Cenozoic evolution of the Ivory Coast- Ghana transform margin : implication for a source-to-sink analysis of the Volta drainage system

Abstract : The clastic sedimentary record of passive margins is a function of lithospheric deformation, eustasy and morphoclimatic evolution of source continental areas. The African passive/transform margin of the Equatorial Atlantic is a unique natural laboratory to explore the feedbacks between those processes during the Cenozoic. Indeed, onshore denudation and drainage history of the margin has been reconstructed by the spatial analysis of dated and regionally correlated geomorphic markers of former base levels. This onshore history can be compared to the offshore sedimentary record from the interpretation of seismic lines calibrated by biostratigraphy. We thus initiated a source to sink approach on the Ivory Coast - Ghana segment of the margin between the Romanche and Chain fracture zones (from Ghana to Benin), which has been mainly fed by the Volta drainage system during the Cenozoic. Offshore data show that Cenozoic deposits often lie over the Cretaceous on a major erosion surface. Paleogene deposits are rarely preserved and affected by strong erosion, probably Oligocene in age. Neogene sedimentation is more continuous even though Mid-Miocene slump deposits demonstrate important regional slope destabilization processes. Plio-Pleistocene delta progradations are well documented along the margin segment. The Oligocene erosion event correlates with a major regional drainage reorganization within the source domain ca. 30 Ma that resulted in the inland growth of the Volta catchment. Since 30 Ma, the drainage configuration is stationary and allows for the detailed correlation of the morphoclimatic record (based on the sequence of lateritic paleolandsurfaces) with offshore stratigraphic architecture. Our approach will be extended to whole equatorial margin and will compare erosion and sedimentation budgets as well as the nature of exported regoliths and deposited clastics.