Algal-microbial carbonates of the Namibe Basin (Albian, Angola): implications for microbial carbonate mound development in the South Atlantic

摘要

Albian carbonate reservoirs are prominent in the subsurface of the South Atlantic. Equivalent exposures in southern Angola (Benguela and Namibe basins) have received relatively little detailed sedimentological work. In the Namibe Basin, carbonates form metre-thick beds interbedded with shallow-marine and continental alluvial fan siliciclastics. Characteristic carbonate mounds (< 5 m high, 1-2 m in diameter) rise above a basal carbonate bed, which consists of oncoid-peloidal rud-grainstones with oysters and echinoderms. Thrombolite mound microfacies include red algae and microbial-algal crusts. The microfacies are marine, and compare with documented occurrences of algal-microbial-oncoidal textures in Albian carbonates of the Congo and Angola. Burial processes dominated diagenesis and have reset carbonate geochemistry from marine values, with the probable exception of Mg concentrations. Up to 22% of primary (intergranular) and secondary (microporosity, mouldic, vuggy, fracture) porosity developed as a consequence of important dissolution and partial cementation.