The Niger Delta, situated at the West African margin of the gulf of Guinea, is a large, arcuate delta of the destructive, wave-dominated type. Five lithostratigraphic units are defined from well-log interpretations corresponding to the Akata, Agbada and Benin Formations. The delta is composed of an overall coarsening and thickening-up regressive sequence. Deltaic sequences are regrouped into two mesosequences with a limit (about 11 My) which coincides with a lowstand wedge. Synsedimentary faults located on both the Eastern and Western Delta probably originate from the basement. Tectonic subsidence at the top of the Akata Formation is subjected to the effects of sediment compaction and basement subsidence. Porosity/depth relations for various facies were obtained from neutron, density and sonic logs. The calculated compaction rates for shales, shaly sandstones, and sandstone are 2.2, 1.65 and 1.2 respectively. Accelerated subsidence at about 11 My and 5 My is explained by an increase in sedimentation due to a rapid rise in the eustatic sea-level and erosion of continental highlands. Tectonic subsidence curves of the substratum are relatively regular, varying from 4 200 to 3 400 m and representing 39-22% of total subsidence. The effects of sediment load is therefore 61-68%. These results suggest the existence of continental crust beneath the studied areas of the Niger Delta. Applying the extensional model of McKENZIE (1978), we obtained a crustal thinning factor of 2.1-1.75. A phenomenon of thermal contraction of the lithosphere following an initial lower Cretaceous rifting of the South Atlantic can best explain the subsidence evolution of the Niger Delta. Heat flow calculated on the distal part of the delta varies from 45 to 85 mWm-2. A mantelic or a crustal origin can be reasonably ruled out; these variations can be better explained by the effects of sediments. These are not in a stabilized thermal state, but in a transitory one. The variations are induced by lateral heat flow conductivity between high and small sedimentation areas. Fluid movements induced by high sedimentation and compaction rates account for thermal exchanges by convection. Gravimetric modelling confirms the presence of continental crust underlying the Niger Delta and its progressive thinning from 35 km (NE) to 17 km (SW). Results obtained from subsidence studies, heat flow calculations and gravimetric models are coherent and comparable. They all show a dissymmetry in crustal thickness. Taking the extension of the oceanic fracture zones of Chain and Charcot towards the continent, we propose a model of differential stretching of the continental crust. Our model explains this dissymmetry from Western to Eastern Niger Delta.
