Cenozoic Paleogeographic, Climate, Drainage and Topographic Maps of Africa

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Ypresian 50+-5Ma

The widespread nature of bauxites (Burke and Gunnell, 2008) suggest very slow erosion over the Paleocene to Eocene, leading to the development of a bevelled (though not necessarily low) surface, often termed the ‘African Surface’. These are taken as indicators of an expanded zone of a warm humid ‘drizzly’ climate, covering most of the plate, leading to generally low sedimentation rates. Northern Africa appears to have been topographically low, as evidenced by the dominance of carbonates, which indicate clear clastic free water, and the wide extent of marine transgressions, e.g. into the Iullemeden (IU) Basin (Moody 1997). Northern Africa, Arabia and the Horn of Africa are rich in evaporites, indicating at least periodic aridity. Sedimentation rates are relatively low on the margins of the continent, with the notable exception of the Niger Delta (ND), which is thought to have been initiated in the Paleocene (Bonne (2014),. The very low sedimentation rates elsewhere on the West Africa margins Macgregor (2012) would suggest that the Niger and Benue systems might be draining large regions that were later captured by younger rivers, including the Nile and the Congo This is supported by observations of northwards drainage in fluvial strata in the Cuvette Centrale (CC, Linol et al 2014). The Orange (OR) depocenter now shows much lower depositional rates than in the Cretaceous, which is interpreted as being due to increased aridity

Priabonian 35+-5Ma

The first Ethiopian volcanics are the first of a series of volcanic events associated with the development of topographic swells, in this case the Afar Plume, though this becomes more significant at 30Ma (Furman et al 2006). Elsewhere, topography and drainage are interpreted to be similar to that interpreted on the Ypresian map. Climates also remains similar to that of the Ypresian. Carbonate, occasionally evaporite, facies dominate in north African margins while slow sedimentation of argillaceous material occurs on southern shelves. The Niger again is the dominant depocenter in terms of sedimentary rate and volumes (Macgregor, 2015) , characterised by a thick shale prone sedimentary pile, suggesting a very wide catchment.

Rupelian 30+-5Ma

The topography of Africa starts to undergo changes at this time, with the commencement of the formation of the ‘basin and swell’ topography of Africa and the modern river systems associated with this new topography (Burke et al 2003) . The first swells that start to form at this time include the Ahaggar (AH), the Afar plume (AF) and a curving axis through the northern part of the South African Plateau (SAP). The Ethiopian traps (ET) erupt on the Afar Plume from 30-31Ma (Furman et al 2006).. Many offshore basins show unconformities at varying levels in the Oligocene, indicating relative uplift of the basin margins (e.g. Angola). These ?mantle cored movements created internal depressions between the growing swells into which rivers initially drained : these then found outlets to oceans through earlier rifts (e.g. Niger/Benue), lines of weakness such as the extension of fracture systems (? Congo, CO) or by cutting gorges through rising topography (Nile, NI). A global climatic change occurs at the base of the Oligocene, with effects across Africa  (Rasmussen et al 1992). The warm temperate belt now seems to extend southwards to cover parts of northern Africa, with a diminishment of the frequency of evaporites The humid tropical belt regresses northwards in southern Africa, where increasingly semi-arid conditions set in as part of a drying trend which continues to Present Day (Senut et al, 2009).

Langhian 15+-3Ma

The African basin and swell system is now becoming more pronounced, particularly in NE Africa, where the initiation of additional swells are suggested by volcanic ages (Swezey, 2009) and by increasing sedimentation in the Nile depocenter. The rising Red Sea rift shoulders also supply significant sediment to the Nile system. The Benue/Niger (ND) system now progrades onto oceanic crust and the Congo progressively switches northwards. Miocene uplifts are interpreted along the West African margin from Equatorial Guinea southwards (Lavier et al 2001). Volcanism continues in Ethiopia, now becoming more areally limited and shield like. Cameroon Line (CL) volcanism is extended onto oceanic crust (Burke, 2001). The climatic belts now look familiar to Present Day and have dried significantly since the Oligocene (Senut et al, 2009). A belt of bauxites run along the paleoequator. The main exception seems to be North-East Africa, where wet, possibly monsoonal conditions are interpreted (Macgregor, 2012).

Zanclean 5+-2Ma

Uplift occurs over large parts of Central and Southern Africa from 11-3Ma (Guillocheau, 2015). There is considerable debate over the magnitude of Neogene uplift of the South African Plateau (SAP), with researchers essentially falling into two camps, one considering that the plateau is essentially a Late Cretaceous feature and the other that there was also kilometre scale Late Neogene uplift, the latter being favoured by the immature nature of the drainage in the region (Roberts and White, 2010) and by significant sediment volumes off the wetter eastern coast (Baby et al 2019) . Stanley et al (2021)) shows that both models remain possible with the available evidence and the maps shown here take a middle ground to this debate, adopting her ‘Hybrid Late’ model .

Other rivers are now resembling their current forms, though a few connections are yet to be made to current upper headwaters. The Pliocene shows a significantly changing climate with probable significant short term variations. Many basins show peak sedimentation rates at this time, including the Niger and Nile (Macgregor, 2012). , attesting to the wetter conditions in the Early Pliocene compared to Present Day. North-east Africa again appears to be wet, while large lakes were developed periodically in Lakes Fezzan (FE) (Drake et al 2008)and Chad (PLC) (Griffin, 2011) . Late on in the Pliocene, increasing aridity will set in in both northern and southern Africa (Senut et al, 2009: Haddon & McCarthy, 2005) , with the current Sahara forming at 2.5Ma.

Holocene

Climates are now highly variable, following glacial and Milankovich cycles, with the Namibe (NA), Kalahari (KA)and Sahara deserts alternatively advancing and retreating in historical times (Burke, 1996; de Menocal 2004) .  Savannah climates are now especially well developed. Africa, particularly southern Africa, now contains the highest regions of high topography in the world that are not associated with plate boundaries, volcanism or collision. This anomalously high topography is concentrated along an NNE-SSW axis between the South African plateau and the Red Sea, including the unusually high and wide East African rift shoulders. Imaging of the mantle from S wave velocity analysis suggest that this is related to a mantle convection cell that rises from the lower mantle below South Africa to an eruption in the southern Red Sea (Adams and Nyblade, 2011) .

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