Tropical and sub-tropical West Africa. Marine and continental changes during the late quaternary / Тропическая и субтропическая Западная Африка. Морские и континентальные изменения в позднем четвертичном периоде

The geological maps show that Africa is almost entirely made up of basement Precambrian rocks. The only exceptions are constituted by northwestern and southern parts of the continent where narrow Phanerozoic mountains belts are stacked against the Precambrian landmass. Structurally, the Precambrian geology of Africa is grossly divided into cratons and mobile belts (Petters, 1991). The two most important cratons are the West African craton and the Congo (Zaire) craton (Fig. 1 (a,b)). Surrounding the cratons are the belts, which have been deformed or metamorphosed during the Early Palaeozoic Pan-African orogeny. Cratonic nuclei are observed in the central, northeastern and northwestern parts of the Congo (Zaire) craton. According to Cahen (1984), the African orogenic history indicates that the bulk of this craton was stabilised after an early Proterozoic orogeny. The history of the tectonic evolution of Africa is based on a collection of radiometric ages. The geochronological subdivisions of the Archean proposed here are those recommended by the International Union of Geological Sciences (in Cahen, 1984). This geochronological framework is divided by successive boundaries. The most significant for Africa occurred at 2.5 billions years for the ArcheanProterozoic boundary, 950 Myr for the Middle-Late Proterozoc boundary. Condie (1989) emphasised two major worlwide orogenic episodes that affect especially the African continent: one in the late Archean (Kibaran) (1500-1000 Myr) and the Pan-African (800-500,000 years) events affected largely the Gondwana continent and seem to play a strong role in the tectonic history of Africa. Some poorly developed Paleozoic sedimentary formations outcrop locally in Ghana, and especially on the mounts of Guinea, Mali, eastern Mauritania, and in various parts of the Sahara and the southern Atlas (Fig. 2). Paleozoic marine transgressions occurred in the Early Silurian, the Mid-Devonian, and the Early Carboniferous. Due to the position of the South Pole in Northwest Africa during the Late Ordovician, there is widespread evidence of glaciation. With the southward movement of this South Pole, another widespread glaciation affected all of southern Gondwana during the Late Carboniferous-Permian. This was followed by phases of igneous activity, which accompanied this continent-wide  phase of rifting from the end of the Karoo sedimentation to the initiation of the East-African Rift System. Some of these volcanic complexes are still active today. The major volcanic activity is located on the East African Rift System, which might possibly be incipient plate boundaries. This 6400 km long Rift marks the eastern limit of the CongoZaire basin and hence the eastern limit of this study. Following the fragmentation of Gondwana, basins formed along the Atlantic margin, which contain a complete Mesozoic-Cenozoic record. These are filled by a sedimentary sequence, which is almost continuous from the Neocomian to the Burdigalian, with only a break during the Oligocene and the Pliocene. The distribution of the various deposits is controlled by the tectonic behaviour of the basement. The first essentially continental infilling of the Rift was separated from the upper marine sediments by a mighty calcareous-evaporitic series which deposited behind the screen of barrier reefs. These upper terms are essentially argillaceous where the detritic occurrences increase on the border of the basin. Major basins of Africa formed during the break-up of Gondwana, and include Mesozoic to Quaternary rocks of limited thickness. Tertiary and Quaternary deposits have accumulated in the centre of the Congolese Basin as the Bateke Series that outcrop largely through eastern Gabon and central Congo. <...>