Histoire de la subsidence post-rift du bassin côtier mauritano-sénégalo-guinéen. Relation avec l'amincissement crustal pendant la période jurassique à Crétacé inférieur. Comparaison avec l'évolution des marges péri-atlantiques au niveau de l'Atlantique Central et Equatorial (côte est des U.S.A., Sud-Sahara, Côte d'Ivoire et Plateau du Demerara)

Abstract

The coastal mauritano-senegalo-guinean basin is made up of a western subsiding domain separated from an eastern stable domain by an hinge zone. Data from petroleum wells were used to study the post-rift subsidence history of these different domains. In the western domain, the post-rift subsidence comprises an earlier rapid period during the Upper Jurassic and the Lower Cretaceous followed by a later slow period during the Upper Cretaceous and the Cainozoic. The rapid subsidence period corresponds to the initial subsidence and the commencement of thermal subsidence, in the model of crustal thinning by extension; it presents rapid accelerations in the Aptian and the Cenomanian. The period of slow subsidence is entirely of thermal origin. In the hinge zone and the eastern domain, the amplitude and the duration of subsidence diminish from the West to the East with variable subsidence factors. As from the Cenomano-Turonian, there was a gradual cessation of subsidence in the whole basin with the development of a platform outbuilding across the outer part of the western domain. Upper Cretaceous and Cainozoic hiatus and unconformity in this domain could be due to the simultaneous gravitationed movements along the continental slope and the eustatic fall of the sea level. Gravimetric data already published show a close correlation between the crustal thinning of the western domain and west of the hinge zone, and the amplitude of the rapid subsidence. Comparison with the peri-Atlantic margins of the rest of the Central Atlantic, particularly the conjugated margins of the Carolina Trough and the Blake Plateau, emphasizes the generality of the structure and the observed relations in the mauritano-senegalo-guinean basin. Finally, the age of the transition from the rapid subsidence to the slow subsidence is the one variable factor. This age is between 140 and 115 Ma on the U.S. Atlantic-continental margin, but between 95 and 90 Ma on the Senegalese margin. A possible cause for the Aptian and Cenomanian accelerations, before the Cenomanian-Turonian low subsidence of the coastal mauritano-senegalo-guinean basin, is the opening of the Equatorial Atlantic. This opening could have modified the crustal extension and the return to thermal equilibrium of the region, a suggested hypothesis by the comparisons with the subsidence of the Demerara Plateau and the off-shore basin of Ivory Coast.