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Distinctive lithotectonic units are recognizable on the south-western African continental margin, namely the prerift, rift-valley, and drift sequences. Four seismic horizons (T, R, P and L) are mapped throughout the area and they mark unconformities which range in age from Mesozoic to Tertiary.

The T-to-R interval is regarded as the rift-valley sequence, the rifting stage being a major component in the evolution of the west African continental margin.

Horizon T, the rift-onset unconformity, forms the base of the Jurassic/Cretaceous succession and where present is considered the top of prerift rocks of diverse age and lithologies. A great number of grabens and half-grabens varying in length and width have been recognized. These trend parallel and subparallel to the present coastline. These are evident on parts 1 to 4 of the composite seismic profile across the south-western African continental margin. Horizon R is an erosional surface generally regarded as the drift-onset unconformity. Where horizon T is not present, horizon R marks the base of the Lower Cretaceous sediments and the top of the prerift rocks. Further to the west, approximately 100 km (62 mi) from the coastline, the T-to-R interval thickens rapidly in a westerly direction. Horizon T descends steeply and soon becomes unrecognizable as a distinct reflector. The eastern pinchout point of this T-to-R wedge is subparallel to the present coastline (illustrated on profile 4 of composite line). Both the eastern grabens and the T-to-R wedge further offshore are related to the "breaking apart" of the continents and therefore are significant as tectonic features.

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