Dynamic interactions between the gulf of Guinea passive margin and the Congo River drainage basin: 1. Morphology and mass balance

A sediment budget between the Congo River drainage basin and the western African margin in the Gulf of Guinea is proposed on the basis of published and unpublished offshore Tertiary isopach maps, and onshore digital elevation analysis. The overall denudation of that area may be as high as 3.5x10(6) km(3) with a maximum of 10% coming from Mesozoic and Cenozoic covers. The southern part of the Congo River basin, related to uplift of the South African and Kalahari shields, appears as the most immature from the morphologic standpoint but provides one third of the present-day sediment production; it represents a maximum denudation of 150,000 km(3). On the easternmost part, East African Rift drainage basins show a more mature relief and a maximum denudation of 270,000 km(3), which can increase to 570,000 km(3) if the Congo drainage basin had extended up to the east branch of the rift. These values are confidently established from the existence of remnant geological surfaces, and can explain the volume of Tertiary sediment in the Gulf of Guinea. During upper Cretaceous, the most important accumulations of sediments correspond to the Ogooe and Kwanza fans, which shows that the organization of continental drainage was different as was potential source of sediments. In the northern part of the present-day Congo River drainage basin, there was a compressional episode during Santonian that may have caused significant relief and erosion along a Benue-Chad axis.