 % in North Bara and 0.54 to 10.67 % in west El Obeid. TiO2 records the fifth range with values of 2.91 to 4.79 % in North Bara 3, and 1.84 to 2.16 % in west El Obeid.

Table 4 .

 4 3: XRF analysis results.

	Sample	Na 2 O MgO	Al 2 O 3	SiO 2	P 2 O5 SO 3	Cl	K 2 O
	No

CaO TiO 2 Cr 2 O 3 MnO Fe 2 O 3 CIA SiO 2 /Al 2 O 3 CaO/ SiO 2

	N.B3-14 0.27	0.93	14.44	70.54	0.26	0.07 0.05 1.39 0.95	3.69	0.05	0.10	6.20	85.7	4.9	0.01
	N.B3-13 0.16	0.77	14.95	69.92	0.23	0.08 0.05 1.26 0.68	4.27	0.06	0.07	6.31	88.8	4.7	0.01
	N.B3-12 0.10	0.69	14.30	70.67	0.21	0.06 0.04 1.18 0.49	4.79	0.05	0.06	5.85	89.8	4.9	0.01
	N.B3-11 0.31	1.08	12.34	74.42	0.26	0.11 0.06 1.41 1.33	2.70	0.03	0.09	5.32	82.9	6.0	0.02
	N.B3-10 0.09	0.67	14.14	71.47	0.17	0.06 0.05 1.18 0.42	4.34	0.05	0.07	6.00	90.0	5.1	0.01
	N.B3-9	0.10	0.76	14.57	70.82	0.14	0.07 0.06 1.20 0.41	4.34	0.05	0.08	6.02	90.0	4.9	0.01
	N.B3-8	0.11	1.07	15.39	68.74	0.11	0.06 0.09 1.15 1.06	3.59	0.05	0.10	7.61	90.6	4.5	0.02
	N.B3-7	0.14	1.20	14.69	69.74	0.09	0.04 0.05 1.12 1.16	4.08	0.04	0.10	6.52	89.9	4.7	0.02
	N.B3-6	0.15	1.50	15.24	68.43	0.10	0.11 0.10 1.05 1.42	3.59	0.05	0.13	7.22	90.4	4.5	0.02
	N.B3-20 0.11	1.55	12.74	54.31	0.08	0.09 0.04 0.73 1.54	3.54	0.04	0.11	6.21	91.8	4.3	0.36
	N.B3-19 0.14	1.85	14.94	56.89	0.07	0.11 0.05 0.93 11.19 3.10	0.04	0.06	7.95	91.0	3.8	0.20
	N.B3-18 0.13	1.86	15.28	58.95	0.07	0.11 0.02 0.93 11.02 2.91	0.04	0.07	7.93	91.4	3.9	0.19
	N.B3-16 0.11	1.54	13.27	67.22	0.09	0.33 0.04 0.98 4.75	3.60	0.04	0.08	7.09	90.4	5.1	0.07
	Ay9	0.15	0.89	14.06	75.03	0.15	0.42 0.05 1.08 0.54	2.08	0.04	0.07	5.50	89.5	6.4	0.01
	Ay8	0.14	0.95	14.19	74.73	0.12	0.56 0.04 1.09 0.59	2.04	0.03	0.04	5.63	89.6	6.3	0.01
	Ay7	0.15	1.23	15.22	72.53	0.07	0.11 0.05 1.07 0.92	2.16	0.02	0.09	6.05	90.3	5.7	0.01
	Ay6	0.17	1.70	14.81	70.66	0.07	0.04 0.03 1.09 2.03	2.14	0.03	0.13	6.70	89.5	5.7	0.03
	Ay5	0.25	1.65	13.89	72.51	0.08	0.05 0.05 1.21 2.41	2.31	0.03	0.09	5.10	86.6	6.2	0.03
	Ay4	0.25	2.18	14.72	63.42	0.09	0.09 0.03 1.01 8.68	2.03	0.02	0.09	7.02	88.5	5.1	0.14
	Ay3	0.32	2.40	14.89	63.82	0.09	0.10 0.07 1.00 7.25	2.08	0.03	0.08	7.52	87.5	5.1	0.11
	Ay2	0.21	2.12	13.93	58.40	0.10	0.10 0.04 0.88 14.46 1.84	0.02	0.10	7.44	89.4	5.0	0.25
	Ay1	0.29	2.28	14.35	61.35	0.12	0.17 0.07 0.90 10.67 2.06	0.02	0.07	6.85	88.2	5.1	0.17

Table 4 .

 4 4. Results of clay mineralogy analysis.

	Sample		Clay minerals
		Smectite %	Illite %	Kaolinite %
	Tag 3	40.2	21.8	38
	Tag 7	35.7	25.1	39.2
	Tag 10	32.1	28.2	39.7
	Tag 12	42.4	20.5	37.1
	Tag 15	50	19.4	30.4
	Tag 17	47.3	23.8	29
	Tag 20	39.7	31.1	29.3
	Tag 25	61.1	12.3	26.6

ACKNOWLEDGEMENTS

I would like to take this space to express my thanks to all of the people who contributed to the successful completion of my PhD.

 [START_REF] Malek | Distribution of the intermediate hosts of bilharziasis in relation to hydrography. With special reference to the Nile basin and the Sudan[END_REF]and in numerous habitats above the lake level, near the shores of the Lake Victoria, particularly in grassy seepages and irrigation furrows (Magendants, 1972). The latter author reported that Webbe (1962) found specimens of B. sudanica (infected by S. mansoni) in backwaters behind the lakeshore, and regarded them as an important source of S. mansoni transmission.

According to Brown et al. (1984), Williams and Hunter (1968) noted that this snail was commonest where Eichhornia (water hyacinth) is dominant. Biomphalaria found in Sudan prefers rather stable conditions with fairly dense vegetation (Brown et al., 1984). This is supported by our field observations that Biomphalaria is often associated with abundant root traces. (Müller, 1774) Geographic range: The genus Melanoides is restricted to the Old World tropics (Pilsbry andBequaert, 1927, in Kock and Wolmarans, 2009). About 30 species of Melanoides occur in Africa, of which only M. tuberculata is widespread. It is also indigenous to India and the Southeast Asian mainland to northern Australia, and was widespread in the present-day Sahara during the late Pleistocene-Holocene (Kock and Wolmarans, 2009).

Melanoides tuberculata

Habitat: The preferred habitat of M. tuberculata is permanent water bodies (standing or running) including rivers, shallow seepages, man-made habitats (Kock and Wolmarans 2009), lakes, lagoons, canal, khors (Brown et al., 1984), streams and pools (Ndifon and Ukoli, 1989). According to Brown (1994) M. tuberculata is not found in temporary waters. Generally, it prefers moderately shaded habitats (marginal, floating and emergent vegetation) but is also often found in low (25-50%) shade (Ndifon and Ukoli, 1989). In Jonglei (Southern Sudan), its habitat was characterized in some sites by the presence of Eichhornia and Vossia (Brown et al., 1984). In our study area, the association of M. tuberculata with numerous root traces supports the above points.

Concerning its preferred temperature, a study from South Africa reported that the majority of samples were recovered from loci that fell within the mean annual temperature interval from 21˚C to 25°C (Kock and Wolmarans, 2009).

In the course of worldwide monitoring, M. tuberculata has been reported to invade brackish and even marine water bodies of up to 33 ‰ salinity, and under experimental condition, survive extreme high condition of salinity up to 45 ‰ (Bolaji et al., 2011).

Gabbiella senaariensis (Kuster, 1852)

Geographic range: Few studies related to Gabbiella senaariensis are available. Generally, its distribution comprises Egypt, Sudan, Uganda, Central African Republic and northern Nigeria (Brown et al., 1984).

Habitat: G. senaariensis is widespread in a variety of relatively permanent waters. In Sudan and South Sudan, it has been found in lakes, in the Nile and Atem Rivers, irrigation canals, ditches and pools. Eichhornia, Vossia and water lilies were present in some sites (Brown et al., 1984). In Egypt it has been found in the Nile Valley, Nile delta, Lake Nasser and Fayoum (Lotfy and Lotfy, 2015).

Bulinus truncatus (Audouin, 1827)

Geographic range: This species is widely distributed in Africa, but the main areas of distribution are Lower Egypt, Sudan and westwards into Mauritania. It has been found in all oases in northern Africa and is widespread in western and eastern Africa (IUCN, 2012). However, its general distribution is Southwest Asia, Arabia, Mediterranean regions and Portugal, Northwest Africa and from the Nile delta southwards to Angola and Malawi (Brown and Rollinson, 1982; Brown et al., 1984).

Habitat: It can be found in various water bodies, flowing and standing, some of which becoming seasonally dry. It is often found in standing waters, irrigation channels and other man-made structures (IUCN, 2012). In Sudan it has been found in clear or muddy waters, including the Nile and Atem rivers, lakes, lagoons, pools and canals. A preferred niche was amongst the deeply floating roots of Eichhornia (Brown et al., 1984). (Krauss, 1848) or Lymnaea natalensis (Krauss, 1848) Geographic range: Radix natalensis is widespread in tropical Africa, but it is rare in the northeastern coastal area. It is known throughout the Congo in Central Africa, widespread in West, East and Southern Africa (except for the ephemeral aquatic systems in Namibia and Botswana), and from the Western Cape Province (South Africa). In North Africa, it is very common in the Nile River and Nile Delta but has only been reported in northwestern Africa from three localities in Algeria, although it was previously known from many prehistoric localities (8-10,000 years ago; IUCN, 2017) as subfossil shells found in the Sahara (Brown et al., 1984). Outside the mainland of Africa, this species is possibly present in Yemen (Al-Safadi, 1990), Oman (Smythe and Gallagher, 1977;Brown and Wright, 1980;Brown and Gallagher, 1985) and Saudi Arabia (Neubert, 1998), but molecular research needs to confirm the taxonomic status of these subpopulations (IUCN, 2017). In Sudan, it is found in Darfur, White Nile, Gezira area and southern Bahr el Jebel. It was first found in the southern region of Sudan (Southern Sudan country) by Longstaff (1914) who obtained no more than 5 specimens; the sites now reported show L. natalensis to be widespread in the Sudd region (Brown et al., 1984).

Radix natalensis

Habitat: It is found in permanent standing and running waters of different sizes (IUCN, 2017) including lakes, rivers, khors, flooded areas and irrigation canals; a wide variety of macrophytes grew at these sites (Brown et al., 1984).

4.2.1.1.B. Semi-aquatic snails

Pila wernei (Philippi, 1851) P. wernei belongs to the wide family of Ampullariidae (IUCN, 2010; Köhler and Glaubrecht, 2006). It is also known under the name of Ampullaria wernei (IUCN, 2010; Pallary, 1902; Longstaff, 1914 in Brown et al., 1984; Philippi, 1851 in Köhler andGlaubrecht, 2006) Geographic range: Pila wernei is generally known from scattered localities over a wide area in Africa, from Mali to Somalia, but it represents a native species in Cameroon, Central African Republic, Chad, Egypt, Ethiopia, Mali, Nigeria, Somalia, South Sudan and Sudan (IUCN, 2010). In Sudan it is found in the White Nile, Bahr el Ghazal (Southern Sudan), Darfur and Bahr el Jebel (Brown et al., 1984).

LIST OF PLATES

. Vertical interpretation

The log-probability curves have been interpreted as vertical population percentages and vertical populations, expressed in grain size variations for each individual section, in order to recognize the vertical variations of these populations. The variation in wind energy through time have been estimated too (Fig. 3.55 -3.58). Moreover, three horizontal population percentages and horizontal populations expressed in size variation diagrams, were plotted along the profile of the studied sections. This helps in the understanding of the lateral variations of transportation processes and therefore, of wind energy along the wind blowing direction (Fig. 3. 59 -3.61).

The saltation population shows variable percentages in the red sand sediments facies (Upper unit) with average value of ≈ 56 %, while it prevails in the mottled facies with an average of ≈ 94%. In terms of grain size, the saltation population is coarser and less sorted in the mottled facies than in the sand sediments facies. The suspension population represents ≈ 9 % in the red sand sediments facies, while it represents an average percentage of ≈ 4 % in the mottled facies.

The domination of the traction and saltation populations in the red sand sediments facies, highly contributes in the development of sand dunes in this area. The very high percentage of the saltation population in the mottled sand sediments facies, indicates the dominance of aeolian processes for this facies, and it may also have contributed in the dune forming in the northern part of the region. Moreover, the relatively coarse grain of the traction population indicates high energy of wind transport.

3.5.3.5.1.A. North Bara3 section

The traction population along the north Bara3 section shows strong variations, since it reaches up to 50 % in the red sand sediments facies (average value : 35 %), while in the mottled facies it only represents ≈ 2 % in average (Fig. 3.55A). When translated into grain size, this traction population of the mottled facies is coarser than that in the red sand sediments facies.

3.5.3.5.1.B. North Bara1 section

In North Bara1, the traction population shows a small percentage (average ≈ 3 %) (Fig. 3.56A).

When expressed in grain size, it shows a slight fining upward trend. The saltation population is fairly dominant all along the section with an average percentage of ≈ 90 %. Its grain size is almost constant, although a slight fining upward trend can be seen (Fig. 3.56B). The suspension population represents an average ≈ 7 % of the grains, and its maximum grain size is about 0.125 mm.

In general terms, this indicates that (1) transport and depositional processes did not change significantly through time, (2) the predominant saltation population resulted in the formation of numerous sand dunes during deposition of both the mottled sand and red sand sediments facies (former sand dunes are expressed by presently undulated topographic surface around North Bara1 and further south, as discussed in the next section), and (3) the wind energy did not evolve significantly through time, during deposition of the studied units. Habitat: Biomphalaria pfeifferi occurs in habitats of widely varying stability; in the stable African Great Lakes [START_REF] Dupouy | Correspondence analysis of shell morphology in the African freshwater snail Bkomphalaria pfeiierk (Kraus 1848) (Pulmonata: Gastropoda[END_REF] and in streams (Magendants, 1972), yet isolated populations living in the Sahara are also recorded (Brown et al., 1984). However, B. pfeifferi prefers permanent water bodies and seems to prefer minimal (0-25 %) shade in their habitats The vertical distribution chart of these groups (Fig. 4.7) shows that the lower part of the section is dominated by the aquatic group with percentages up to 65%, followed by the tropical indicators (20%), and the savanna group (15%), species form the arid group being absent. The upper part of the section is dominated by the arid indicator group with an average percentage of about 75 % that increases upward, the aquatic group representing about 20%, whereas the tropical and savanna groups are very rare. The middle part contains balanced percentages of these groups, although one sample presents a high percentage of pollens of the savanna group. All analyzed samples of the En-Nahud 1 and En-Nahud 2 sites were gastropod shells from the red sand sediments (Upper unit), which included the semi-aquatic species Pila wernei and terrestrial land-snails Limicolaria flammea. The results show δ 18 O values from -11.5 ‰ to -0.012 ‰ and δ 13 C from -8.62 ‰ to - 1.33 ‰ (Figs. 4.16 and 4.17).

The evolution of the C and O isotopes in both En-Nahud sections (Figs. 4.16 and 4.17) shows that the samples of the lowermost part of red sand facies are depleted in the heavier 18 O (-12 < δ 18 O < -11) compared to samples from the upper part of the same unit (-2.5 < δ 18 O < 0), which shows gastropod shells enriched in 18 O and depleted in 16 O. The evolution of the C isotopes is less clear, but shows a decrease of the 13 C through time.

Interpretation

The wide range of δ 18 O values reflects climatic variation through time. The highly negative values of δ 18 O in the lower part of the red sand sediments facies can be interpreted as due either to a high flux of regional water vapors enriched in 16 O and depleted in 18 O, or to little evaporation, and therefore moderate temperature. This suggests that the area has experienced a wet climate at that time (Figs. 4.18 and 4.19). The depleted δ 18 O values in gastropds in the Nile Valley is considered an evidence of high rainfall episodes [START_REF] Abell | Oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in Afar region of Ethiopia[END_REF]) derived from a distant oceanic source [START_REF] Ayliffe | Stable carbon and oxygen isotopic composition of early-Holocene gastropods from Wadi Mansurab, north-central Sudan[END_REF]. The latitude effect may have played an important role, since the more distant from the coast, the lower the δ 18 O values [START_REF] Rozanski | Isotopic patterns in modern global precipitation[END_REF]. The study area is relatively far from the Atlantic Ocean (more than 3000 km) therefore, the isotopic composition of the rainwater in this area may be normally depleted. However, since the variation is recorded at the same site, it means climatic change through time, rather than a latitudinal effect.

All analyzed samples of the East El-Khowei site were gastropod shells. The lower three samples, from the palustrine sandy limestone unit, are the semi-aquatic Pila wernei, and the aquatic Biomphalaria pfeifferi and Melanoides tuberculate gastropods. The upper five samples are the terrestrial land snails Limicolaria flammea.

Interpretation

The highly negative values of δ 18 O for samples from the sandy limestone unit (≈ 10 to 6 ky BP) are consistent with low evaporation rates and therefore, humid condition for the palustrine sandy limestone (Fig. 4.21).

The upper part of the east El Khowei section shows shells of terrestrial snail enriched in 18 O and depleted in 16 O. This change in the isotopic compositions between the lower and upper parts indicates that the climate shifted to dry conditions between deposition of the palustrine carbonates and deposition of the red sand sediments, i.e. between ≈ 6 and 1 ky BP. Among the Hydrological information about these lakes would be very powerful for a paleohydrological reconstruction of Eastern Sahara taking into account megalake Chad, West Nubia, El Ga'ah-Sodari lakes and the El Khowei-En Nahud swamps. A topographic profile has been drawn from Digital Elevation Model along a total distance of ≈ 2200 km, showing the maximum lake levels, the groundwater table during the AHP, and the present groundwater table (Fig. 5.2.). The very high lake level at Megalake Chad, the relatively high lake level in the West Nubia and El Ga'ah lakes, and the remarkable groundwater table raise during the AHP provide clear evidence that Eastern Sahara experienced abundant rainfall during the AHP, and that the region may have remained during about 4000 year (10-6 ka BP) within the tropical rainy belt.

Archeology

The lake stages and environmental condition in the Western Nubian Paleolake correlate well with the distribution of early human settlements, characterized by dotted wavy-lines pottery dated between 6300 and 3500 14 C yr BP [START_REF] Hoelzmann | Environmental change and archeology: lake evaluation and human occupation in the Eastern Sahara during the Holocene[END_REF]. Such dotted wavy line potteries have a wide distribution west of the Nile across the Sahara-Sahel Belt (Mohammed-Ali, 2003) and have been observed in several sites along Wadi Howar (Jesse, 2007). In the present study, the existence of some archeological remnants (potteries, grinding stones, cutting tools, etc) on the northwestern shoreline of El Ga'ah Lake and El Firja depression lake (North of Sodari), provides good evidences that these lakes developed at the same time as the West Nubian and Wadi Howar paleolakes, and that they experienced similar human settlements. The latter are controlled by several factors, but hydrology and climate are the most important controlling factors on the distribution these archeological sites, since the density of human population in Africa is primarily regulated by the availability of water [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. Moreover, we found historic potteries (Merowetic and Islamic eras) associated with the Neolithic dotted wavy-lines potteries around the El Ga'ah and El Firja lakes. The fact that such historic potteries are not reported from the Western Nubia and Wadi Howar lakes, suggests that these lakes dried up before the beginning of these historic era. On the other hand, this may provide evidence of the southward human migration as a dynamic response to the southward shift of the rainy belt after the late AHP, as documented by [START_REF] Kuper | Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa's Evolution[END_REF].

Grain size and Major elements geochemistry

As presented in the results of major elements geochemistry (4.4.1), the ratio of SiO2 to Al2O3 is modrately high in both sections of North Bara3 and West El Obeid, with little variability along both the Lower unit (mottled sand facies) and the Upper unit (red sand facies). This moderately high ratio is interpreted as due to the domination of silicate minerals. Figs. 5.3 and 5.4 represent a comparison between the mean grain size and the SiO2 /Al2O3 ratio for the two sections. It appears that the mean grain size is within the range of sand : medium sand and fine sand at North Bara3 and West El Obeid, respectively. The sandy nature of the Lower and Upper units is in good agreement with the SiO2/Al2O3 ratio, which evidences a high SiO2 content. These data strongly suggest aeolian deposition in an arid environment for both units.

VI. CONCLUSION

Summary

Primary observations on temporal and spatial variations of the sedimentological records in the study area suggest that the Kordofan region has been submitted to significant climatic and environmental fluctuations.

The sedimentological and stratigraphic investigations allowed to distinguish eight different sedimentological facies belonging to four stratigraphic units. The identified facies are: mottled sand or silty sand, lens shaped conglomerate, alternating sand and silts, sandy carbonate, carbonate, argillaceous sand, red sand facies, and clinoform bearing, coarse clastic facies.

Carbon dating permitted to date the stratigraphic units and therefore, to correlate them and to establish the succession of sedimentological and climatic events that occurred in the terminal Pleistocene to Holocene period.

Sedimentological variations, sub-fossil contents, sedimentary and pedogenetic features were used to reconstruct the depositional environments, sedimentary evolution and the evolving paleogeography in the study area.

The sedimentology, palynology, gastropod sub-fossil shells paleontology, stable isotopes, major elements geochemistry, and clay mineralogy were used as proxies to reconstruct the paleoclimatic evolution of the study area throughout the latest Pleistocene-Holocene period.

These studies made it possible to place the resulting paleoclimatic reconstruction in the context of the climatic evolution of the East Saharan Region.

Main results

The sedimentological and paleoclimate results are summarized in figure (6.1). The main findings of this study are:

 Prior to ≈ 10 ka BP, the depositional environment was dominated by aeolian activities, and the climate seems to have been arid to hyper-arid.  Between ≈ 10 and ≈ 6 ka BP, the northern part of the study area was marked by lacustrine/palustrine depositional environment and the climate was humid. In the southern part of the area, this humid period may have extended until ≈ 3 ka BP and the sedimentation was dominated by fluviatile deposits.  Between ≈ 6 and ≈ 1 ka BP, the northern part of the area was marked by a non-deposition period, interpreted as a hiatus due to strong aeolian activity preventing deposition and causing erosion, associated with an arid climate. Meanwhile, the southern area was wetter than the northern one, and the hiatus lasted from ≈ 3 ka BP to ≈ 1 ka BP.  After ≈ 1 ka BP, the depositional environment was dominated by aeolian activity in the North, while the southern part was dominated by both aeolian and fluviatile processes. Consequently, the climate was generally dry but wetter to the South.