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

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Marine Geology
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Aug 15
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We reconstruct the stratigraphy and paleowater depths along two seismic profiles across the Congo and Angolan margins from the Eocene to the present. The results indicate that in the Eocene, both margins had deep-water carbonate shelves with shelf breaks lying at about 500 m water depth. At the Eocene-Oligocene transition, submarine erosion removed as much as 1000 m of sediments from the upper slope and shelf break of the margin. In the Miocene, the rate of terrigenous sediment supply increased dramatically and progradation of clinoform-shaped deposits resulted in the shallowing to the water depth 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 margin by at least 500 m and consequent erosion of the shelf.The evolution of the West African margin postrift stratigraphy and paleowater depth do not result only from the interaction between thermal subsidence, absolute sea-level change and sediment supply. They are also controlled by global oceanographic and climatic change related to the Tertiary greenhouse to icehouse transition and to the Miocene epierogenic uplift of Africa. Global cooling at the Eocene-Oligocene transition affected the sedimentation of the margin by replacing the carbonate sedimentation of the Eocene by terrigenous siliciclastic sedimentation in the Oligocene and Miocene. The related onset of intermediate depth Antarctic oceanic currents may have triggered the submarine erosion and mass wasting at the Eocene-Oligocene transition. 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 Antarctica. However, the sedimentation in Congo contains no such pulse. Rather, it correlates with the gradual Miocene tectonic uplift of Africa. Miocene sedimentation on the Angolan profile may be more sensitive to global climatic change since it receives the discharge of the large Congo River drainage system. Sedimentation in the Congo profile is less affected by the Congo River. The sedimentation here may reflect the supply from the regional rivers that erode the edge of the uplifted margin and, therefore, the local erosion caused by the Miocene uplift of the margin. However, we note that large-scale shifts in the depocenter of the Congo River may confound this interpretation. (C) 2001 Elsevier Science B.V. All rights reserved.


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