Sedimentology and chemostratigraphy of the Bwipe Neoproterozoic cap dolostones (Ghana, Volta Basin): A record of microbial activity in a peritidal environment
Résumé
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, and 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. delta C-13 values are nearly constant at -3.7%(0) and delta O-18(PDB) are about -6%(0). Sr-87/Sr-96 ratios range between 0.7061 and 0.7073. The delta C-13 signatures are nearly similar to those of coeval cap dolostones from the northern part of the Basin. The y dolostones change upwards to limestones with secondary textures, as well as more negative delta C-13 and higher Sr-87/Sr-86 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 C-13-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.