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Sedimentology and Stratigraphy 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 salt-dominated evaporites of the South Atlantic precipitated during the Late Aptian, during a short interval around the lithospheric break-up period of the margin. Despite the importance of this stratigraphic event in the margin evolution, the geologic framework leading to the 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 0.5 to 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 understanding of the salt basin. Well logs interpretations and core descriptions highlight eleven evaporitic cycles limited by few meter-thick organic-rich shales that can be correlated over the entire study area. The first cycle records halite accumulation emplaced in topographic lows bordered by very shallow facies such as algal mats. Following cycles 2 to 8 have a combined thickness of up to several hundred meters-thick and share similar evolution trend of the evaporite facies. Each cycles start with a coarse crystalline halite grading upward to coarse crystalline carnallite and fine-grained halite inter-beds. The sequences end by a fine-grained carnallite and some with localized caps of bischofite-tachyhydrite accumulations. Such petrographic evolution evidence the general trend of salinity increase during salt precipitation. Most of the meter-thick halite - carnallite beds can be correlated over 300 km from the South Gabon to the Cabinda, suggesting their precipitation from an extended single brine body with laterally uniform salinity saturation. Conversely, the bischofite-tachyhydrite deposits are within localized depocenters and their spatial distribution within the cycles suggests a shift of the basin depocenter from the SE to the NW during the course of the evaporitic period. The upper cycles 8 to 11 are more isopachous over the study area than the lower cycles and record a different trend of facies evolution. They are dominated by halite with more widely spaced carnallite beds which tend to be thicker, and some exhibit moderate lateral variation in thickness. The change in characteristics between the lower to upper cycles may suggests a hydrological modification in the basin probably related to macro-structural influences. The results of the study evidence the significant interplay between the evaporite sedimentology and the geodynamic evolution of the margin. Ongoing geochemical analysis should provide further insights on the depositional setting of the different evaporates.