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.