Climatic and tectonic control on the Cenozoic evolution of the West African margin

We reconstruct the stratigraphy and paleowater depths along two seismic pro files across the Congo and Angolan margins from the Eocene to the present. The results indicate that in the Eocene, both margins had deep-water carbon ate shelves with shelf breaks lying at about 500 m water depth. At the Eoce ne-Oligocene transition, submarine erosion removed as much as 1000 m of sed iments from the upper slope and shelf break of the margin. In the Miocene, the rate of terrigenous sediment supply increased dramatically and prograda tion of clinoform-shaped deposits resulted in the shallowing to the water d epth of modern margin. We also found that there was a tectonic reactivation of the West African margin in the Miocene. This caused the uplift of the m argin by at least 500 m and consequent erosion of the shelf. The evolution of the West African margin postrift stratigraphy and paleowat er depth do not result only from the interaction between thermal subsidence , absolute sea-level change and sediment supply. They are also controlled b y global oceanographic and climatic change related to the Tertiary greenhou se to icehouse transition and to the Miocene epierogenic uplift of Africa. Global cooling at the Eocene-Oligocene transition affected the sedimentatio n of the margin by replacing the carbonate sedimentation of the Eocene by t errigenous siliciclastic sedimentation in the Oligocene and Miocene. The re lated onset of intermediate depth Antarctic oceanic currents may have trigg ered the submarine erosion and mass wasting at the Eocene-Oligocene transit ion. The increase in sedimentation off Angola seems to be dominated by the effect of global cooling in the Oligocene and in the Miocene. A major pulse of sedimentation on the Angolan margin correlates with the middle Miocene delta O-18 shift marking the beginning of permanent glaciation of Antarctic a. However, the sedimentation in Congo contains no such pulse. Rather, it c orrelates with the gradual Miocene tectonic uplift of Africa. Miocene sedim entation on the Angolan profile may be more sensitive to global climatic ch ange since it receives the discharge of the large Congo River drainage syst em. Sedimentation in the Congo profile is less affected by the Congo River. The sedimentation here may reflect the supply from the regional rivers tha t erode the edge of the uplifted margin and, therefore, the local erosion c aused by the Miocene uplift of the margin. However, we note that large-scal e shifts in the depocenter of the Congo River may confound this interpretat ion. (C) 2001 Elsevier Science B.V. All rights reserved.