Geologic structure and evolution of the Keta basin, Ghana, West Africa

Abstract

The Keta basin, covering approximately 2,200 km 2 , lies along the east coast of Ghana. It is one of the chains of Mesozoic and Tertiary sedimentary basins in the Gulf of Guinea. The basin is filled with 870 m of Paleozoic marine and nonmarine sediments that were deposited in an interior basin that once occupied the present site of the Keta basin. These sediments are unconformably overlain by 3,600 m of Mesozoic-Tertiary deposits. Three major post-Paleozoic depositional cycles are recognized in the basin. Jurassic dolerite sill from well samples in the basin are correlative with dolerite exposures in other coastal regions of Ghana and West Africa. Radiometric ages of these dolerites (162 to 192 m.y.) agree with the presumed initial time of separation of Africa and South America. Bouguer gravity anomalies show that the Keta basin is dominated by two major northeast-trending anomalies, with an axial high of 36 mgal to the north and a low of −15 mgal to the south, where the sediments are thickest. The prominent northeast-southwest orientation of the basinal structure, faults, and gravity anomalies coincide with the orientation of the Romanche Fracture Zone. The structure exerts considerable influence on sediment accumulation in the basin. Seismic data from the adjacent continental shelf and gravity data from the basin suggest that the Romanche fracture extends into the West African continent as the Fenyi-Yakoe fault. The Keta basin is a graben that is modified by basement faulting.