Jean-Louis Grimaud
email: jgrimaud@umn.edu

Dominique Chardon

Václav Metelka

Anicet Beauvais

Ousmane Bamba

Neogene cratonic erosion fluxes and landform evolution processes from regional regolith mapping (Burkina Faso, West Africa)

Keywords: Regolith, Pediment, Landform evolution processes, Sediment routing system, Source to sink

The regionally correlated and dated regolith-paleolandform sequence of Sub-Saharan West Africa offers a unique opportunity to constrain continental-scale regolith dynamics as the key part of the sediment routing system. In this study, a regolith mapping protocol is developed and applied at the scale of Southwestern Burkina Faso. Mapping combines field survey and remote sensing data to reconstruct the topography of the last pediplain that formed over West Africa in the Early and Mid-Miocene (24-11 Ma). The nature and preservation pattern of the pediplain are controlled by the spatial variation of bedrock lithology and are partitioned among large drainage basins. Quantification of pediplain dissection and drainage growth allows definition of a cratonic background denudation rate of 2 m/My and a minimum characteristic timescale of 20 Ma for shield resurfacing. These results may be used to simulate minimum export fluxes of drainage basins of constrained size over geological timescales. Background cratonic denudation results in a clastic export flux of ~ 4 t/km 2 /yr, which is limited by low denudation efficiency of slope processes and correlatively high regolith storage capacity of tropical shields. These salient characteristics of shields' surface dynamics would tend to smooth the reverine export fluxes of shields through geological times.

Introduction

Interactions between landform evolution and regolith production and mobility over shields exert first-order controls on the source, pathways and fluxes of sediments and solutes over very large emerged surfaces on geological timescales (e.g., [START_REF] Fairbridge | Cratonic erosional unconformities and peneplains[END_REF][START_REF] Millot | Planation of continents by intertropical weathering and pedogenetic processes[END_REF]. Constraining these interactions on continental scales is therefore relevant to quantifying the contribution of shields, as opposed to that of orogens, to global sediment budgets and biogeochemical cycles in the context of longterm Cenozoic climate cooling (Ollier and Pain, 1996a;[START_REF] Molnar | Late Cenozoic increase in accumulation rates of terrestrial sediment: How Might Climate Change Have Affected Erosion Rates?[END_REF][START_REF] Willenbring | Long-term stability of global erosion rates and weathering during late-Cenozoic cooling[END_REF][START_REF] Goudie | Weathering and the global carbon cycle: geomorphological perspectives[END_REF][START_REF] Willenbring | Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales[END_REF][START_REF] Larsen | The contribution of mountains to global denudation[END_REF]. Large-scale studies of regolith transfers are also necessary for -source to sink‖ analyses of coupled drainage areas and sedimentary basins. The present contribution aims to quantify long-term landform evolution, regolith mobility and erosional export fluxes over a large region representative of shields' sediment routing systems.

Tropical shields are mantled by lateritic regoliths derived from intense rock weathering. Such lateritic covers are subject to remobilization by slope and fluvial processes, and the reworked regoliths are commonly re-weathered after transport (Ollier and Pain, 1996b). Renewed periods of regolith production by weathering and remobilization by pedimentation lead to the formation of composite landscapes consisting in a mosaic of lateritic paleo-landsurface remnants of various generations.

Such landscapes are spectacularly preserved throughout Sub-Saharan West Africa, a region of more than 4.5 million km 2 over which a long-recognized Cenozoic regolithpaleo-landsurface sequence has been dated [START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF] and correlated [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

Here we develop a field-and remote sensing-based regolith-landform mapping protocol applied over Southwestern Burkina Faso (Fig. 1). The investigated area is large (ca. 300 x 300 km) and exposes the most widespread, type geologic and morphoclimatic configuration of the West African surface (granite-greenstone terrains and flat sandstones in the Guinean and Soudanian climatic zones i.e., between 10 and 13°N; Fig. 1). Given the constrained chronological framework of regolith-landform production over the sub-region, the selected area is therefore suitable for the characterization of landscape and regolith dynamics and the quantification of long-term (10 6 -10 7 yr) erosion representative of shields surfaces. Regoliths are studied here both as in-situ produced or transported sediments and as paleo-landscape remnants. The obtained regolith-landform map allows evaluation of the nature, distribution and preservation of regolith mantles derived from a pediplain elaborated during the Early and Middle Miocene (ca. 24-11 Ma) over the sub-region. Based on this map, a topographic reconstruction of the pediplain is used to visualize regolith redistribution on slopes during pedimentation and to evaluate landscape and drainage evolution after its abandonment 11 Ma ago. Quantification of post-11 Ma dissection of the pediplain leads to estimation of a type-erosion flux for shields, emphasizing the low capacity of slope and alluvial processes to remove and export regolith mantles.

Geomorphological and geological background

1. The West African geomorphic sequence and its regoliths

The following summary of the West African sequence of stepped lateritic paleolandsurfaces (Fig. 2a) is based mostly on the works of [START_REF] Michel | L'évolution géomorphologique des bassins du Sénégal et de la Haute-Gambie, ses rapports avec la prospection minière[END_REF][START_REF] Michel | Les bassins des fleuves Sénégal et Gambie : étude géomorphologique[END_REF][START_REF] Michel | Les glacis cuirassés d'Afrique occidentale et centrale[END_REF], [START_REF] Eschenbrenner | La séquence de cuirasses et ses différenciations entre Agnibiléfrou et Diébougou (Haute-Volta)[END_REF], [START_REF] Boulangé | Descriptions morphoscopiques, géochimiques et minéralogiques des faciès cuirassés des principaux niveaux géomorphologiques de Côte d'Ivoire[END_REF], [START_REF] Grandin | Aplanissements cuirassés et enrichissement des gisements de manganèse dans quelques régions d'Afrique de l'Ouest[END_REF], [START_REF] Boulangé | La distribution des bauxites sur le craton ouest-africain[END_REF] (see [START_REF] Chardon | Planation, bauxites and epeirogeny: one or two paleosurfaces on the West African margin?[END_REF][START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

Each member in the sequence has a distinct regolith cover and geomorphic character. The first two members of the sequence are the bauxitic and so-called Intermediate surfaces, which bear thick in-situ formed regoliths capped by bauxites and ferricretes, respectively. Bauxites are the end-product of a period of enhanced chemical weathering that started in the Late Cretaceous and culminated in the Mid-Eocene.

Bauxites seal a topography called the African Surface, which makes the present-day envelope of the West African relief. The Intermediate surface corresponds to a differentiated landscape carved in the African bauxitic surface. The following three stepped paleo-landsurfaces of the sequence are glacis (French term for pediments) called the High, Middle and Low glacis. Glacis surfaces are commonly covered by a detrital layer issued from degradation of earlier landforms. Each glacis has undergone weathering after pedimentation, indicating repeated transitions from arid or semi-arid pedimentation to seasonally contrasted or wet tropical weathering. Weathering periods generally ended with the formation of a ferricrete cementing the glacis surfaces and their detrital cover. Today, glacis occupy an overwhelming part of the West African landsurface [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF][START_REF] Grimaud | Dynamique long-terme de l'érosion en contexte cratonique : l'Afrique de l'Ouest depuis l'Eocène[END_REF]. The sequence is best preserved in the Sahelian and Soudanian climatic zones. The glacis, having undergone relief inversion, show evidence of degradation further south under the humid climate of the forest zone [START_REF] Grandin | Aplanissements cuirassés et enrichissement des gisements de manganèse dans quelques régions d'Afrique de l'Ouest[END_REF]. Though originally defined in the French-speaking countries, the sequence or elements of the sequence have been formally or implicitly identified and mapped in other countries of the sub-region (e.g., [START_REF] Fölster | Late Quaternary erosion phases in SW Nigeria[END_REF][START_REF] Grandin | Aplanissements cuirassés de la péninsule de Freetown (Sierra Leone)[END_REF][START_REF] Bowden | On the composition and fabric of the footslop laterites (duricrusts) of the Sierra Leone, West Africa, and their geomorphological significance[END_REF][START_REF] Durotoye | Quaternary sediments in Nigeria[END_REF][START_REF] Thomas | Timescales of landform development on tropical shieldsa study from Sierra Leone[END_REF][START_REF] Thomas | Geomorphology in the Tropics: A Study of Weathering and Denudation in Low Latitudes[END_REF][START_REF] Teeuw | Regolith and diamond deposits around Tortiya, Ivory Coast, West Africa[END_REF] see [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF][START_REF] Grimaud | Dynamique long-terme de l'érosion en contexte cratonique : l'Afrique de l'Ouest depuis l'Eocène[END_REF].

The Lower to Mid-Eocene age of peak weathering and abandonment of the bauxitic surface had long been stratigraphically bracketed (e.g., [START_REF] Millot | Geology of clays[END_REF]. In absence of any stratigraphic constraints or radiometric data, the three glacis were thought to reflect Quaternary glacialinterglacial climatic cycles having led to the dissection of the Intermediate landscape of supposedly Latest Pliocene age [START_REF] Michel | L'évolution géomorphologique des bassins du Sénégal et de la Haute-Gambie, ses rapports avec la prospection minière[END_REF]. Absolute age constraints on the paleo-landsurface sequence were provided by radiometric 39 Ar-40 Ar dating of supergene cryptomelane (K 1-2 Mn 8 O 16 , nH 2 O) around Tambao in Northeastern Burkina Faso [START_REF] Hénocque | 40Ar/39Ar dating of West African lateritic cryptomelanes[END_REF][START_REF] Colin | First 40 Ar/ 39 Ar geochronology of lateritic manganiferous pisolites; implications for the Palaeogene history of a West African landscape[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]summarized in Beauvais and Chardon, 2013;Fig. 1). Data retrieved from the weathering profiles of each surface relict define five Ar-Ar age groups and 3 Ma;Fig. 2b) bracketing periods of chemical weathering of the paleosurfaces. The lower limit of these age groups corresponds to the abandonment age of each paleosurface, i.e., the stabilization of the weathering front by the end of the weathering period preceding duricrusting and subsequent incision of each paleosurface (Fig. 2b). Abandonment ages are ca. 45 Ma for the African bauxitic surface and 24 Ma for the Intermediate surface. The High glacis surface developed between ca. 24 and 11 Ma with a predominance of weathering between 18 and 11 Ma. In Syama (Southern Mali; Fig. 1), Ar-Ar ages on supergene alunite and jarosite [START_REF] Vasconcelos | 40 Ar/ 39 Ar analysis of supergene jarosite and alunite: Implications to the paleoweathering history of the western USA and West Africa[END_REF] confirm predominance of weathering conditions during elaboration of the Intermediate surface until the end of the Oligocene and weathering of the High glacis during the Mid-Miocene (Fig. 2b). Weathering and later abandonment of the Middle and Low glacis have occurred around 7-6 and 3 Ma, respectively (Fig. 2).

Geological context

The study area belongs to the Paleoproterozoic portion of the West African craton exposed South of the Sahara (e.g., [START_REF] Feybesse | The Paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF]Fig. 1). This area comprises Birimian (2.2-2.1 Ga) granite-greenstone terrains over two-thirds of its surface and the Neoproterozoic sandstones of the Taoudeni basin, which were intruded by dolerite sills (Fig. 3). The basement comprises the Banfora, Houndé and Boromo greenstone belts and intervening granitoids [START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (~2.2-2.0 Ga), Western Burkina Faso[END_REF][START_REF] Metelka | A geophysically constrained litho-structural analysis of the Eburnean greenstone belts and associated granitoid domains, Burkina Faso, West Africa[END_REF]Fig. 3b). Greenstones consist mainly of mafic volcanics (basalts and andesites) and volcanosediments, whereas granitoids comprise TTG (tonalite -trondhjemitegranodiorite) and granitic plutons. The escarpment delimiting the Banfora plateau marks the southeastern boundary of the Taoudeni basin (Fig. 3a). The southern part of the plateau bears the highest summits of the region and the height of the escarpment decreases towards the NE. The study area mostly belongs to the Mouhoun drainage system (the main stream of the Volta drainage system, i.e., the Black Volta) and is fringed by the catchments of the Upper Niger River to the west, the Comoé River to the southwest and the Nazinon River to the east (part of the Nakambé drainage system i.e., the White Volta; Figs. 1,3).

3. Earlier works on the study area

The pioneering study of [START_REF] Daveau | Cuirasses et chaînes birrimiennes en Haute-Volta[END_REF] around Houndé and Gaoua (Fig. 3) led to the identification of stepped lateritic landsurfaces and to the description of what will be recognized later as the High glacis. Later on, the southern part of the study area and its extension in the Ivory Coast became one of the first regions where the entire West African regolith-landform sequence was thoroughly documented [START_REF] Eschenbrenner | La séquence de cuirasses et ses différenciations entre Agnibiléfrou et Diébougou (Haute-Volta)[END_REF]. [START_REF] Boeglin | Géochimie, degré d'évolution et lithodépendance des cuirasses ferrugineuses de Gaoua au Burkina Faso[END_REF] and [START_REF] Boeglin | Évolution minéralogique et géochimique des cuirasses ferrugineuses de la région de Gaoua (Burkina Faso)[END_REF] later focused on an area of ca. 50 x 30 km around Gaoua (Fig. 3), where they mapped the High glacis relicts and performed a detailed geochemical study of their ferricretes. More recently, [START_REF] Bamba | L'or disséminé dans les Albitites de Larafella (Burkina Faso), évolution dans les altérites et les cuirasses ferrugineuses : Métallogénie -Pétrologie -Géomorphologie[END_REF] and [START_REF] Bamba | Ferricrete genesis and supergene gold behaviour in Burkina Faso, West Africa[END_REF] have refined detailed mapping of the three glacis around the Poura mine (left bank of the Mouhoun River, West of Tô; Fig. 3) to study surface remobilization of gold. Lately, [START_REF] Butt | Relief inversion in the geomorphological evolution of sub-Saharan West Africa[END_REF], reporting on gold prospects of the area, argued for the stepped character of the glacis and the detrital nature of their ferricretes, which had been documented by geomorphologists since the late 1950's (e.g., [START_REF] Michel | L'évolution géomorphologique des bassins du Sénégal et de la Haute-Gambie, ses rapports avec la prospection minière[END_REF][START_REF] Daveau | Cuirasses et chaînes birrimiennes en Haute-Volta[END_REF] but largely ignored since then.

Field relationships and regional-scale regolith-landform mapping

1. Rationale

As opposed to the other regolith-landform associations of the lateritic paleolandsurfaces sequence, High glacis remnants have both distinctive field and remotelysensed mappable properties at the resolution of the digital data and given the size of the map area (see below). Our work therefore focused on the High glacis regolith-landform associations, which were characterized and mapped for evaluation of the pedimentation and weathering processes controlling the development of the High glacis paleolandsurface. The topography of this paleo-landsurface was then reconstructed as a datum for the quantification of regional erosion after its abandonment. Higher paleolandsurface relicts of the sequence (i.e., bauxitic and Intermediate) were already inherited landforms incorporated to the High glacis landscape and are still preserved in the current landscape. As seen below, their duricrusts are also found as reworked elements in the High glacis regoliths. Field characteristics of the higher landsurfaces are therefore described and their occurrences reported in the regolith-landform map. Relicts of the Middle and Low glacis (i.e., lower landsurfaces) occupy lower parts of today's landscape that have been excavated in the High glacis surface. Their field characteristics are briefly described below.

2. Field typology of landforms and associated regoliths

2. 1. Field survey

Our field observations were carried out at stations (Fig. 3b), which consist of areas of several km 2 , where landscape analysis was undertaken by interpreting sceneries using 1/50 000 and 1/200 000 scale topographic maps. Mapping of regolith-landform units was undertaken at key stations. Landscape interpretations/chronologies and identification of paleo-landsurface relicts were complemented by in-situ examination of the regolith along tracks through the landscape and at road cuts. This work included the identification of the textures of the exposed lateritic weathering profile horizons and the description of the sedimentary facies of the transported regoliths.

2. 2. Higher landsurfaces

Bauxite relicts form mesas on the highest summits. On the basement, those mesas are restricted to mafic substrates and their size is generally limited (i.e., less than 100 m wide) (Fig. 4a). In the Taoudeni basin, bauxitic plateaus are larger and mostly restricted to dolerites (Figs. 4b,4c). The bauxites have typical pink-whitish pisolithic textures (Fig. 4d) or less evolved, nodular-mottled textures (Fig. 4e). In the Taoudeni basin, particularly on dolerites, the Intermediate ferricrete caps gently dipping convexconcave surfaces forming the downslope extension of the bauxitic plateaus (Figs. 4b,4c). These ferricretes (Figs. 4f,4g) show gradational variation in composition and texture with the bauxites. This is interpreted to result from iron leaching that leads to relative aluminum enrichment of the bauxites and net concomitant iron accumulation in the ferricretes [START_REF] Boulangé | Relation between lateritic bauxitization and evolution of landscape[END_REF]. Intermediate ferricretes on sandstones display typical nodular textures (Fig. 4g). On the basement, the Intermediate ferricrete is rarely found in-situ but rather as pebbles or cobbles reworked in the High glacis ferricrete (see below). Instead, a residual -Intermediate relief‖ surrounds bauxitic duricrusts and may be locally paved with a cemented scree of bauxite cobbles (Fig. 4a). Bauxites are underlain by weathering profiles that are at least 80 m thick, as estimated on the stripped slopes of mesas, whereas Intermediate weathering profiles exceed 20 m in thickness.

2. High glacis

The best visible High glacis relicts are kilometer-scale plateaus (the largest ones being concave) shallowly dipping away from the topographic massifs made of mafic volcanics, which may carry higher relict paleo-landsurfaces (Fig. 5a). Such High glacis relicts are often separated from the massifs by a peripheral hollow due to incision of their upslope parts (see also [START_REF] Beauvais | Analysis of poorly stratified lateritic terrains overlying a granitic bedrock in West Africa, using 2-D electrical resistivity tomography[END_REF]. This configuration indicates that the High glacis has occupied large piedmonts that have been dissected. Dissection of the High glacis ferricrete has carved 1 to 20 m-high scarps dominating slopes cut into soft regoliths and covered by ferricrete debris (Figs. 5,6a,6b).

Most High glacis plateau ferricretes consist of a conglomerate ranging from gravel to cobble, with a predominance of gravels mostly made of ferruginous nodules (Fig. 6). Matrix silts and sands may bear quartz, particularly on volcano-sediments and granitoids. Coarse gravels and cobbles are made of Intermediate-type ferricretes, vein quartz and bauxites (Figs. 6a-6e). Conglomerates are commonly matrix-or blocksupported, suggesting emplacement by mudflow or debris flow process. Channels are observed at the base of the ferricrete and decimeter-to meter-scale oblique stratifications are recognized in the downslope parts of some large High glacis relicts (Fig. 6d). The coexistence of mud or debris flow and alluvial facies in the High glacis cover is indicative of interplay of slope and fluvial processes during pedimentation [START_REF] Dohrenwend | Pediments in Arid Environments[END_REF].

The noticeable absence of a mottled horizon under the glacis conglomerate or exposure of deep portions of weathering profiles right under that cover (Figs. 6a,6c) indicates that these weathering profiles have been truncated before deposition of the conglomerate. Duricrusting (i.e., iron impregnation or cementation) is mostly restricted to the conglomerates cover (Figs. 6a,6b). Still, soft ferricretes, i.e., pedogenic impregnation of a mottled clay lateritic horizon [START_REF] Tardy | Petrology of laterites and tropical soils[END_REF], are common under the cemented conglomerates. This suggests renewed weathering along the upper fringe of the truncated lateritic profiles under the conglomerates. At certain locations, the ferricrete made of cemented debris flows may even be underlain by a type-succession of weathering profile horizons (i.e., soft ferricrete, mottled layer, fine saprolite, coarse saprolite; Figs. 6e,6f), suggesting the full development of a weathering profile after pedimentation.

On granitoids located away from topographic massifs (e.g., west of Djigoué and south of Bobo Dioulasso, Tô region), the High glacis ferricretes are generally thinner (0.5 -2 m) and mantle smooth interfluves (Figs. 5c,5d). Such ferricretes have vermicular and nodular textures and are devoid of clasts. They appear less ferruginous and less indurated than their plateau counterparts. These ferricretes show gradational contacts with underlying soft ferricretes and are part of in-situ weathering profiles.

2. 4. Lower landsurfaces

The restricted size of higher paleo-landsurfaces remnants is primarily due to the development of the High glacis. But rejuvenation of the slopes of the residual reliefs carrying those remnants and the formation of peripheral hollows (Fig. 5) is attributed to the following periods of glacis formation. The landsurface between High glacis relicts and the modern alluviums is almost exclusively occupied by the Middle and Low glacis. These glacis may be stepped or may combine to form a single polygenic landsurface.

The regolith of the Middle and Low glacis is comparable to that of the High glacis, but their ferricrete is generally thinner to absent and may be covered over large areas by loose material made of reworked saprolite, sand and ferruginous gravels.

Regional-scale regolith-landform mapping protocol

On basement terrains, High glacis units were manually mapped in GIS on the basis of their airborne gamma ray spectrometry and their photo-interpretation signature (Google Earth), both validated by the field survey / maps (Figs. 3,7). Airborne gamma ray spectrometry was acquired during the 1998-1999 SYSMIN (System for Mineral Products) project [START_REF] Metelka | A geophysically constrained litho-structural analysis of the Eburnean greenstone belts and associated granitoid domains, Burkina Faso, West Africa[END_REF]. Those data display uranium (U), potassium (K) and thorium (Th) contents of the upper 30 cm of the Earth surface [START_REF] Dickson | Interpretation of aerial gamma-ray surveysadding the geochemical factors[END_REF]. The spatial resolution of the data on the basement is 125 m (with an original line spacing of 500 m). Spatial resolution of 250 m (line spacing of 1000 m) over the Taoudeni basin has required resampling to 125 m in the area to obtain a harmonized image. Both the raw and enhanced (Th/K ratio) gamma-ray spectrometry data where used (Fig. 7) during the analysis and combined with the shaded relief maps of the SRTM (Shuttle Radar Topographic Mission) data (90 m spatial resolution). The High glacis ferricretes have a distinctive photo-interpretation texture that spatially matches blue-green colors in raw gamma-ray data. This attests of a strong depletion in potassium, and a relative enrichment in thorium and uranium as a result of weathering processes [START_REF] Dickson | Interpretation of aerial gamma-ray surveysadding the geochemical factors[END_REF][START_REF] Wilford | Application of airborne gamma-ray spectrometry in soil/regolith mapping and applied geomorphology[END_REF][START_REF] Metelka | A geophysically constrained litho-structural analysis of the Eburnean greenstone belts and associated granitoid domains, Burkina Faso, West Africa[END_REF]. However, uranium has a complex behavior during weathering and regolith formation [START_REF] Dickson | Interpretation of aerial gamma-ray surveysadding the geochemical factors[END_REF][START_REF] Wilford | Application of airborne gamma-ray spectrometry in soil/regolith mapping and applied geomorphology[END_REF][START_REF] Dequincey | Chemical mobilizations in laterites: evidence from trace elements and 238U-234U-230Th disequilibria[END_REF]. Therefore, the Th/K ratio was preferred to further enhance the signature of the High glacis ferricretes (in yellow-red) and remove any regional gradients in the data (Figs. 7b,7d).

The spatially homogeneous radiometric signature of the High glacis relicts suggests that their ferricretes have been geochemically homogenized by weathering despite the variability of their substrate. Indeed, trace element concentrations systematically converge as a function of the Fe 2 O 3 content of the ferricrete taken as an index of their geochemical maturity [START_REF] Boeglin | Géochimie, degré d'évolution et lithodépendance des cuirasses ferrugineuses de Gaoua au Burkina Faso[END_REF][START_REF] Boeglin | Évolution minéralogique et géochimique des cuirasses ferrugineuses de la région de Gaoua (Burkina Faso)[END_REF].

Conversely, the ambiguous radiometric signature of the Middle and Low glacis may be interpreted as a lower maturity of their ferricrete and a mixed signature of the material covering their ferricrete.

The strong Th and exceptionally low K signatures of the Taoudeni basin (Figs. 7a,7b) are explained by the mineralogy of the sandstones that are originally quartz rich and correlatively poor in clays. The larger pixel size (250 m) of the gamma ray spectrometry may have further modified the Th signature of High glacis ferricretes by sampling other surface material with high Th content (e.g., Bauxite and Intermediate duricrusts). Therefore, the high values (red colors) of Th/K ratio image (Fig. 7b) may not only be due to High glacis ferricretes. Hence, regolith-landform mapping over the Taoudeni basin was mainly based on photo-interpretation in Google Earth and field observations. Bauxite and Intermediate surface relict occurrences were mapped by photointerpretation, field observations and after [START_REF] Petit | Carte Géomorphologique du Burkina Faso au 1/1 000 000[END_REF]. River plain sediments were mapped mainly on the basis of their negligible relief and the characteristic geometries of stream networks in the gamma-ray imagery. Because of the higher mobility of U compared to Th, river plain sediments raw gamma-ray signature may indeed vary i.e., color of the source material (variable content of radioelements), very light to white color (high U / Th / K signal), or very dark color (low U / Th / K signal). It might reflect the composition of the source material, sediment mixing, or strong attenuation by vegetation or water (Fig. 7a).

4. Interpretation of the High glacis regolith-landform map

For the purpose of the interpretation, the mapped regolith-landform units were superimposed on the topography in Fig. 8 and on the geological map from [START_REF] Metelka | A geophysically constrained litho-structural analysis of the Eburnean greenstone belts and associated granitoid domains, Burkina Faso, West Africa[END_REF] in Fig. 9. Comparison of these two maps highlights major contrasts in the pattern of High glacis relicts' distribution on either side of the main watersheds, even on the same geological substrate. This is particularly exemplified on TTG across the southwestern and eastern Mouhoun watersheds (Figs. 8,9a). Density of High glacis relicts generally decreases when approaching the main streams (Fig. 8). Relicts are best preserved on TTG and granite (45 and 20% of the total surface of the relicts, respectively). Greenstones underlay less than 25% of the total area of the preserved High glacis ferricrete (Figs. 3b,9c). Fifteen percent of the map area of the TTG and more than 20% of the granitoids are covered by High glacis ferricretes, whereas only 10% of the greenstones are covered (Fig. 9d).

Type-patterns of High glacis regolith-landforms may be distinguished in the study area (Figs. 5,9,10). In high relief volcanic terrains, High glacis relicts are well preserved on piedmonts (Fig. 10a), equally on greenstones and granitoids. Volcanosedimentary-granitic terrains of moderate relief (e.g., Djigoué area) display High glacis relicts that once coated wide N-S trending valleys, which were in turn dissected by second-order narrower E-W oriented valleys (Fig. 10b). Over TTG terrains of the Comoé catchment (e.g., Dandougou-Bobodiolasso area), a very low relief and very low slope High glacis surface is preserved on sparse residual hills where the ferricrete may be dismembered into boulders resting on the soft ferricrete (Fig. 10c). The resulting landscape is a convexo-concave plain of low amplitude that is gently dipping towards the main stream (Fig. 10c). Equivalent TTG terrains in the Nakambe catchment reveal a better-preserved High glacis paleo-landsurface (see above), with planar to weakly concave relicts delineating a plain of very low-relief surface envelope (Fig. 10d).

The occurrence of strong and thick plateau ferricretes upon granitoids on the piedmonts of volcanic reliefs contrasts with the weak and thin ferricretes preserved on the same granitoids of the low land (Figs. 5a-5d). This reflects the high iron content and thickness of the piedmont conglomerates made of reworked bauxitic and Intermediate duricrusts that formed on iron-rich mafic rocks. On the contrary, low land ferricretes are solely issued from in-situ weathering of iron poor granitoids. The limited lateritic cover of mafic volcanics massifs further argues for the transfer of upland iron-rich duricrusts as colluviums to the High glacis piedmonts. This transfer has resulted in net iron accumulation on glacis surfaces further down in the landscape [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie : étude géomorphologique[END_REF][START_REF] Michel | Les glacis cuirassés d'Afrique occidentale et centrale[END_REF][START_REF] Beauvais | Analysis of poorly stratified lateritic terrains overlying a granitic bedrock in West Africa, using 2-D electrical resistivity tomography[END_REF].

The above analysis indicates that the High glacis landsurface was a pediplain, defined as a surface of coalescent pediments [START_REF] Maxson | Terminology of surface forms of the erosion cycle[END_REF]. The spatial distribution of these pediments and their regolith as well as the preservation pattern of the High glacis pediplain are spatially controlled at various scales. A first, local control of this distribution is exerted by the lithology of the bedrock. This sets the relief and therefore the pedimentation process, as well as the weathering mode of the pediplain. On a regional scale, pediplain dissection and erosion mode appear to have been compartmentalized among drainage basins.

Reconstruction of the Late Mid-Miocene pediplain and quantification of its erosion

1. Methodology

The high density of High glacis relicts warrants reconstruction of the pediplain they were part of. The reconstructed topography is a triangulated surface interpolated from a set of geo-referenced points generated from the polygons of the High glacis relicts using the DSI method (Gocad software; [START_REF] Mallet | Discrete smooth interpolation in geometric modelling[END_REF]. Each High glacis polygon was converted to a grid of 500 m-spaced points inside its limits. The elevation of the SRTM digital elevation model, degraded to 500 m resolution, was then sampled for each point to form the High glacis relicts' elevation dataset. The interpolated surface was created from this dataset through a series of iterations. The number of surface's triangles, which reflected the distribution of the dataset for the first incremental surface, was increased at each iteration step, while the High glacis surface was forced to remain above the modern topography. This allowed for the inclusion of the reliefs dominating the pediplain and preserved the composite nature of the High glacis landscape. The resulting topography is shown on Fig. 11. Erosion post-dating the abandonment of the pediplain was obtained by subtracting the modern topography from that of the reconstructed pediplain (Figs. 11a-c). The drainage network and a slope map were then automatically extracted from both the modern and pediplain digital elevation models for comparison (Figs. 11d-h).

2. Late Mid-Miocene topography and landscape dynamics

The pediplain displays the same first-order relief pattern as the current topography. The main divides such as the inselbergs and the main escarpment already existed at the time the pediplain was functional and did not migrate since then (Figs. 11a-b). The main river network was therefore already established since the Late Mid-Miocene with the exception of a few local river rearrangements (Figs. 11d,11h). Our reconstruction does not allow for testing whether the Upper Mouhoun (flowing on the Taoudeni basin) was connected to the lower Mouhoun at the time of the High glacis (Fig. 11h). This rearrangement [START_REF] Hubert | Sur un important phénomène de capture dans l'Afrique occidentale[END_REF][START_REF] Palausi | Contribution à l'étude géologique et hydrogéologique des formations primaires au Soudan et en Haute-Volta[END_REF] occurred after the upper Mouhoun River, which used to flow northeastward into the Niger River since the Eocene, formed the Gondo Plain internal delta [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF][START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]Fig. 1).

The spatial variability of the pediplain relief on the basement is representative of the type-landscapes shown in Fig. 10 (comparisons with Figs. 9a,12). One distinguishes (i) an inselberg-studded pediplain around volcanic terrains, (ii) almost flat lands over TTG, and (iii) a mix of the first two landscape types over volcano-sediments and associated granite plutons. In the Taoudeni basin, the pediplain consists of wide and shallowly dipping piedmonts connecting sandstone or higher landsurfaces plateaus to the main streams.

3. Interpretation of the pediplain dissection pattern

The pediplain has a low drainage density compared to that of the modern landscape, with typical pediment widths of 10-20 km (up to 30 km over the sandstones; Figs. 11,12). Dissection of the High glacis landscape accompanied an increase in the drainage density. The modern valley sides rarely exceed 5 km in width (Figs. 3,8).

Dissection of the pediplain also resulted in an increase in slopes (Fig. 11g), which is in agreement with the evolution from a pediplain towards narrower / deeper valleys. In other words, the Middle and Low glacis, which contributed to the dissection of the pediplain, never attained the width of the High glacis.

Post-11 Ma dissection of the pediplain is non-uniformly distributed, with denudation depths ranging from 0 to 100 m and corresponding denudation from 0 to 10 m/my (Fig. 11c). Extreme denudations (> 60 m, > 5 m/My) over basement terrains are concentrated on the steep slopes of residual reliefs (Figs. 6,10,11). Along the two main streams, denudation is moderate to high (30 -60 m; 2.5 -5 m/My). The denudation is rather uniformly distributed in the Comoé drainage, meanders being delineated by denudation patterns in the vicinity of the Mouhoun River (Fig. 11c). The lowest erosion range (< 10 m, < 1 m/My) is recorded east of Tô, southeast of Bobo-Dioulasso and around Gaoua and Djigoué (Fig. 11c), in agreement with the high degree of preservation of the pediplain relicts (Fig. 8). The volume eroded from the pediplain over the map area represents a spatially averaged denudation of ca. 20 m, corresponding to a denudation rate of about 2 m/My. Dissection of the Late Miocene pediplain affected the whole region and did not propagate upstream by scarp or knickzone retreat as advocated by [START_REF] King | The Morphology of the Earth[END_REF] and as implicitly included in river profile inversion works based on stream power incision models (e.g., [START_REF] Paul | The African landscape through space and time[END_REF]. As an illustration, the bounding escarpment of the Taoudeni basin is stable since at least 11 Ma (Fig. 11) and most probably 45 Ma (see [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. Higher erosion rates coincide with the main drains outside the High Mouhoun, suggesting a control of the stream power (e.g., [START_REF] Whipple | Dynamics of the stream-power river incision model; implications for height limits of mountain ranges, landscape response timescales, and research needs[END_REF] on pediplain dissection. Moderate stream power in the rest of the river network would have preserved the pediplain from complete stripping. Scarps bounding High glacis relicts often coincide with second-order divides of the modern topography, indicating a component of slope retreat in the dissection of the pediplain. Meandering patterns of the Mouhoun River (Figs. 8,11c) further suggest that channel migrations may have contributed to slope retreat. The typical retreat distance of the High glacis relicts' edges over the study area (1-10 km) suggests a retreat rate of 100-1000 m/My. Therefore, under the erosion dynamics at play since ca. 10 Ma and assuming an initial valley side width of 20 km (Fig. 12), a minimum of 20 My would be required to entirely resurface the High glacis pediplain.

Discussion

Our results complement denudation and incision rates estimated from differential elevation of sparse High glacis remnants and local base levels that have mean values of 5-7 m/My but non-uniform distributions over the sub region from 2 to 15 m/My [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF][START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. This suggests a partitioning of erosion among geomorphic provinces, with implications on the spatial variability of the sediment routing system. This variability may be a function of the regional topographic relief and/or positive epeirogeny, which seem to focus higher denudation [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF][START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. Importantly, this variability is also and primarily due to contrasted river network evolution among subdrainage basins separated by stationary knickzones [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]this work).

Our approach has consisted in regional scale, volumetric quantification of denudation from a dated paleo-landscape datum, which enhances accuracy of denudation measurement over a West African province of lowest Neogene erosion. The obtained averaged denudation rate of 2 m/My is close to the lower limit of Cenozoic cratonic denudation rates measured worldwide [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Given the relevance of the size and morpho-geological context of the study area, we propose that this rate determines the background denudation ‗noise' of shields. The Late Neogene increase in clastic sedimentation documented worldwide and particularly in African deltas such as that of the Niger [START_REF] Séranne | Early Oligocene stratigraphic turnover on the West Africa continental margin: a signature of the Tertiary greenhouse-to-icehouse transition?[END_REF][START_REF] Jermannaud | Plio-Pleistocene sequence stratigraphic architecture of the Eastern Niger Delta: A record of eustasy and aridification of Africa[END_REF]Fig. 1) should however be somehow included in the 2 m/My of background denudation averaged since the Early Late Miocene. This issue constitutes a sizable research challenge.

The material derived from dissection of the pediplain and removed since the Late Mid-Miocene was made of regolith, i.e. the High glacis cover and its underlying laterites. Indeed, no outcrop of significant surface / height has been exhumed since the abandonment of the pediplain. The fact that only the main stream channels locally flow atop the bedrock and that an averaged denudation of less than 20 m since abandonment of the pediplain is estimated further argues for denudation being restricted to the regolith. Denudation is however not strictly weathering-limited because if glacis typically cut preexisting and potentially old regoliths (i.e., Late Cretaceous to Eocene and Oligocene for the bauxitic and Intermediate weathering episodes, respectively, Figure 2), they rarely attained the bedrock. Therefore, although weathering is instrumental in producing regolith mantles available to future stripping [START_REF] Fairbridge | Cratonic erosional unconformities and peneplains[END_REF], low abrasion power of the pedimentation process combined with a probably limited transport capacity of the river network should be regarded as the limiting factor of tropical shields denudation.

The minimum characteristic timescale of 20 My for shield resurfacing indicated by post-Mid-Miocene drainage growth reinforces the view that shields are nonequilibrium landscapes, which increase their relief through geological time by preserving old landforms [START_REF] Thomas | The role of etch processes in landform development II. Etching and the formation of relief[END_REF][START_REF] Twidale | A model of landscape evolution involving increased and increasing relief amplitude[END_REF][START_REF] Bishop | Long-term landscape evolution: linking tectonics and surface processes[END_REF][START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. A long resurfacing timescale also implies the high sediment retention capacity of shields as those sediments stem from weathering mantles dating back from at least the Early Paleogene and have been slowly recycled through the landscape since then. This suggests that weathering and slope processes play a major role, comparable to or greater than sediment transport by rivers [START_REF] Métivier | Stability of output fluxes of large rivers in South and East Asia during the last 2 million years: implications on floodplain processes[END_REF][START_REF] Jerolmack | Shredding of environmental signals by sediment transport[END_REF] The background denudation rate of 2 m/My may be converted into a volumetric export rate of 2 x 10 -3 km 3 /km 2 /My and a clastic yield of 4 t/km 2 /yr, considering an effective grain density of 2,500 kg/m 3 (estimated from a bulk density of 2,000 kg/m 3 and a 20 % porosity) for the stripped regolith. This value corresponds to the lower limit of modern West African solid river loads (3.5 -300 t/km 2 /yr) and to the 4 t/km 2 /yr measured at the outlet of the Volta catchment [START_REF] Milliman | River Discharge to the Coastal Ocean: A Global Synthesis[END_REF].

Therefore, cratonic erosion fluxes derived from a 2 m/My denudation rate appear to be realistic for low-capacity shield drainage systems on geological timescale. Conversely, this could suggest that the lower limit of large tropical shields catchments yields is governed by a cratonic background denudation noise. Importantly, background cratonic denudation of 2 m/My may be used to simulate the minimum export fluxes of drainage basins of constrained size over geological timescales. The minimum clastic yield of the major African catchments seems to be currently dominated by such a cratonic noise. Our results, once combined with landscape / weathering chronologies, point to the shields' high storage capacity of sediments, consisting exclusively of regoliths that have been mainly recycled in the landscapes since the Late Cretaceous. Such a slow regolith turnover is due to the low efficiency of slope denudation processes by pedimentation and would tend to smooth riverine export fluxes of shields over geological time scales.

Conclusions

Figures Captions

Figure 1: Simplified geology of sub-Saharan West Africa. The study area is shown by a red frame (map modified from [START_REF] Feybesse | The Paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF]. Africa (compiled after [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie : étude géomorphologique[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]Beauvais and[START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]modified after Grimaud et al., 2014). (b) Synthesis of Ar-Ar age data obtained on supergene Mn oxides. Colored time slices represent the main periods of weathering correlated with the successive paleolandsurfaces, based on dating of cryptomelane in Tambao (after [START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF][START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Ages reported from Syama were obtained on alunite and jarosite (after [START_REF] Vasconcelos | 40 Ar/ 39 Ar analysis of supergene jarosite and alunite: Implications to the paleoweathering history of the western USA and West Africa[END_REF]. Note that only alunite and jarosite ages with error of less than 5 Ma are reported. Terminology of the paleo-landsurfaces: Bx -Bauxite; Int -Intermediate; HG -High glacis; MG -Middle glacis; LG -Low glacis. Syama and Tambao are located on Fig. 1.

  in -shredding‖ and particularly smoothing or buffering climatic signals in the sedimentary record. Still, the ubiquitous character of the West African lateritic paleo-landsurfaces sequence and its chronology would suggest that Cenozoic landscape rejuvenation and sediment fluxes were punctuated by major climatic periods. Subdued clastic exports are indeed expected for time intervals of enhanced weathering during the Lower and Mid-Eocene, Late Oligocene, Mid Miocene, Latest Miocene and Latest Pliocene (e.g., Fig. 2b).

  Regional regolith-landform mapping allows characterizing surface dynamics, weathering patterns, paleotopography and dissection mechanisms of the last pediplain formed over West Africa from Early to Mid-Miocene. The nature and preservation / dissection patterns of the pediplain are controlled by the spatial distribution of bedrock lithologies and are partitioned between large drainage basins. Quantification of the post Mid-Miocene dissection of the pediplain determines a cratonic background denudation noise of 2 m/My and a minimum shield resurfacing characteristic timescale of 20 My.

Figure 2 :

 2 Figure 2: (a) Lateritic paleo-landsurface sequence and incision chronology of West

Figure 3 :

 3 Figure 3: Topography and simplified geology of the study area (see Fig. 1 for location).

Figure 4 :

 4 Figure 4: Illustrations of field characteristics of the higher lateritic paleo-landsurfaces

Figure 5 :

 5 Figure 5: Illustrations of the High glacis relict landforms. (a) -(b) Google Earth view

Figure 6 :

 6 Figure 6: Illustrations of High glacis regoliths. (a) Base of a High glacis ferricrete

Figure 7 :

 7 Figure 7: Ternary image of the airborne gamma-ray spectrometry survey (a) and the

Figure 8 :

 8 Figure 8: Regolith-landform map superimposed on the STRM digital elevation model

Figure 9 :

 9 Figure 9: (a) Regolith-landform map superimposed on the geology that is simplified

Figure 10 :

 10 Figure 10: Maps and cross-sections of type-landscapes on basement terrains. Frames of

Figure 11 :

 11 Figure 11: (a) Modern topography based on SRTM digital elevation model smoothed to

Figure 12 :

 12 Figure 12: Geomorphological map of the High glacis pediplain before its abandonment

  Figure 1 Click here to download high resolution image

Acknowledgments

This work was funded by WAXI, the CNRS and the ANR TopoAfrica (ANR-08-BLAN-572 0247-02). The manuscript benefited from the comments of R. Teeuw and an anonymous referee as well as from editorial suggestions by A. Plater. We thank M. Jessell, D. Rouby, L. Baratoux and D. Huyghe for discussions and support and A.

Fofana for participation in the fieldwork. We acknowledge AMIRA International and the industry sponsors, including AusAid and the ARC Linkage Project LP110100667, for their support of the WAXI project (P934A) as well as the Geological Surveys / Departments of Mines in West Africa as sponsors in kind of WAXI.