LAUSR

Abstract Petrographic studies, geochemistry, δ13C and δ18O isotopes have been used to characterize the well-preserved carbonate depositional environments of the Bobassa and Ndimba unit in the 640-565 Ma Ombella-M'poko Formation, Central African Republic. The carbonate facies are mainly composed of calcite, dolomite and quartz with some accessory minerals (opaque minerals, muscovite and pyrite). The Ca/Mg ratios show that the Bobassa carbonates are slightly metamorphosed dolomitic limestones, while those of Ndimba correspond to slightly metamorphosed highly dolomitic limestones and cipolins. The Bobassa unit recorded a depositional evolution from fluvial to peritidal environments in a possible tectonically active platform as evidenced by changes of lithology and slump structures. The Ndimba was deposited in a tidal-influenced fluvio-lacustrine environment under dry conditions. Geochemistry revealed that the Bobassa and Ndimba units recorded variable redox conditions from anoxic to oxic. The PAAS-normalized REE + Y distributions indicate that the both units were slightly altered by freshwaters and detrital materials in reduced and acidic environments. The lowest δ18O and highest δ13C values suggest warm surface waters and deeper water column respectively. The carbon isotope profiles of Bobassa and Ndimba units show values varying between −0.89 and −0.34‰ that are not stratigraphic coeval with the cap carbonates δ13C profiles deposited in the aftermath Marinoan Glaciation, but rather are correlated to the middle to upper part of the Schisto-Calcaire Group in Gabon and Republic of Congo and the time-equivalent Lukala Subgroup in DRC.