In the Lower Congo region, the Ediacaran Schisto-Calcaire Subgroup consists of five carbonate-dominated formations (C1 to C5). They record tectono-eustatic sea-level fluctuations controlled by several short-time extensional tectonic events occurred in the whole basin, followed by the development of the Araçuaï-West Congo Orogen between 630 Ma and 560 Ma. The uppermost units of the C2 Formation, i.e. C2d and C2e members, consist of open marine to peritidal/sabkha cycles of 1–4 m in thickness formed during in a Highstand Systems Tract (HST). The unexposed transition between the C2 and C3 formations is interpreted as a ‘final’ HSTphase which initiated the burial of the carbonate ramp by prograding siliciclastics or an early Transgressive Systems Tract (TST) phase. The carbonates of the C3 Formation represent open marine shallowing-upward cycles of 3–8 m in thickness, with deposition at the top of massive oolitic barrier shoals during a TST which flooded the entire the Neoproterozoic West Congo Basin. During the highstand, contributions of river water and land-derived material inputs occured, intermittently according to the semi-arid to arid conditions that prevailed in the restricted inner ramp and in the sabkha facies belts. In term of geochemistry, the disturbed d13C trends of the post-Marinoan C2 and C3 carbonates rather reflect early diagenetic variations related to (i) the mixing of carbonate rocks with 13C and 18O depleted fluids including decarboxylation during early organic diagenesis and deep burial, or (ii) the meteoric alteration during carbonate stabilization, than temporal signals of the global ocean chemistry. This observation does not negate the stratigraphic utility of d13C ratios for intrabasinal correlations. |
