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1 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
2 Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA
The history of continental separation in the equatorial Atlantic is important to our understanding of the events which have led to the establishment of the present patterns of water circulation. Orientations of oceanic basement lineaments determined from bathymetric, seismic, magnetic and satellite altimetry data, and the distribution of seismic reflectors in deep-water sediments indicate that during its early opening stages the Atlantic was bounded to the south by the Guinea Fracture Zone. Using stage poles obtained from South Atlantic spreading patterns, basement ages and the palaeobathymetry of the equatorial region have been derived. The proximity of magnetic anomaly M0 to the present continental slopes suggests that the deep-water basins began to form in the Aptian. During the early stages of basin development water circulation was greatly restricted by fracture zone ridges, leading to the formation of thick sequences of carbonaceous shales. Outflow of dense, saline water from the equatorial basins may have been an important factor in controlling deposition along the Atlantic margins, contributing to the development of unconformities within the Cretaceous sedimentary record. By Santonian time the equatorial rift had reached a width of c. 1200 km, water depths close to the continental margins exceeded 5000 m and the transfer of surface water between the North and South Atlantic was well established. The conjugate Sierra Leone and Ceara Rises were built up during the late Cretaceous and existed as separate features by Early Oligocene time. Both the Romanche and Vema Fracture Zones have acted as important conduits for the transfer of bottom-water from the western to the eastern equatorial basins, with seismic profiles providing evidence for vigorous bottom water flow during the Eocene and later Tertiary. In the Sierra Leone Basin the circulation of bottom water may have reversed during the late Tertiary as a result of the movement of the eastern portion of the Romanche Fracture Zone north of the equator.
