MESO-CENOZOIC SOURCE-TO-SINK OF THE AFRICAN MARGIN OF THE EQUATORIAL ATLANTIC
Résumé
The objective of the Transform Source to Sink Project (TS2P) is to link the dynamics of
the erosion of the West African Craton to the offshore sedimentary basins of the African margin
of the Equatorial Atlantic at geological time scales. This margin, alternating transform and
oblique segments from Guinea to Nigeria, shows a strong structural variability in the margin
width, continental geology and relief, drainage networks and subsidence/accumulation patterns.
We analyzed this system combining onshore geology and geomorphology as well as
offshore sub-surface data.
For the Cenozoic, mapping and regional correlation of dated lateritic paleo-landscape
remnants allows us to reconstruct two physiographic configurations of West Africa. These geometries
show that the present-day drainage organization stabilized by at least 29 Myrs ago
(probably by 34 Myr) revealing the antiquity of the Senegambia, Niger and Volta catchments
toward the Atlantic as well as of the marginal upwarp currently forming a continental divide.
The drainage rearrangement that lead to this drainage organization was primarily enhanced by
the topographic growth of the Hoggar swell and was associated with a major stratigraphic turnover
along the Equatorial margin. Elevation differences between paleo-landscape remnants
give access to the spatial and temporal distribution of denudation for 3 time-increments since
45 Myrs. From this, we estimate the volumes of sediments and associated lithologies exported
by the West African Craton toward different segments of the margin, taking into account the
type of eroded bedrock and the successive drainage reorganizations.
For the Mesozoic, we produced paleogeographic maps at the scale of West Africa spanning
the continental domain and offshore basins since 200 Ma. Mapping spatial and temporal
distribution of domains either in erosion (sources) or in accumulation (sinks) document the
impact of the successive rifting of Central and Equatorial Atlantic on the physiography of the
area. We use low temperature thermochronology dating along three transects perpendicular
to the margin (Guinea, Ivory Coast and Benin) to determine periods of maximum denudation.
We compare these data to accumulation histories in passive margin basins and discuss
their stratigraphic expression according to the type of margin segment they are preserved in.