Abstract: The Post-rift Paleogeographic Evolution of the South Atlantic Basins of Brazil and West Africa and the Influence of Hinterland Uplift on Drainage and Sedimentary Depocenters

Norvick, Martin S. and Schaller, Hannfried - BHP

The salt basins of coastal Brazil and West Africa have common elements in stratigraphy and structure (1), including a multi-phase rift history, mobile salt in the early post-rift, carbonates in the Albian, and thick turbidite sedimentation in the Upper Cretaceous to Mid-Tertiary that drove salt diapirism and down-slope gravity sliding and folding. Many of the basins are characterised by a deep water fold belt, which formed where extension on the shelf is translated into compression at the toe of the sediment pile near the continent-ocean boundary. Evolution of post-rift depocentres can be related to mantle plume activity, uplift and changes in drainage systems. These effects are presented on a set of palinspastic reconstructions ( 2), which allow correlation of the sedimentary events with drift history, tectonics and the location of climatic belts.

Initial events were the formation of new oceanic crust in the mid-Aptian, flooding of the evaporite basin with kerogenous marls in the Early Albian, progradation of Albian carbonates and a further anoxic flooding event in the Turonian. The region lay in the hot arid zone at this time with little seaward drainage or clastic input. The first major river system was the proto-Oguoue in northern Gabon, which was active from the Cenomanian to the Miocene. During the Santonian to Maastrichtian, there was magmatism, regional uplift and clastic supply in the hinterland of the Santos basin. This sediment source and depocentre migrated northeastwards with time supplying deep water clastics into the basin and initiating salt movement. At the same time on the African margin, folding and localised magmatism occurred in the Abakaliki fold belt and Benue Trough. These compressional events were caused by reversal of the movement direction of the Central African Shear, related to changes in relative rates of spreading in the South and Central Atlantic.

During the Early Tertiary, transpressional faulting affected the hinterland of the Santos and Campos basins. Clastics were shed from the uplifted areas on to the Campos margin; during the Oligocene lowstand, these clastics were recycled as turbidites. There was widespread mantle-plume activity in the Eocene along the Brazilian margin; all of this vulcanicity moved offshore with time. Large rivers evolved in West Africa during the Early Tertiary, providing deltaic and turbidite depocentres at the mouths of the Niger, proto-Oguoue, proto-Congo and Kwanza. Of these, only the Niger delta lay in the humid tropical belt, which may explain the magnitude of the clastic supply and its large land-derived organic component. The diapirism and gravity-induced folding in the West African basins were probably driven by down-building induced by these clastic pulses, and the differences in structural style were caused by rapidity of sedimentation and differences in the mobile phase (mud in the Niger delta, salt in the other basins).

In the Neogene, coastal uplifts occurred along both margins, shutting off certain rivers (southern Brazil, proto-Oguoue) and concentrating drainage into others (modern Congo). In the Gabon, Congo and Kwanza basins, these uplift events also caused erosion in the onshore, slowing of sediment supply into the deep water and waning of salt diapirism and gravity tectonism. Finally, the Cameroon volcanic plume segmented the Niger delta, and the high topography in the humid tropics provided a Plio-Pleistocene acceleration in run-off and clastic supply.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil