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

Abstract

Reconstructing the evolving geometry of large river catchments over geological time scales 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 is 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 stratigraphic turnover along the Equatorial margin of West Africa.

Paleogeographic and structural evolution of northwestern Africa and its Atlantic margins since the early Mesozoic

“…Temperature-time paths of apatites suggest that the Hoggar shield underwent exhumation since the Late Eocene (40-30 Ma; Rougier et al, 2013;English et al, 2016). The products of its erosion are emplaced as mega fans of the "Continental Terminal" around the uplifting massif (Chardon et al, 2016). Doming of the Hoggar shield led to the individualization of the present-day intracontinental basins of northwestern Africa by fragmentation of a single large Saharan basin into the Taoudeni, South Algerian, Murzuq, Chad, and Iullemmeden basins.…”

“…Intense weathering is favored by peak greenhouse climate, allowing bauxites to develop all emerged lands (Beauvais and Chardon, 2013;Chardon et al, 2016). The preservation of bauxites over the Nigerian shield and the Bida rift (Valeton, 1991) precludes the connection between the equatorial Atlantic Ocean and the Tethys (Chardon et al, 2016). Because continental chemical weathering is favored, correlative carbonates and phosphates are found in all West African basins, and clastic fluxes through the river systems are subdued (e.g., Lang et al, 1990;Valeton, 1991;Johnson et al, 2000).…”

“…2). Intense weathering has affected West Africa since the Late Cretaceous and left relicts of lateritic paleolandscapes, which have been used for reconstructing denudation histories and drainage evolution of West Africa (Beauvais and Chardon, 2013;Chardon et al, 2016). The present-day West African drainage system has stabilized in the Early Oligocene, following the onset of hotspot-related growth of topographic massifs such as the Hoggar and the Tibesti (Chardon et al, 2016), which led to the basin-and-swell physiography of the continent (Burke, 1996).…”

“…Intense weathering is favored by peak greenhouse climate, allowing bauxites to develop all emerged lands (Beauvais and Chardon, 2013;Chardon et al, 2016). The preservation of bauxites over the Nigerian shield and the Bida rift (Valeton, 1991) precludes the connection between the equatorial Atlantic Ocean and the Tethys (Chardon et al, 2016).…”

“…Marine sedimentation is recorded repeatedly until the Late Paleo ceneEarly Eocene, particularly in the Iullemmeden and Chad basins (e.g., Radier, 1959;Greigert, 1966;Kogbe, 1980;Reyment, 1980;Moody and Sutcliffe, 1991). Those series are overlain by fluvial sediments of the Late Eocene-Early Oligocene "Continental Terminal" (Lang et al, 1986(Lang et al, , 1990Chardon et al, 2016;Fig. 2).…”