Geochemical characterization of the Marinoan “Cap Carbonate” of the Niari-Nyanga Basin (Central Africa)

Abstract

Despite recent works on the Schisto-calcaire Group of the West-Congolian Supergroup in Central Africa, Neoproterozoic Cap Carbonate have not been well documented in the Niari-Nyanga basin that extends from Gabon to Angola. The couplet “Cap Carbonate (SCIa) and Upper Diamictite”, that is a marker of the “Snowball Earth event” was studied in 5 sections from the Niari-Nyanga basin and in one section from the external zone of the Mayombe folded belt (MFB). Our paper presents a high-resolution data set based on results from petrography, geochemistry, mineralogy and stable isotope studies performed on Cap Carbonate samples. The estimation of the illite crystallinity indexes indicates different degrees of post-sedimentary transformations evolving from deep diagenesis in the syncline basin to an epimetamorphism in the external zone of the MFB. Thin-sections of rocks sampled in this latter section display metamorphic features materialized by recrystallization of minerals in a S1 cleavage plane. Despite such an observation, all our samples clearly show preservation of primary petrographic structures such as biolaminations, fenestrae and peloids. Geochemical data leads us to highlight that the Cap Carbonate is composed by a variety of carbonate types. These results support the fact that the Cap Carbonate was derived from a combination of biological and chemical processes: organomineralization and chemical precipitation from supersaturated solutions, with a negligible impact of post-depositional transformations. For all the studied sections, stable isotope profiles display a drop in δ13C values, from about −2.6‰ to −4‰, associated with δ18O values ranging from −6‰ to −10‰. Moreover, the similarity of the isotopic profiles of the studied Cap Carbonate (from the Niari-Nyanga basin and the MFB) and the close fit to the expected Neoproterozoic δ13C and δ18O seawater values constitute a strong indication of a weak impact of a diagenetic to epimetamorphic evolution on the carbon isotopic signature of the SCIa Cap Carbonate. In the light of this data set, our isotopic records must be considered as a reliable tracer of the Marinoan Glaciation within the West-Congolian Supergroup.