Dominique Chardon
email: dominique.chardon@ird.fr

Jean-Louis Grimaud

Delphine Rouby

Anicet Beauvais

Frédéric Christophoul

Stabilization of large drainage basins over geological timescales: Cenozoic West Africa, hot spot swell growth and the Niger River

Reconstructing the evolving geometry of large river catchments over geological timescales is crucial to constraining yields to sedimentary basins. In the case of Africa, it should further help deciphering the response of large cratonic sediment routing systems to Cenozoic growth of the basin-and-swell topography of the continent. Mapping of dated and regionally correlated lateritic paleolandscape remnants complemented by onshore sedimentological archives allows the reconstruction of two physiographic configurations of West Africa in the Paleogene. Those reconstructions show that the geometry of the drainage stabilized by the Late Early Oligocene (29 Ma) and probably by the end of the Eocene (34 Ma), allowing to effectively link the inland morphoclimatic record to offshore sedimentation since that time, particularly in the case of the Niger catchment -delta system. Mid-Eocene paleogeography reveals the antiquity of the Senegambia catchment back to at least 45 Ma and suggests that a marginal upwarp forming a continental divide preexisted Early Oligocene connection of the Niger and Volta catchments to the Equatorial Atlantic Ocean. Such a drainage rearrangement was primarily enhanced by the topographic growth of the Hoggar hot spot swell and caused a major stratigraphic turnover along the Equatorial margin of West Africa.

Introduction

Reconstructing the evolving geometries of large drainage basins over geological time scales (10 6 -10 7 yr) is key to linking the sediment routing system to the deformation and landform evolution processes of continents. It is also crucial to constrain clastic sedimentary fluxes towards sedimentary basins. Along passive continental margins, stratigraphic architectures and sedimentary records would indeed be primarily sensitive to inland catchment size and therefore to drainage rearrangement (e.g., Rouby et al., 3 2009). But conversely, the sedimentary record in itself may not be used as an unequivocal proxy of change in inland drainage given the multiplicity of factors controlling sedimentation and their interactions. The drainage of the African continent has evolved with the growth of a continental-scale basin-and-swell relief (Fig. 1), which results from the interplay of marginal upwarps sustained or rejuvenated since Gondwana continent break-up and intraplate Cenozoic hot spot swells [START_REF] Burke | The African Plate[END_REF][START_REF] Summerfield | Tectonics, geology, and long-term landscape development[END_REF]. Within such an evolving topographic framework that is still poorly constrained, the age and acquisition mode of the main river courses remain controversial. Key questions are to know whether, when, and how short coastal rivers became connected to large inland drainage basins [START_REF] Summerfield | Tectonics, geology, and long-term landscape development[END_REF]. Solving those questions is particularly relevant for interpreting the increase of clastic sediment fluxes around the African continent following Eocene peak greenhouse and its links with lithospheric deformation [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] Burke | Africa's petroleum systems: four tectonic 'aces' in the past 600 million years[END_REF].

A large body of research exists on river network evolution of numerous drainage basins of the African continent. Inland deltas, elbows and steps (knickzones and knick points) along river courses have been considered as recording rapid headward erosion by coastal rivers having "recently" captured inland drainages via piracy (e.g., [START_REF] Goudie | The drainage of Africa since the Cretaceous[END_REF]. Such interpretations are solely based on the present day geometry of river networks and topography. At best, much ambiguity therefore remains on drainage reorganization scenarios given the paucity of evidence for past river courses and their age [START_REF] Summerfield | Tectonics, geology, and long-term landscape development[END_REF]. The high steepness and stepped character of modern African rivers' lower reaches may be interpreted as a consequence of the maintenance of rivers across continental margins during continental uplift (e.g., [START_REF] Bond | Evidence for Some Uplifts of Large Magnitude in Continental Platforms[END_REF][START_REF] Burke | The African Plate[END_REF].

Alternatively, models of denudational isostatic uplift of the seaward slope of marginal upwarps suggest maintenance of dual (inland and seaward) drainage systems long after rifting [START_REF] Summerfield | Plate tectonics and landscape development on the African continent[END_REF][START_REF] Gilchrist | Differential Denudation and Flexural Isostasy in Formation of Rifted-Margin Upwarps[END_REF]. Such a configuration would lead steep aggressive costal rivers to eventually capture the interior drainage [START_REF] Gilchrist | Post-Gondwana Geomorphic Evolution of Southwestern Africa -Implications for the Controls on Landscape Development from Observations and Numerical Experiments[END_REF][START_REF] Summerfield | Tectonics, geology, and long-term landscape development[END_REF]. In order to test these interpretations, arguing either for antiquity or youthfulness of the drainage, geological markers documenting past drainage configurations are required.

Here we investigate the Cenozoic drainage development of West Africa (Fig. 2) by mapping dated and regionally correlated lateritic paleolandscape remnants that are widely and densely distributed over the sub region. This allows reconstructing two Paleogene stages of the West African paleogeography showing that the main river courses of the sub region stabilized since at least the Latest Early Oligocene (~29 Ma).

This result opens new perspectives on linking the continental geomorphic record to the sedimentary evolution of the Equatorial margin of Africa. We also document the key role of the growth of the Hoggar hot spot swell in reorganizing the relief regionally and leading to the installation of the modern drainage.

Regional context and earlier works

The drainage of West Africa is organized around three topographic massifs: the Guinean Rise forming the continental divide the closest to the coast and the Hoggar and Jos Plateau Cenozoic hot spots (Fig. 2). Rivers directly flowing to the Atlantic Ocean are increasingly longer to the east of the Guinean Rise, the continental divide being located ~1400 km from the coast at the northernmost headwaters of the Volta river system (Fig. 2). The Niger River catchment may be divided in three sub-drainage areas.

The main area drains mostly the Hoggar swell and the Jos plateau. The High Niger catchment is NE-trending and comprises the drainage upstream the elbow in the river course. The Benue River catchment constitutes the third sub-drainage area (Fig. 2). The Gambia and Senegal catchments fringe the northwestern watershed of the High Niger and flow towards the Central Atlantic Ocean (Fig. 2). The High Niger and Sourou inland deltas are large functional inland alluvial plains (Fig. 2).

Since exploration by [START_REF] Chudeau | Missions au Sahara -Tome II[END_REF], the elbow between the High and Low Niger (Fig. 2) has been considered to result from a Quaternary capture of the inland delta, which would have been drained by river(s) flowing north-or northwestward into the Taoudeni basin or the Hodh depression until capture by the Low Niger [START_REF] Chudeau | La capture du Niger par le Taffassasset[END_REF][START_REF] Furon | L'ancien delta du Niger[END_REF][START_REF] Furon | Introduction à la géologie du Soudan occidental[END_REF][START_REF] Urvoy | Les bassins du Niger. Etude de géographie physique et de paléogéographie[END_REF]. But neither sediment accumulations nor paleoriver courses exist that would support the existence of such rivers (Beaudet et al., 1977a). The delta was interpreted to result from damming of the High Niger River by Late Quaternary sand dunes and [START_REF] Urvoy | Les bassins du Niger. Etude de géographie physique et de paléogéographie[END_REF] and [START_REF] Tricart | Géomorphologie dynamique de la moyenne vallée du Niger (Soudan)[END_REF] favored "very recent" aggradation-driven overspill to explain its connection to the Low Niger. However, Beaudet et al. (1981a) argued that the delta has used a persistent spillway at the location of its current downstream sill. Other authors such as [START_REF] Voute | Geological and morphological evolution of the Niger and Benue valleys[END_REF] inferred a recent Quaternary connection between the Lowermost Niger River and the rest of the Niger catchment upstream its confluence with the Benue (Figs. 1 and2). According to [START_REF] Hubert | Sur un important phénomène de capture dans l'Afrique occidentale[END_REF] and [START_REF] Palausi | Contribution à l'étude géologique et hydrogéologique des formations primaires au Soudan et en Haute-Volta[END_REF], the Sourou delta (also known as the Gondo plain, Fig. 2) was formed by the Sourou River that flowed northeastward into the Low Niger before its connection to the Volta drainage in the Latest Quaternary.

The prevailing view on the West African river network, and particularly the Niger, is therefore that of a youthful drainage resulting from Quaternary rearrangement(s). This view is still deeply entrenched in the literature [START_REF] Goudie | The drainage of Africa since the Cretaceous[END_REF] although it remains largely conjectural due to the lack of geological markers of past river courses and in absence of age constraints on such markers. Recent advances made by dating [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 mapping remnants of a unique sequence of spatially correlated Cenozoic lateritic incision markers at the scale of West Africa [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] offer an opportunity to test the age of the drainage and its potential evolution on the Cenozoic timescale.

Lateritic paleolandscape markers and field investigations

The present day topography of West Africa results from the dissection of the African Surface, a landscape being the product of a protracted period of lateritic weathering that culminated and ended during Eocene peak greenhouse [START_REF] Boulangé | La distribution des bauxites sur le craton ouestafricain[END_REF][START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Remnants of this landscape bear thick (> 80 m) bauxite-capped weathering profiles, which are preserved throughout West Africa as mesas (Fig. 3) defining the topographic envelope of the sub region. Lateritic bauxites developed on all pre-Cenozoic rocks exposed in West Africa during the Paleocene and Eocene. Stratigraphic and sedimentologic investigations have established the synchronicity and link between inland bauxite production and Early to Mid-Eocene marine chemical sedimentation in the basins of the sub region. This allowed constraining a stratigraphic age for the bauxites [START_REF] Millot | The geology of clays[END_REF] and correlating the African surface with Paleogene marine series now buried in those basins (Fig. 4).

In the literature, the term "African Surface" generally has a genetic implication.

The African Surface was originally defined by L.C. King as a flat planation surface of continental extent that had been leveled down to sea level until the Early Tertiary [START_REF] King | On the Age of the African land-surfaces[END_REF]1967). In King's paradigm, escarpment retreat from the coasts towards deep continental interiors as a result of pedimentation was the shaping agent of a flat African continent. Later work showed that the African Surface could have a significant original topography and relief and even comprised the Great Escarpment of Southern Africa (e.g., [START_REF] Partridge | Geomorphic evolution of southern Africa since the Mesozoic[END_REF]. For [START_REF] Burke | The African erosion surface: A continental scale synthesis of geomorphology, tectonics, and environmental change over the past 180 million years[END_REF], the topography and the main escarpments of the continent result from the uplift and incision of the African Surface as a consequence of the growth of the plate-scale basin-and-swell relief (Fig. 1).

Such a model implies a low-lying and flat continent and therefore a low-lying and flat African Surface before 30 Ma i.e., at the time mantle dynamics is inferred to have started imprinting the continent's topography.

In this work, we argue that West African bauxitic remnant landscapes do not define a planation surface as defined by King. Instead of pedimentation, weathering enhanced by efficient drainage of the lateritic regolith that was maintained by slow river incision to produce a hilly bauxite-capped landscape of moderate local relief (<100 m) and wavelength (5-20 km) [START_REF] Chardon | Planation, bauxites and epeirogeny: One or two paleosurfaces on the West African margin?[END_REF]Fig 3). As a smooth but differentiated landscape efficiently drained by a river network [START_REF] Grandin | Aplanissements cuirassés et enrichissement des gisements de manganèse dans quelques régions d'Afrique de l'Ouest[END_REF][START_REF] Chardon | Planation, bauxites and epeirogeny: One or two paleosurfaces on the West African margin?[END_REF], the African Surface had a significant long-wavelength (≥100 km) topography, relief and slopes and could not have been flat at sea level elevation except along coasts. This is further documented by the present study (see below) and shows that the African surface cannot be used as a simple flat elevation datum for measuring surface uplift.

Outside the Cenozoic sedimentary basins, stepwise dissection of the African Surface produced mosaic landscapes made of successive remnants of lateritic landforms with specific geomorphic, sedimentological and petrological characteristics [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] Boulet | La géomorphologie et les principaux types de sols en Haute-Volta septentrionale[END_REF][START_REF] Eschenbrenner | La séquence de cuirasses et ses différentiations 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] Gunnell | Radiometric ages of laterites and constraints on long-term denudation rates in West Africa[END_REF][START_REF] Burke | The African erosion surface: A continental scale synthesis of geomorphology, tectonics, and environmental change over the past 180 million years[END_REF]; recent reviews by [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][START_REF] Grimaud | Neogene cratonic erosion fluxes and landform evolution processes from regional regolith mapping (Burkina Faso, West Africa)[END_REF]. The Intermediate Surface designates a landscape whose remnants are preserved below bauxitic relicts of the African Surface and above younger lateritic pediments (Fig. 3). Relicts of the Intermediate Surface bear ≥ 40 m-thick weathering profiles topped by a ferricrete that is easily distinguishable from the bauxites of the African surface or the pediments. Outside Cenozoic sedimentary basins, Intermediate Surface relicts typically delineate wide river valleys between bauxitic relief remnants forming interfluves (Fig. 3). The Intermediate Surface therefore comprises residual reliefs inherited from the Bauxitic African Surface; it has a differentiated topography and local relief and -like the African Surface -may not be considered as an originally flat regional topographic marker.

The end of bauxitic weathering coincides with the onset of the dissection of the African Surface. Similarly, the weathering and ultimate development of the Intermediate ferricrete ended with incision of the Intermediate Surface [START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. These paleolandscape abandonment ages are constrained by Ar-Ar age groups of K-Mn oxides sampled in the weathering profile of each relict surface at the Tambao type locality in Burkina Faso [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]Fig. 2) [START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

Given their type geomorphology and regolith and the wide distribution of their relicts over the sub region [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], the African Surface (referred to as S1) and the Intermediate Surface (S2) are decisive paleolandscape datum that are used here to investigate the regional relief and drainage configuration before their respective abandonments. Our study primarily relies on field surveys allowing assessing the spatial relationships between S1 and S2 remnants, Cenozoic sediments and the modern drainage network and alluviums over key areas of the sub region (Figs. 4 to 6). Given the dense spatial distribution of S1 and S2 landscape remnants, we also constructed the geometry of S1 and S2 surfaces at the scale of the sub region in order to extract their large-scale drainage networks.

Regional topographies and drainages reconstruction protocol

1. Construction of S1 and S2 present-day geometry

To reconstruct S1 and S2 surfaces geometries, we built an irregular triangulated mesh using the DSI method designed for full 3D representation of complex geological objects [START_REF] Mallet | Discrete Smooth Interpolation in Geometric Modeling[END_REF]Figs. 7a and 7d). Spatially varying resolution is a motivation for using triangulated surfaces as compared to 2D grids, because they allow low surface nodes densities (larger triangles) in smoothly varying areas and higher densities (smaller triangles) in high curvature areas. Additionally, a range of constraints is used to ensure geometrical consistency of the surfaces (e.g., no self-intersections), as well as consistency between surfaces (e.g., no intersection). In practice, DSI solves for the optimal location of the surface nodes to minimize a weighted sum of the surface roughness and constrain misfit [START_REF] Mallet | Discret modeling of natural objects[END_REF][START_REF] Mallet | Geomodeling[END_REF][START_REF] Caumon | Surface-based 3D modeling of geological structures[END_REF]. Strict constraints restrict the degree of freedom of surface nodes: a node ascribed to a surface remnant data point is frozen to a given location in space. In addition to this data-points compliance, soft constraints are honored in a least-squares sense: nodes are forced to lie above the present day topography as well as above or below the other triangulated surface (S1 above S2 above topography). This procedure allows ensuring the hydrological consistency of the surfaces and accounting for the composite nature of the S2 Intermediate landscape that comprises residual S1 reliefs (see Supporting Information). Finally, the progressive increase in triangulated surface resolution during construction, from an initial state based solely on S1 and S2 remnant data-points, allows for both the data compliance and geometrical consistencies to be preserved in domains where nodes are created (triangles are subdivided) beyond the initial resolution of the surface remnant data sets. We verified the robustness and sensitivity of our workflow using a synthetic case calibrated to the dimensions of typical West African surfaces' topography and relief (see Supporting Information).

2. Denudation of S1 and S2

Reconstructed S1 and S2 surface geometries represent the present-day geometry of past regional topographies that have potentially undergone lithospheric deformation after their abandonment (Figs. 7a and7d). Among the sources of deformation, the easiest one to account for is the flexural response of the lithosphere to erosion and sedimentation since their abandonment. We therefore corrected S1 and S2 triangulated surfaces from the flexural isostatic deformation of the lithosphere due to denudation and sediment deposition that distorted their geometry since their abandonment (see [START_REF] West | Links between climate, erosion, uplift, and topography during intracontinental mountain building of the Hangay Dome, Mongolia[END_REF] for a comparable approach).

At the locations of paleolandscape remnants, by definition, no denudation occurred since their abandonment. Those paleolandscape remnants constitute densely distributed summits of the region whose envelope, compared to the present day topography, constrains the amount of incision since surfaces abandonments (e.g., Fig. 3).

Flexural isostasy wavelength of the old (~2 Ga) and cold West African lithosphere is expected to be much longer (several hundreds of km) than the typical horizontal distance between paleolandscape remnants (i.e., a few tens of km; Figs. 7a and7d). The characteristic wavelength of incision of S1 and S2 is thus small enough to assume that the local relief, and therefore the denudation produced by incision, is not affected by flexural isostatic deformation. Hence, we computed denudation maps of S1 and S2 by subtracting the present-day topography from their surface geometries (Figs. 8a and8c).

3. Flexural isostatic correction of surfaces geometries

We computed the change in crustal thickness due to post-S1 and post-S2 erosion and sedimentation and the resulting flexural isostatic response (Figs. 8b and8d). To do this, we used the formulation of Braun et al. (2013) that calculates the deflection of a thin elastic plate assuming the effective elastic thickness (EET) is linked at depth to a given isotherm. We used a 128x128 grid, 2600x1900 km in size. We assumed a 36 km thick crust with a density of 2800 kg.m -3 , a 150 km thick lithosphere with a mantle density of 3300 kg.m -3 and a basal temperature of 1300°C. We have tested effective elastic thicknesses ranging from 30 to 50 km (isotherms 250° to 450°). For a 50 km EET, isostatic uplift ranges from 0 to 600 m for S2 and -200 to 800 m for S1 (Figs. 8b and8d). For lower values of EET, uplift amplitudes are higher (up to 650 m for S2, -300 to 1000 m for S1) with identical map patterns. A simple Airy isostatic compensation calculation using the same grid leads to the same regional uplift pattern as that produced with a 50 km EET, but with a noise at the cell scale that is not realistic with regard to a flexural isostasy wavelength. We therefore chose to use the 50 km EET for correcting S1 and S2 surfaces geometries from denudational flexural isostasy (Figs. 8b and8d). 8b and8d represent rock uplift i.e., the displacement of rocks with respect to the geoid [START_REF] England | Surface Uplift, Uplift of Rocks, and Exhumation of Rocks[END_REF], whereas it is surface upliftdefined as the denudation subtracted from rock uplift -that should be used for correcting S1 and S2 surface geometries. But those surface geometries are constrained by S1 and S2 paleolandscape remnants, which have undergone no denudation since their abandonment. Accordingly, rock uplift was used directly for correcting S1 and S2 (Figs. 7b and7e). Corrections generally tend to reduce the large-scale topographic amplitude of surface geometries without modifying their first-order relief map pattern (Figs. 7a and 7b; 7d and 7e). Drainages were then extracted from the corrected surface geometries (Figs. 7c and 7f; see Supporting Information). These drainages (Figs. 7c and 7f) are consistent with higher-resolution field investigations made over key areas (Figs.

Maps of Figures

4 to 6), validating the construction and isostatic correction procedures. The resulting Late Oligocene (S2) and Mid-Eocene (S1) regional physiographic stages are then interpreted from paleogeographic maps (Fig. 9), which combine the extracted drainages and the onshore paleoenvironmental sedimentary record compiled from various sources [START_REF] Faure | Reconnaissance géologique des formations sédimentaires postpaléozoïques du Niger oriental[END_REF][START_REF] Greigert | Description des formations crétacées et tertiaires du bassin des Iullemeden (Afrique occidentale)[END_REF][START_REF] Greigert | Essai de description des formations géologiques de la République du Niger[END_REF][START_REF] Monciardini | La sédimentation éocène au Sénégal[END_REF][START_REF] Charpy | Contribution à l'étude lithostratigraphique et chronostratigraphique du Tertiaire du bassin de Côte d'Ivoire[END_REF][START_REF] Lang | Le sidérolithique du Tertaire Ouest-africain et le concept de Continental Terminal[END_REF][START_REF] Conrad | The Continental-Terminal, Its Role in the Geodynamic Evolution of the Senegal-Mauritania Basin during the Cenozoic[END_REF][START_REF] Reijers | Stratigraphy and sedimentology of the Niger Delta[END_REF] the present work).

Geomorphic configuration along key portions of the drainage

1. Iullemmeden basin

Differential elevation of S1 and S2 tends to decrease from the exposed West African Paleoproterozoic basement towards the southern margin of the Iullemmeden basin [START_REF] Colin | First Ar-40/Ar-39 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]Figs. 2 and4). There, fluvial sediments of the Continental Terminal overly S1 bauxitic weathering profiles developed upon Pre-Cenozoic bedrocks [START_REF] Greigert | Description des formations crétacées et tertiaires du bassin des Iullemeden (Afrique occidentale)[END_REF][START_REF] Gavaud | Les grands traits de la pédogenèse au Niger méridional[END_REF], Beaudet et al., 1977b; our own field observations). The Continental Terminal also tops Mid-Eocene marine series in the rest of the basin and a temporal and spatial correlation is established between the S1 surface and those series (Fig. 4). Instead of dipping under the basin infill, S2 is preserved as conspicuous mesas capping the Continental Terminal and its adjoining basement [START_REF] Dresch | Observations morphologiques dans le Sahel du Niger[END_REF][START_REF] Faure | Reconnaissance géologique des formations sédimentaires postpaléozoïques du Niger oriental[END_REF][START_REF] Greigert | Description des formations crétacées et tertiaires du bassin des Iullemeden (Afrique occidentale)[END_REF][START_REF] Gavaud | Les grands traits de la pédogenèse au Niger méridional[END_REF]Beaudet et al., 1977b;[START_REF] Kogbe | Origin and composition of the ferruginous oolites and laterites of Northwestern Nigeria[END_REF]; our own field observations; Fig. 4). Chronostratigraphic constraints on the Continental Terminal are Oligocene mammals, fishes and woods [START_REF] Radier | Contribution à l'étude stratigraphique et structurale du détroit soudanais[END_REF][START_REF] Radier | Contribution à l'étude géologique du Soudan oriental. Deuxième partie : Le bassin crétacé et tertiaire de Gao[END_REF] as well as Mid-Eocene to Oligocene pollens [START_REF] Lang | The Continental Terminal in West Africa[END_REF].

Weathering and duricrusting of S2 at 29-24 Ma as constrained by Ar-Ar geochronology of K-Mn oxides in Tambao [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]Fig. 4) are consistent with biostratigraphy and allow restraining the age of the Continental Terminal from Lutetian to Rupelian (49-29 Ma).

S2 plateaus define a very smooth regional surface (Fig. 4), which has been dissected by the Niger River and its tributaries (Fig. 5a). Incision of S2 led to the exposure of its weathering profile developed on sandstones and argillites of the Continental Terminal [START_REF] Réformatsky | Quelques observations sur les latérites et les roches ferruginisées de l'ouest de la colonie du Niger français[END_REF][START_REF] Gavaud | Les grands traits de la pédogenèse au Niger méridional[END_REF]; our own field observations; Fig. 5a). The dense spatial distribution and large size (up to tens of km) of S2 plateaus as well as their clear-cut photointerpretation signature validated by field control prompted us to reconstruct the geometry of S2, which displays two main features (Fig. 5b). The most prominent one is a ca. 200 km wide, NW trending asymmetrical trough with a short southwestern flank steeper than its northeastern flank and a hinge line roughly coinciding with the present day course of the Niger River. The second feature is a N-S trending valley roughly coinciding with the trace of the Dallol Bosso that widens towards its junction with the NW trending trough (Fig. 5b). We interpret this topographic pattern to reflect the main drainage axes of the region before abandonment of the S2 landscape in the Late Oligocene. The map shows that those drainage axes were already the Niger and the Dallol Bosso at their current location at the time. This interpretation is reinforced by the fact that the S2 ferricrete cements alluvial gravels or sand blankets (even where atop weathered argillites) on plateaus adjoining those modern rivers, forming the alluvial terraces along those paleo-river courses (see also [START_REF] Greigert | Description des formations crétacées et tertiaires du bassin des Iullemeden (Afrique occidentale)[END_REF]Beaudet et al., 1977b). The map also suggests that the present-day Sokoto and Zamtara Rivers were already tributaries of the Niger River in the Late Oligocene (Fig. 5b).

2. High Niger valley and inland delta

In the northernmost High Niger catchment (Fig. 2), S1-S2 differential elevation may attain 400 m and decreases towards the internal delta where S1 and S2 tend to merge [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 is confirmed by field observations at the northern margin of the delta, where S1 and S2 duricrusts cap a laterally continuous weathering profile developed on Neoproterozoic sandstones (Fig. 6a). Around Ségou, both duricrusts are seen to mantle low (< 30 m-high) residual hills dominating a pediplain (Fig. 6b). Comparable hills emerge from the inland delta alluviums further to the northeast (data points in Fig. 6a). Given the low (< 40 m) relief of the composite S1/S2 landscape, both types of duricrust may not only occupy hills but are also seen in man-made pits in the pediplain (Figs. 6c and6d). In the Niger River course itself, a bauxitic weathering profile capping the Neoproterozoic sandstones even crops under alluviums (Fig. 6e). Mapping of S1 and S2 duricrust remnants on the slopes of the Niger valley around the delta (Fig. 6a) indicates that the composite S1-S2

weathering profile carpets the entire valley (Fig. 6f). Therefore, the valley likely had its current cross-profile by the end of bauxitic weathering and has undergone very limited relief evolution since then (Fig. 6f). In other words, the High Niger drainage axis already existed by the Mid-Eocene (i.e., ~45 Ma). The stability of the valley is probably structurally controlled for it occupies a syneclise in the Neoproterozoic sandstones [START_REF] Urvoy | Les bassins du Niger. Etude de géographie physique et de paléogéographie[END_REF]Fig. 6f). The Late Oligocene S2 weathering profile capping the Neoproterozoic sandstones is directly overlain by the Quaternary inland delta (Figs. 6e and6f). This precludes preservation of Pre-Quaternary Cenozoic fluvial sediments in the High Niger valley as inferred by [START_REF] Urvoy | Les bassins du Niger. Etude de géographie physique et de paléogéographie[END_REF] and [START_REF] Erhart | Les latérites du moyen Niger et leur signification paléoclimatique[END_REF], who mistook the S1 / S2 weathering profile developed on the Precambrian sandstones for the Continental Terminal (e.g., [START_REF] Bassot | Carte géologique du Mali à 1/1 500 000[END_REF]. The High Niger valley has therefore remained a sediment bypass zone since at least the Mid-Eocene.

3. Niger River elbow region

The Niger River elbow region in between the inland delta and the Iullemmeden basin (Fig. 2) has been surveyed by Beaudet et al. (1977aBeaudet et al. ( , 1977bBeaudet et al. ( , 1981a) ) and [START_REF] Michel | Les modelés et dépôts du Sahara méridional et Sahel et du sudouest africain[END_REF]. Those authors have shown that the S2 ferricrete capping the mesas of the region (i.e., the relicts of the "fundamental topography" of Beaudet et al., 1977a) systematically dips towards the modern river and cement alluvial gravels in the vicinity of its course, arguing for its antiquity. Fieldwork also led Beaudet et al. (1981a) to the same conclusion for the Tilemsi River, a tributary of the Niger draining the western flank of the Adrar des Ifoghas massif (Fig. 2). Those observations are consistent with our investigation upstream and downstream the elbow region (Figs. 5 and6). The 29-24 Ma minimum age of S2 argues for the stability of the Niger drainage since the Late Oligocene.

4. Sourou valley

Our field surveys reveal that the Sourou delta (Fig. 2) emplaced in a shallow trough carpeted with a weathering profile developed on Neoproterozoic sandstones and topped by bauxites and Intermediate ferricretes. S1 and S2 duricrusts cap hills emerging from the delta alluviums and occur along the margins of the plain. S1 and S2 relicts see their elevation and differential elevation increase eastward from the Sourou valley [START_REF] Boulet | La géomorphologie et les principaux types de sols en Haute-Volta septentrionale[END_REF][START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF], the valley being bounded to the west by the Bandiagara plateau (See Figs. 2 and6f). Our observations and interpretation are consistent with the bauxite occurrences map of [START_REF] Petit | Carte géomorphologique du Burkina Faso à 1/1 000 000[END_REF] and well logs in the delta [START_REF] Koussoubé | Hydrogéologie des séries sédimentaires de la dépression piézometrique du Gondo (bassin du Sourou) -Burkina faso / Mali[END_REF]. But the Cenozoic sediments inferred by [START_REF] Urvoy | Les bassins du Niger. Etude de géographie physique et de paléogéographie[END_REF] to underlie the delta (the "Continental Terminal" of the later authors) is precluded because the S1/S2 weathering profile was again mistaken for pre-delta sediments. Therefore, the morphosedimentary structure of the Sourou valley is indicative of a drainage axis existing since at least the Mid-Eocene (i.e., minimum age of the bauxites), which remained a sediment bypass until installation of the delta in the Quaternary.

The Sourou River extends via a wind gap into a river flowing eastward to the Low Niger (Fig. 2). As opposed to the uppermost north-flowing High Volta, a south-flowing Sourou could not have generated the delta considering the very limited size and relief of its catchment. The delta is indeed fringed to the northeast by the Niger watershed (Fig. 2), which would have remained stationary since the Late Oligocene (section 4.1; Fig 5b).

Furthermore, the Sourou River still receives one-sixth of the High Volta waters and still flows northward up to 50 km north of their confluence [START_REF] Palausi | Contribution à l'étude géologique et hydrogéologique des formations primaires au Soudan et en Haute-Volta[END_REF]. We therefore interpret the connection of the Sourou to the Volta as resulting from overspill of the delta formed by the Sourou River that used to flow into the Niger. Delta formation and ultimate drainage reversal of the river must have resulted from surface uplift in the area of the current wind gap, which is also attested by efficient erosion of the S1 and S2 landscape remnants in between the Sourou and the Niger elbow (e.g., Beaudet al., 1981b). imposed oceanic outlets to all drains. Some water gaps have thus been forced, such as that linking the High Niger to the Senegal catchment north of Bamako (Fig. 7b). A divide was also generated NW of Gao between the Low and High Niger (Fig. 7b). As field evidence attests to the existence of a continuous Niger River course by the end of S2 landscape stage in that area (section 4.3), a "Gao divide" may be reasonably considered as irrelevant given the resolution of the method. As a way of consequence, the water gap connecting the High Niger to the Senegal drainage is precluded because the Niger could not overspill into the Senegal drainage and, at the same time, flow across the Gao sill to form a major river valley across the Iullemmeden basin (Fig. 5b).

Reconstitution of two

Extracted drainage displays a Sourou River flowing south into a Volta drain (Fig. 7b). However, this connection occurred only in the Quaternary (section 4.4). Therefore, the divide generated between the Sourou and the Niger valleys near Gao (Fig. 7b) may be ignored in the interpretation (Fig. 7c). Nonetheless, this divide must somehow reflect uplift-driven formation of the current wind gap isolating the Sourou River from the Niger (Fig. 2) and later drainage reversal of the Sourou River (see below). One must therefore consider a Sourou River already flowing northeastward into the Low Niger in the Late Oligocene (Fig. 7c).

2. Mid-Eocene stage (S1)

Corrected S1 surface geometry displays a marginal upwarp forming the continental divide with a steep seaward slope and a gentle northward slope, which includes large valleys plunging NE i.e., the High Niger, the Sourou, and a Paleo-Volta (Figs. 7e and7f). A modern-like Senegambia catchment comparable to that of the Late Oligocene stage (Fig. 7c) is also marked (Fig. 7f), as attested by extensive mapping of S1 and S2 by [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]. A divide isolates the Sourou from the Niger valley and a water gap connects the Sourou valley to an upper drain of the Paleo-Volta drainage (Fig. 7e). This configuration is comparable to the drainage extracted from S2 geometry (Fig. 7b). Both this divide and water gap may therefore be precluded for the same reasons as those invoked in the S2 case, especially considering the fact that the southern coastline of the Tethys was located SW of Gao in the Mid-Eocene (i.e., at the time S1 was being ultimately weathered and duricrusted; Fig. 7f). Hence, the Sourou River must have flown directly to the sea near Gao by the Mid-Eocene (Fig. 7f). The water gap allowing the lower High Niger to flow into the Senegal drainage would imply a loop in the river very near to the seashore, allowing back flow across the marginal upwarp. Such a configuration is unlikely given the very low relief and slope of S1 and the lack of evidence for paleo-river course remnants in that area (Beaudet et al., 1977a).

One must therefore consider that a Mid-Eocene High Niger used to flow northeastward to the sea (Fig. 7f).

Interpretation

1. Oligocene paleogeography: antiquity of the West African drainage

The modern Niger drainage was established by the Late Oligocene and already comprised the High Niger and its tributaries draining the Hoggar swell (Fig. 9a). The uppermost High Niger watershed was probably located at the southernmost limit of its uncertainty domain shown in Fig. 7c given the reconstituted S2 longitudinal profiles of the High Niger and short coastal River drains, all pointing to a continental divide near its current location [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. The Senegambia catchment had also already acquired its current shape and its main drains (Fig. 7c). The fact that S2 paleolandscape remnants systematically dip towards the main present day river courses of the region [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF] and the similarity between the drainage extracted from S2 corrected geometry and that of the current drainage (Figs. 9a and9b) further attest to the stability of the main drains since the Late Oligocene. Connection of the Sourou River to the Volta and limited headward migration of the uppermost Niger watershed would be the only adjustments of the drainage since the Late Oligocene (Figs. 9a and9b).

2. Mid-Eocene paleogeography

In the Mid-Eocene, the drainage of West Africa was organized on both slopes of a marginal upwarp isolating a marine intracratonic basin connected to the Tethys [START_REF] Guiraud | Phanerozoic geological evolution of Northern and Central Africa: An overview[END_REF] from the Central and Equatorial Atlantic Ocean (Figs. 9c). Short rivers drained the seaward slope of the marginal upwarp, whereas longer drains, such as the High Niger, the Sourou and a Paleo-Volta flowed northward (Fig. 9c). The alluvial facies of the nascent Niger delta suggests that a (short) seaward-flowing Benue drain fed the delta (Fig. 9c). As both slopes of the marginal upwarp are carved by major valleys at a high angle to its trend, any river crossing its crest would inevitably connect both seas, forming seaways that are not substantiated by the geological record. This, together with the fact that S1 bauxites and marine Paleogene sediments are mutually exclusive [START_REF] Millot | The geology of clays[END_REF], argues for a continental divide coinciding with the crestal region of the marginal upwarp in the Mid-Eocene (Figs. 9c). The uncertainty on that divide for the uppermost High Niger (Figs. 7f and9c) may be reasonably restrained towards its southwestern limit as suggested by the geometry of S1 along paleo-long profiles of both the High Niger and short coastal rivers [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF].

3. Establishment of the modern drainage

Connection of the Niger catchment to the Equatorial Atlantic Ocean is at least 29 Ma old i.e., the age of the oldest preserved weathering of the S2 landscape that fossilizes the current river courses (e.g., Fig. 5). Actually, acquisition of the modern drainage configuration most probably dates back to 34 Ma assuming it triggered building of the coastal Niger delta at the Eocene -Oligocene boundary (biostratigraphic age in [START_REF] Doust | Niger delta[END_REF]. The Niger catchment has indeed remained the delta's most prominent supplier. Despite episodic overspill into the Benue's catchment during the recent Quaternary, the Chad basin (Figs. 1 and2) remained an active sag basin trapping sediments throughout the Cenozoic [START_REF] Burke | Chad Basin -Active Intra-Continental Basin[END_REF][START_REF] Talbot | Environmental responses to climatic change in the West African Sahel over the past 20 000 years, in The Sahara and the Nile[END_REF].

Furthermore, the Benue drainage appears to have remained isolated from the Congo basin (Fig. 1) by the Cameroon volcanic Line (e.g., [START_REF] Ségalen | Les sols et la géomorphologie du Cameroun[END_REF][START_REF] Fritsch | Chronologie relative des formations cuirassées et analyse géographique des facteurs de cuirassement au Cameroun[END_REF]Fig. 2) since the Eocene. Therefore, the Benue River catchment must have kept its current limited size throughout the Cenozoic and could not contribute significantly to the Niger delta compared to the Niger River.

Modification of the regional physiography between the Mid-Eocene (~45 Ma) and the Late Early Oligocene (~29 Ma) resulted in a major inland shift of the continental divide, east of the Guinean Rise, of up to 1000 km and required entrenchment of the marginal upwarp to allow opening of at least two major drainage basins (Niger and Volta) on the Equatorial Atlantic Ocean (Figs. 9a,9c and 10).

Emplacement of alluvial systems of the Continental Terminal followed post-Ypresian (~48 Ma) sea retreat (Figs. 10a and10b). The absence of evaporites in the Iullemmenden fan (excluding the hypothesis of a playa-bearing closed depression), paleo-current directions in the fan (our own field data), and map configuration in Fig. 5b suggest that the course of the Niger River was established during building of the fan.

Aggradation-driven overspill of the Iullemmeden Fan is therefore a plausible mechanism for connecting the Niger to a Benue drain. Nevertheless, a capture by headward erosion of a Benue tributary may not be ruled out, but would have had to occur before Iullemmeden fan installation. Rearrangement of the Volta drainage involved establishment of a water gap across the marginal upwarp as well as drainage reversal, implying seaward tilt of the catchment (Figs. 9 and 10). Several lines of evidence reported below suggest that such a tilt may have resulted in the formation of an internal delta against the ancient divide before connection of the Volta into the seaward drainage. This sill coincides with one of the highest knickzones in West Africa (KZ; Fig. 2) that exists since at least 11 Ma [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. This knickzone also currently acts as the spillway for the vast plain occupying the Paleozoic Volta basin (Fig. 2) and where airborne magnetic data allowed imaging a major meandering paleochannel transecting the main current drains (Jessell et al., 2015). Such a paleochannel suggests the existence of a transient flood plain formed at the time of drainage reversal before overspill into the Atlantic drainage.

Late Oligocene paleogeographic reconstruction (~29-24 Ma; Fig. 9a) post-dates the emplacement of large alluvial systems of the Continental Terminal (i.e., megafans), which had undergone deformation and erosion after settling of the S2 ferricrete (Figs. 10a and10b). In the eastern part of the study area, this deformation is documented by (i) the occurrence of Oligocene fluvial sediments equivalent to the Continental Terminal at the top of the Atakor range in the Hoggar swell above 2500 m elevation [START_REF] Rognon | Précisions chronologiques sur l'évolution volcano-tectonique et géomorphologique de l'Atakor (Hoggar): Apport des données radiométriques (K/Ar) et paléobotaniques (bois fossiles)[END_REF]Figs. 2 and 11), (ii) erosion of the Continental Terminal at the northern margin of the Iullemmeden basin and (iii) the geometry of the basin indicative of warping and southward migration of depocenter on the propagating piedmont of the swell (Fig. 11).

Such deformation and denudation patterns sign the amplification and propagation of an epeirogenic wave [START_REF] Faure | Relations dynamiques entre la croûte et le manteau d'après l'étude de l'évolution paléogéographique des bassins sédimentaires[END_REF], which is still active as attested by anomalies in river profiles showing southwestward growth of the swell [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]Fig. 9b).

Swell growth is interpreted to have produced reversal of the Volta drainage and later slope decrease of the High Niger and Sourou rivers that triggered the formation of their internal deltas [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF] (Fig. 10). The short Sourou River could not keep pace with this deformation and underwent flow reversal and overspill into the Volta, whereas the High Niger had a higher stream power thanks to a larger catchment draining highlands under intense precipitations and was able to maintain its course.

Practically, epeirogenic wave propagation due to the growth of the Hoggar swell explains the artificial "Gao divides" and "captures" of the High Niger by the Senegal and the Sourou by the Volta on S1 and S2 corrected geometries (Figs. 7b and7e) that have recorded this deformation.

To summarize, sea retreat from Northwestern Africa starting in the Mid-Eocene, Continental Terminal fan emplacement and their warping and incision, drainage reversal of the Volta valley and the development of internal deltas are successive consequences of the Hoggar hot spot swell growth (Fig. 10).

Discussion

The present work shows that the drainage map pattern of West Africa remained essentially stationary since the Late Early Oligocene (29 Ma) and most probably the Earliest Oligocene (~ 34 Ma). Antiquity of the drainage argues against the view that typically stepped longitudinal profiles and elbow map patterns of African rivers are Quaternary features of aggressive coastal drains having captured intracratonic basins by headward erosion [START_REF] Goudie | The drainage of Africa since the Cretaceous[END_REF]. Very long term drainage stability also questions earth surface process models arguing for dynamically and therefore "permanently" reorganizing drainage networks in cratonic and passive margin settings [START_REF] Willett | Dynamic Reorganization of River Basins[END_REF]. Finally, the West African example shows that classically invoked features such as abrupt elbows and/or knick points in river courses may not necessarily be diagnostic of drainage capture by river piracy due to headward erosion [START_REF] Bishop | Drainage rearrangement by river capture, beheading and diversion[END_REF].

Mid-Eocene (~ 45 Ma) paleogeography of West Africa suggests that the installation and stabilization of the current drainage between 34 and 24 Ma followed regional-scale drainage rearrangement. The mechanism(s) responsible for this rearrangement may not all be unequivocally identified. Nonetheless, long-wavelength lithospheric deformation related to the growth of the Hoggar swell has been instrumental in enhancing modification in regional slope(s) that contributed to that rearrangement. Therefore, river piracy via headward erosion may not be required to achieve the documented drainage rearrangement. The present work attests to the permanency of long portions of river valleys since the Early Paleogene (High Niger, Sourou, Volta) although some may have undergone flow reversal. Such longevity is explained by the fact that these valleys are structurally controlled, as early pointed out by [START_REF] Urvoy | Les bassins du Niger. Etude de géographie physique et de paléogéographie[END_REF] and [START_REF] Palausi | Contribution à l'étude géologique et hydrogéologique des formations primaires au Soudan et en Haute-Volta[END_REF].

The stability of West African drainage map patterns warrants investigation of river long profiles evolution over geological time scales [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. But uplift histories retrieved from river profiles inversion [START_REF] Paul | The African landscape through space and time[END_REF] would not be valid as extrapolated back in time beyond the Late or Early Oligocene (30-34 Ma), which corresponds to the time of regional drainage rearrangement. Besides, river inversion procedures are based on stream power law incision models that explicitly rely on headward migration of knickzones, which have been shown to be stationary along West African rivers during the Neogene [START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF]. Moreover, the growth of several hot-spot swells over Northern and Eastern Africa from the Mid-Eocene onward [START_REF] Burke | The African Plate[END_REF] is likely to have triggered drainage reorganizations comparable to that documented here around the Hoggar. Therefore, the main rivers draining those swells must have composite profiles whose inversion in terms of uplift may be challenged.

Drainage stabilization since the Early Oligocene sets a new framework for understanding the West African craton-margin source-to-sink system. Indeed, large catchments such as the Niger, the Volta and Senegambia have kept a constant geometry since 29-34 Ma. The Niger catchment has been the Niger delta's most prominent supplier since that time. Therefore, increasing Neogene clastic sedimentary flux to the delta that has attained a peak in the Pliocene [START_REF] Robin | Cenozoic siliciclastic fluxes evolution around Africa[END_REF] has responded to the growth of the Hoggar swell and potentially to climatically driven change in erosion efficiency, but not to drainage reorganization. Accordingly, the Latest Pliocene drop in clastic fluxes to the delta [START_REF] Jermannaud | Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger Delta: A record of eustasy and aridification of Africa[END_REF][START_REF] Robin | Cenozoic siliciclastic fluxes evolution around Africa[END_REF] may solely be due to aridification that dried out the Niger's tributaries draining the Hoggar swell (e.g., Fig. 2). Finally, stability of the Senegambia catchment offers the opportunity to investigate relationships between the sedimentary record of the Senegal basin and the denudation history of the catchment since the Mid-Eocene (i.e., age of the S1 bauxites; Fig. 10). By contrast, a major change in the sedimentary record of the Equatorial margin of Africa is expected consecutively to the connection of its two most prominent catchments (Niger and Volta) to the Equatorial Atlantic Ocean after an Early Paleogene (pre ~34 Ma) period of subdued clastic inputs by short rivers. The main cause for this stratigraphic turnover is the initiation and amplification of the Hoggar hot-spot swell.

Long-wavelength lithospheric deformation due to hot spot swell growth may trigger large-scale drainage rearrangement as exemplified here by West Africa since the Mid-Eocene (~48-40 Ma). But the West African example also shows that once the new drainage configuration is set, ongoing swell growth may not necessarily lead to further significant river network rearrangement. Maintenance of the drainage his allowed by a very gentle (~0.3 ‰) regional slope from the Atakor's piedmont to the coast, which may be sustained by very-long wavelength swell growth (Fig. 11). On the upper southern slope of the swell where the S2 topography has been eroded the most (~1.5 km; Fig. 11), more than 600 m of denudational isostatic uplift is expected since the Latest Oligocene (~24 Ma) (Fig. 8b). This suggests that erosion of the growing swell contributed to at least one-third of the maximum finite uplift due to hot spot swell growth (~2 km; Fig. 11), the remaining two-third of the uplift being produced by dynamic asthenospheric support, isostatic compensation of magmatic underplating and potential plume thermal effect.

Our work argues for the stability of rivers crossing the West African marginal upwarp since the Early Oligocene but also shows that the upwarp earlier formed a continental divide. Periods of dual (northward and southward) drainage may therefore alternate with periods of through-going rivers tapping sediments from the continental interior and would plead for renewed margin upwarping even long after (~90 Ma) continental breakup. Still, our study shows that at least 1500 km wavelength, hot-spot related lithospheric deformation (Fig. 10) may be efficient in triggering seaward tilt of the continental surfaces leading to the connection of internal drainage to coastal rivers across low-elevation passive margins.

Conclusion

The main current pattern of the West African drainage such as the Niger and Volta river systems stabilized in the Late Early Oligocene (>29 Ma) and probably at the Eocene-Oligocene boundary (34 Ma). Antiquity of the drainage opens new perspective on linking offshore stratigraphic records to landform evolution of large catchments of known stationary geometry over the last ~34-29 Ma and even since ~45 Ma for the longitudinal profiles of cratonic rivers as diagnostic of Quaternary drainage rearrangements due to aggressive headward erosion, and suggests that river piracy may be much less common than inferred in shield contexts. Reconstituted Mid-Eocene (~45 Ma) geography suggests that the drainage reorganized and stabilized after a period of dual drainage on both slopes of a marginal upwarp acting as a continental divide. Very long wavelength growth of the Hoggar hot spot swell since the Late Eocene caused regional drainage rearrangement that led to the modern physiography of West Africa.

Importantly, a major regional stratigraphic turnover is expected along the Equatorial margin of Africa consecutive to the opening of at least two major catchments (Niger and Volta) on the Atlantic Ocean following a period of subdued clastic sedimentary fluxes provided by short rivers. The section shows the spatial and temporal continuity established between the S1 bauxitic erosional surface and the Early to Mid-Paleogene marine sediments. Basin geometry is modified after [START_REF] Greigert | Description des formations crétacées et tertiaires du bassin des Iullemeden (Afrique occidentale)[END_REF]. [START_REF] Michel | Les modelés et dépôts du Sahara méridional et Sahel et du sudouest africain[END_REF][START_REF] Bassot | Carte géologique du Mali à 1/1 500 000[END_REF]Beaudet et al., 1981b) and GoogleEarth surveys. (b) S1 Bauxite-capped hill with slopes draped by the S2 Intermediate ferricrete (near Sansanding, 5 km east of the River; Fig. 6a). Hilltop (314 m) dominates the pediplain by less than 30 m (width of the view is ca.

3 km). (c) Pisolithic bauxite from a pit in the pediplain (same area, 3 km from the River, 284 m). (d) Contact between the Intermediate ferricrete and its underlying carapace (quarry located 10 km upstream of Ségou, 292 m). (e) Cross-section in Sansanding (Fig. 6a) based on a series of boreholes (modified after [START_REF] Furon | Notes de stratigraphie soudanaise. De l'extension des grès, conglomérats ferrugineux et calcaires lacustres dans les vallées du Niger et du Bani[END_REF] showing the S1 bauxitic weathering profile underlying the Niger alluviums. (f) Cross-section of the High Niger River valley (dashed line in Fig. 6a). S1 bauxites (in red) and S2 ferricretes (in blue) having undergone relief inversion are shown. S1/S2 weathering profile's thickness takes into account the local relief and regional observations.

  Paleogene relief and drainage stages 6. 1. Late Oligocene stage (S2) Corrected S2 surface geometry strikingly mimics today's topography, with the High Niger, Low Niger and Sourou valleys being clearly delineated and a drainage divide similar to the current one (Figs. 2, 7b and 7c). The drainage extraction procedure

Figure captions Figure 1 .

 captions1 Figure captions Figure 1. Topography, "basin-and-swell" relief, and main river systems of Africa. Cross-hatched patterns are divides delineating basins' septa. Dashed rivers are dried out. T -Taoudeni basin; Iu -Iullemmeden basin; Ch -Chad basin; Co -Congo basin.

Figure 2 .

 2 Figure 2. Topography and drainage of West Africa showing selected geological elements. Rivers are shown in white (dashed where dried out since the Early Quaternary). A -Aïr; AI -Adrar des Ifoghas; KZ -Knickzone of the Lower Volta.

Figure 3 .

 3 Figure 3. Stacked topographic cross-sections of the granite-greenstone terrains of the Yamoussoukro -Dimborko area (Central Ivory Coast; Fig. 2) showing the disposition of relicts of the bauxitic Surface (S1) and the ferricrete relicts of the Intermediate (S2) Surface (adapted from Grandin, 1976). The Bauxitic landscape has a low relief (< 70 m) compared to that of the Intermediate landscape (up to 350 m) that includes bauxite relicts. The sections trend mostly NW and encompass a total area ca. 100 x 70 km. Greenstone belts form the topographic massifs.

Figure 4 .

 4 Figure 4. Regional cross section of the Cenozoic Iullemmeden basin and its basement illustrating the relationships between the S1 bauxitic and S2 Intermediate paleosurfaces and alluvial sediments of the Continental Terminal (line of section is located on Fig. 2).

Figure 5 .

 5 Figure 5. Relationships between the S2 Intermediate Surface and its weathering profile, fluvial sediments of the Continental Terminal, and the Niger drainage in the southern Iullemmeden basin (see Fig. 2 for location). (a) Illustration of a S2 Intermediate landscape remnant and its underlying weathering profile developed from argillites (near Filingué, Fig. 5b). The 35-m high cliff exposes, from bottom to top, a fine-grained saprolite (yellowish), a mottled zone (brown), a carapace (reddish) and a ferricrete (slab pavement). Top of whitish coarse saprolite locally crops out at the base of the cliff. Note large S2 remnant in the background. (b) Contour map of the Intermediate landsurface (S2) ferricrete processed from stations comparable to that shown in (a). Data were automatically contoured and manually extrapolated in areas of low data density near the northwestern and southeastern edges of the map. Note the Tambao locality where weathering and abandonment ages of the S2 surface were constrained by Ar-Ar dating[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].

Figure 6 .

 6 Figure 6. Remnants of the Bauxitic Surface (S1) and ferricretes of the Intermediate Surface (S2) in the Niger inland delta region. (a) Map view. Squares are our own field observations and circles are from the literature[START_REF] Michel | Les modelés et dépôts du Sahara méridional et Sahel et du sudouest africain[END_REF][START_REF] Bassot | Carte géologique du Mali à 1/1 500 000[END_REF]

Figure 7 .

 7 Figure 7. (a) Interpolated geometry of the Intermediate (S2) Surface. Data points are shown in black. (b) Drainage extraction from S2 geometry corrected for erosional isostasy. (c) Corresponding drainage interpretation. Divides are drawn in red with their uncertainty in grey. (d-f) Same maps as (a-c) for the Bauxitic (S1) Surface. Areas of marine flooding are shown in blue (after the paleogeographic synthesis of Fig. 9). Water gaps shown by red circles and wind gaps / divides shown by white circles in (b) and (e)were rejected in the interpretation in (c) and (f) on the basis of field observations or other considerations (see text for further explanation). Uncertainties on divides also take into account the density of data points used to construct the surfaces geometries.

Figure 8 .

 8 Figure 8. Sources for the isostatic correction of the Intermediate Surface (S2) and the Bauxitic Surface (S1) from denudation and sediment accumulation since their abandonments ca. 24 and 45 Ma ago, respectively. (a) Denudation map computed from the subtraction of S2 topography to the present day topography. (b) Computed flexural rock uplift in response to that denudation considering a thin sheet model of lithosphere with an elastic thickness of 50 km. (c-d) Same maps as (a-b) for the S1 Surface. The grid represents the ~60 km-size cells used for inverting denudation into flexural uplift. The main features revealed by the denudation and uplift maps are (i) a marginal upwarp whose main present-day remnant is the Guinean rise, (ii) the Hoggar swell and, in the case of S1, (iii) the Iullemmeden basin in which the S1 surface is buried (compare with Figure 2 for locations).

Figure 9 .

 9 Figure 9. Cenozoic paleogeographic / drainage configurations of West Africa. (a) Oligocene -Miocene boundary (~24 Ma). (b) Present day. (c) Mid-Eocene (~45 Ma).Area of active propagating uplift is drawn after[START_REF] Grimaud | Very long-term incision dynamics of big rivers[END_REF].

Figure 10 .

 10 Figure 10. Sketch view of three stages in the relief / drainage evolution of West Africa and its continental margin. (a) Mid-Eocene (~45 Ma) period of maximum flooding. A continuous marginal upwarp marks the continental divide. (b) Early Oligocene (~34-29 Ma). Feeding of Continental Terminal fans on the Hoggar's piedmont; establishment of the modern Niger River Course; reversal of the Paleo-Volta drainage with development of an internal delta (inferred) and eventual connection to the Equatorial Atlantic Ocean. (c) Modern period (Quaternary). Further hot-spot swell propagation leads to warping and incision of the alluvial fans as well as slope decrease of the High Niger and Sourou rivers leading to the formation of inland deltas and eventual overspill of the Sourou into the Volta drainage.

Figure 11 .

 11 Figure 11. Synthetic cross-section of West Africa from the Hoggar to the Ivory Coast margin (see Fig. 10 for location). Offshore part of the section is adapted from Helm (2009). Onshore erosional surfaces are prolonged in the sedimentary basins as sequence boundaries, which are considered as their time equivalent.

  . A 55-45 Ma age group confirms the Early-to Mid-Eocene age of the late bauxitic landscape and its abandonment around 45 Ma, whereas a 29-24 Ma age group constrains the Latest Oligocene abandonment age of the Intermediate landscape

Acknowledgments

This work stems from extensive regional field reconnaissance and compilation work over the years, which have been supported by Toulouse University, the IRD, the TopoAfrica project (ANR-08-BLAN-572 0247-02), the CNRS and the West African eXploration Initiative (WAXI). We acknowledge current support from Total SA (TS2P project) and the gOcad research consortium. We are grateful to J. Braun for access to Flex3D that allowed for the flexural isostatic corrections. We thank G. Brocard for his review of an earlier version of the manuscript, the referees for their constructive suggestions, S. Bonnet for advise on drainage extraction and D. Huyghe for support. J.-L. Rageot, G. Besançon and Z. Garba are thanked for fruitful discussions and support during a memorable early field trip in Niger. 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. The data used are available upon request to the authors.

NNE Iuellemmeden Basin

Atakor Paleoproterozoic basement (MARGINAL UPWARP)

Gulf of Guinea