Continental Margin Off Western Africa: Cape St. Francis (South Africa) to Walvis Ridge (South-West Africa)

Abstract

Approximately 17,000 km of continuous gravity, magnetic, and seismic-reflection profiles were recorded to determine the structure of the continental margin from Cape St. Francis to Walvis Ridge, and of the adjacent Agulhas and Cape deep-ocean basins. These and previous sea-floor and land data suggest that basement structures are the result of the breakup of Gondwanaland and the dispersion of the fragments to their present positions. This breakup may have been initiated as early as the Carboniferous Period, but most of the dispersion has taken place since Middle Jurassic. Igneous activity during the early phase may have led to the emplacement of ridges along the continental margin. Later, block faulting and volcanism along the fracture zones that delineate the flow lines of the drifting continents produced Walvis Ridge, Cape Rise, and the Agulhas Plateau. One of these fracture zones, the Agulhas fracture zone, dominates the structural grain of the continental margin and deep-ocean floor off the African southern coast. Sediments as thick as 7 km buried the fragmented continental basement and adjacent oceanic basement off the west coast and formed a broad continental rise and abyssal plain within Cape basin. The source of much of this clastic debris is believed to be the Orange River. In contrast, sedimentation off the southern coast since the breakup of Gondwanaland has been very limited, mostly being restricted to the narrow St. Francis basin atop the shelf. The adjacent continental slope and Agulhas basin have only a thin sediment cover. Much of the sediment that was present on the slope has slumped into the narrow Agulhas fracture zone at the base of the slope. Numerous swells on the southwestern end of the Walvis Ridge, undulating topography of the ocean floor in the western part of the Cape basin, swells on the upper continental rise, and the rough topography of the Agulhas Plateau were formed by the movement of the South Atlantic Bottom Water that enters Cape basin on the west side, flows along the southern flank of Walvis Ridge, and then is deflected southward by the continental slope. Pleistocene eustatic changes in sea level considerably modified the shelf, upper slope, and the eastern end of Walvis Ridge by wave action, turbidity currents, and the Benguela Current.