Abstract: Comparison between Offshore Angola and the Gulf of Mexico in Terms of Salt Tectonics

Spencer, J. - Spirit Energy; G. Tari* - Amoco; P. Jeronimo - Sonangol; B. Hart - Amoco

A critical exploration issue concerns the applicability of salt structure concepts developed for the mature Gulf of Mexico producing basin to the emerging Angola offshore exploration trend. To conduct a regional analysis, cross sections and a 1:500,000 scale salt province map were constructed for the Angolan segment of the West African passive margin, to allow comparison with published Gulf of Mexico data. In addition, local salt geometries in the basins were comparatively analyzed.

These salt basins have the same overall dimensions, but the Angolan margin being about half as wide. In a vertical sense, the maximum thickness of the post-salt sedimentary succession is significantly less in offshore Angola (< 10 km) than in the Gulf of Mexico basin (>20 km). These basic differences are attributable to the fact that the formation of the West African passive margin is younger (Lower Cretaceous) than that of the Gulf of Mexico (Middle Jurassic). Furthermore, the Gulf of Mexico basin experienced large terrigenous sedimentary input right from its initiation, whereas major clastic sedimentation began only during the Cenozoic in the Congo basin with the draining of the Zaire River into the Atlantic Ocean.

As the corollary of the thinner sedimentary cover, the basement does appear to control the style of salt tectonics in offshore Angola as opposed to the Gulf of Mexico basin. Major basement promontories bounded by transform faults in offshore Angola are responsible for the segmentation of the margin into subbasins. The resulting concave and convex shapes appear to promote convergent and divergent basinward salt flow, respectively. This is contrast with the Gulf of Mexico basin where the overall concave geometry may be associated with convergent radial salt flow. Also, a chain of pre-salt volcanic seamounts between the Kwanza and Benguela basins exert a profound effect on the regional-scale geometry of basinward salt flow.

Regional similarities of the basins include the progressive complication of salt-related structures basinward, the presence of significant allochthonous salt sheets and an actively deforming salt front at the oceanward edge of the basins (Sigsbee and Angolan escarpments).

At the local scale, many Gulf of Mexico salt geometries have analogs in the Angola basin, including salt rollers, diapirs, unilateral and bilateral salt nappes, canopies and welds. Differences are noted, however, in supra-salt flank geometries suggesting that while the fundamental mechanisms of salt redistribution are similar, the Angola salt structures record greater post-depositional extension and concomitant contraction than is typical for their Gulf of Mexico counterparts. Accordingly, salt models which emphasize lateral extension via gravity gliding and/or spreading may best describe the Angola salt basin, while models which focus on vertical subsidence and passive flation/deflation processes may be preferred for much of the Gulf of Mexico salt province.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil