ABSTRACT: Tectonics and Sedimentation along the West African Margin

MEYERS, JAYSON B., and BRUCE R. ROSENDAHL, Project PROBE, Marine Geology and Geophysics, University of Miami-RSMAS, Miami, FL

Deep-imaging seismic reflection, gravity, and magnetic data from the PROBE Study have been combined with other information to ascertain: (1) the regional trends of structures associated with rifting and early sea floor spreading along the West African margin, and (2) the interrelationships between tectonics and large-scale depositional processes. Continental synrift sedimentation in Gabon lasted from the Neocomian to Barremian. Similar aged sediments have not been identified in the Douala basin, but occur on the conjugate margin in Brazil's Sergipe-Alagoas basin. After a short depositional hiatus and a period of peneplanation in the early Aptian, the margin was transgressed by shallow marine deposits, followed by salt deposition. Deeply buried beneath postrift sediments are seaward-di ping wedges that are probably composed of volcanic flows and volcaniclastic material. The wedges seem to occur at the transition between vertically thinned continental crust and what appears to be normal oceanic crust. We believe the wedges were created in the Aptian during the hiatus between synrift and the postrift depositional phases. This suggests that the salt basins of West Africa and Brazil were separated by a zone of sea floor spreading.

After salt deposition, the deep part of the basin remained anoxic until the end of the Cretaceous. The margin was transgressed by clastic and carbonate deposits into Eocene times. The postsalt units were influenced by salt tectonics and shoal carbonates formed on salt-induced highs. Siliciclastics filled the lows. On the slope, where salt beds were inclined during thermal subsidence, salt diapirs domed at the footwalls of gravity-driven growth faults. The base of the salt is terraced in a step-like fashion, probably because of differential subsidence of rift blocks.

From the Miocene to Recent, as the thermal subsidence rate decreased, deposition was dominated by transgressive deltaic deposition. In Gabon, the Ogooue Delta prograded the shelf over 100 km seaward. This delta is bounded to the north and south by the South Fang and N'Komi Fracture Zones, respectively. In Cameroon, the Sanaga delta prograded the shelf more than 50 km seaward, onto and over oceanic crust. This delta is bounded to the north by the Ascension Fracture Zone and the Cameroon Volcanic Line, and to the south by the Kribi Fracture Zone. This style of interfracture zone focusing on clastic deposition and the resulting differential loading that occurs between fracture zones seems to have resulted in a kind of "keyboard" pattern of subsidence along the margin.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)