A peritidal evaporite environment in the Neoproterozoic of South Gabon (Schisto-Calcaire Subgroup, Nyanga Basin)

Abstract : The Neoproterozoic (630 Ma) Nsc3 or SCIII Formation (Schisto-Calcaire Subgroup) of the Nyanga Basin is a muddy peritidal carbonate (dolomite) shelf succession that is exposed in the old Mouila quarry (Kéri quarry), South Gabon. Its 20 m-thick lower member is dominated by microbially laminated dolomites and domal stromatolites representing shallow subtidal to supratidal environments. Abundant pseudomorphs after gypsum and anhydrite relicts indicate carbonate precipitation in sulphate-rich hypersaline environments. Dolomitization of the muddy microbial sediments probably occurred by the evaporite reflux of groundwaters. Silica precipitates constitute the ultimate diagenetic phase. Dolomites of the NSc3 Formation retain a pattern of oxygenic isotopic variation that reflects the depositional environment. Shallow subtidal microbialites and stromatolites record the lightest values (δ18O = −5.1‰) consistent with the isotopic values of Neoproterozoic marine waters whereas strongly dolomitized facies display heavier oxygen isotope values (δ18O = −1.3‰) with the heaviest values in subaerially exposed caps (enrichment of 3.8‰). Fungal hyphae and spores have been discovered in these upper parts. The δ18O pattern through the plurimetric recognized sequences (thicknesses from 2 to 6 m) suggests a constant exposure in a sabkha-like environment. Although the timing of the series is poorly constrained, the C-isotopic values of the Nsc3 carbonates with values of +2 and +3‰ approximate depositional conditions. Minor variations in C-isotopes have been observed. The evolution of the succession from the stromatolitic Lower Member to the oolitic Upper Member of the Nsc3 Formation could record the progradation of a low-energy evaporitic tidal flat that migrated seaward from oolitic shoal crests.