Microfacies, paleoenvironments and development pattern of Neoproterozoic cap carbonates in the Niari–Nyanga Basin (Congo and Gabon Republics, Central Africa)

Abstract

In the Niari–Nyanga Basin, the Neoproterozoic SCI Formation of the Schisto-Calcaire Group includes a lower member (SCIa), made up of carbonates and corresponding to the cap carbonate that unconformably overlies the Marinoan glacial diamictite. Petrographic analysis of carbonate and terrigenous rocks from seven stratigraphic sections of the Niari–Nyanga Basin led to define six microfacies (MF0 to MF5). The vertical and lateral variations of the cap carbonate microfacies suggest a shallow-water carbonate system with a dominating microbial carbonate factory. The area of carbonate production migrated through time to the South-West prior to the final demise of the carbonate system in the basin. Such a transition is marked by a sharp negative excursion in bulk carbon and oxygen isotope ratios, that is widely correlatable across the basin, at the top of the SCIa dolostones. The presence of depositional features, such as oolites, peloidal stromatolites with fenestral structures, together with evidences of early leaching of carbonate grains within the SCIa member and at the base of the SCIb member strongly suggest a peritidal marine carbonate production in the Niari–Nyanga Basin, during the deposition of the cap carbonate.