Stratigraphic Architecture and Evolution of the Cretaceous Evaporites of the West African Margin, Insight From the Proximal Domain of the South Gabon, Congo, and Cabinda Area

Abstract

The Cretaceous evaporites of the South Atlantic act as a major Ancient salt giant. The salt precipitated at the Late Aptian, during a short interval (<1 Ma) commonly linked to the lithospheric break-up period of the margin. Despite the importance of such a stratigraphic event in the understanding of the margin evolution, the geologic framework leading to such accumulations of salt in the South Atlantic is still debated. From the South Gabon to the Cabinda, the proximal domain of the West African Margin has limited diapiric deformations and the depositional stratigraphic architecture of the 1 km-thick evaporites is still preserved. The mining and oil industries performed many wells in this area, offering a unique dataset that we investigated for the comprehension of the salt basin. Well logs interpretations highlight twelve equivalent-order evaporitic sequences that can be correlated over the entire studied area. Each of them starts with organic-rich dolomitic shale grading upward to crystalline halite. Carnallite beds interbedded with halite constitute the end-members of depositional sequences, except for four of the evaporitic sequences that end with bischofite-tachyhydrite precipitations. The first evaporitic sequence displays localized shallow-water saline deposits emplaced in topographic lows. The second and third sequences have an homogeneous but relatively thin distribution over the margin, suggesting a large scale flooding of the basin. The following sequences 4 to 8 remain widespread but evidence a depocenter migrating from the Cabinda (sequences 4 and 5), to the central Congo (sequence 6), to the South Gabon (sequences 7 and 8). This trend is supported by the similar migration of isolated bischofite depocenter in the sequences 6, 7 and 8. We interpret the migrating pattern as resulting from halokinectic deformations together with the tectonic activity of major fault transforms affecting the proximal domain of the margin. Finally the sequences 9 to 12 remain more homogeneously widespread and relatively thick over the studied area, highlighting the re-establishment of a large-scale subsidence of the margin. The results evidence the significant interplay between the evaporites sedimentology, the halokinesis and the structural inheritance of pre-salt deposits in the West African margin. Ongoing sedimentologic and geochemical investigations on core samples should provide further insights on the depositional setting of the different evaporite sequences.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018