Cretaceous separation of Africa and South America: the view from the West African margin (ODP Leg 159)

Abstract

The opening of the Equatorial Atlantic Gateway (EAG) during the Cretaceous was accompanied by the disruption of the sedimentary basins that had developed on the conjugate margins of Africa and South America. Drilling along the Côte d'Ivoire–Ghana Transform Margin (ODP Leg 159) provided a transect across the northern rim of this gateway. The interplay of tectonic and oceanic processes along the gateway created a complex continental margin that evolved in three stages interrupted by dramatic changes in sedimentary facies, waterdepths, and subsidence rates. The earliest stage records the formation of small basins with restricted connection to the world ocean and rapid infill with siliciclastic deposits in an Early Cretaceous intracratonic rift or wrench tectonic setting. This stage ended with an uplift event and the formation of a regional unconformity. During the late Albian to middle Coniacian, the oceanward side of the margin subsided below the calcite compensation depth (CCD) and a deepwater connection between Central and South Atlantic became established. Deepening of the basement ridge and its landward slope, in contrast, were delayed and detrital limestones intercalated with carbonaceous shales accumulated at shelf to slope depths. During the ensuing, latest Cretaceous to present stage, passive margin subsidence led to continuous deepening of the basement ridge and on its landward slope. Condensation and gradually decreasing organic contents point to an intensified exposure to deepwater circulation. The replacement of the zonal circulation system through the Mesozoic Tethys and Central Atlantic with a modern, oxidizing meridional circulation system through the EAG appears to be intimately related to the changing depositional conditions over large parts of the Cretaceous Atlantic.