Introduction to sedimentary basins

Summary

An older and a younger group of sedimentary basins can be distinguished in West Africa, and the basins are of two main kinds. Intracontinental basins are relatively broad and shallow with a thin sediment fill, generally not much more than 5 km. Coastal basins are normally rather narrow and deep, with sediments up to 10 km thick or more.

The older group comprises the Taoudeni and Volta Basins, both of the intracontinental type, developed on the West African craton. Lower parts of the sequences in them are the platform sediments that were deposited contemporaneously with the sediments and volcanics of the adjacent mobile belts deformed in the Pan African. Palaeozoic sediments record an overall retreat of shelf seas from the craton, following the Pan African event.

The younger basins developed mainly in the Mesozoic. The large and subcircular lullmedden and Chad Basins are intracontinental, and so are the more linear Bida Basin and Benue Trough, though the latter is a rifted basin and something of a special case. Continental margin basins include the Senegal Basin and Niger Delta, along with the smaller coastal basins. The seas retreated from the intracontinental basins early in the Tertiary, as the Alpine orogeny commenced in southern Europe. There was regression in the coastal basins at the same time, but sedimentation continued in them offshore.

Several fracture and lineament systems have been identified in West Africa, but there is no evidence of major lateral displacements since the Pan African, and certainly not during the Mesozoic or Tertiary.