Sedimentology and chemostratigraphy of the Bwipe Neoproterozoic cap dolostones (Ghana, Volta Basin): A record of microbial activity in a peritidal environment

Neoproterozoic glacial and post-glacial sediments from the Volta Basin (West Africa) form a stratigraphic Triad of tillites, carbonates and cherts. The carbonates that cap the tillites were studied in detail at Bwipe (Ghana), in the western part of the basin. They are made of finely-laminated dolostones with well-preserved sedimentary features, suggesting that dolomite formation was penecontemporaneous of deposition in a warm, arid peritidal environment, with microbial activity. Rare-earth element distribution display seawater-like patterns. Redox-sensitive trace elements indicate suboxic conditions. High Ba/Al ratios can be related to high organic productivity. a13C values are nearly constant at 6e. super(87)Sr/ super(86)Sr ratios range between 0.7061 and 0.7073. The delta super(13)C signatures are nearly similar to those of coeval cap dolostones from the northern part of the Basin. The dolostones change upwards to limestones with secondary textures, as well as more negative delta super(13)C and higher super(87)Sr/ super(86)Sr ratio. Therefore, only the dolostones witness the post-glacial conditions in seawater. It is proposed that, due to a bloom of microbial productivity following ice thaw, organic matter likely accumulated at the water-sediment interface and was consumed by sulphate-reducing bacteria, leading to the precipitation of delta super(13)C-negative dolomite. This microbially mediated model is supported by present-day field evidence from hypersaline lagoons in Brazil and by previously published culture experiments. It is consistent with the sedimentological and geochemical data from the Ghana cap dolostones and can be applied to other Neoproterozoic cap dolostones worldwide.Original Abstract: Les sediments glaciaires et post-glaciaires du bassin des Volta (Afrique occidentale) constituent une Triade formee de tillites, carbonates et silexites. Les cap carbonates qui surmontent directement les tillites ont ete etudies en detail a Bwipe (Ghana), dans l'Ouest du bassin. Ce sont des dolomies finement laminees ayant conserve des caracteres sedimentaires primaires, indiquant que la formation de la dolomie est penecontemporaine du depot et s'est produite en conditions peritidales, sous un climat chaud et aride et en presence d'activite microbienne. Les spectres de terres rares sont de type eau de mer. Les teneurs en elements traces sensibles aux conditions redox indiquent des conditions suboxiques. La valeur elevee de Ba/Al suggere une productivite e