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

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• 作者：Anne Né ; dé ; lec ; Pascal Affaton ; Christian France-Lanord ; André ; Charriè ; re ; Javier Alvaro
• 关键词：Neoproterozoic ; Dolostone ; C Isotopes ; Peritidal environment ; Sulphate-reducing bacteria ; Ghana
  Mots clé ; s ; Né ; oproté ; rozoï ; que ; Dolomie ; Isotopes de C ; Environnement pé ; ritidal ; Bacté ; ries sulfato-ré ; ductri
• 刊名：Comptes Rendus Geoscience
• 出版年：2007
• 出版时间：March 2007
• 年：2007
• 卷：339
• 期：3-4
• 页码：223-239
• 全文大小：2102 K

文摘

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. δ¹³C values are nearly constant at −3.7‰ and δ¹⁸O_PDB are about −6‰. ⁸⁷Sr/⁸⁶Sr ratios range between 0.7061 and 0.7073. The δ¹³C 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 δ¹³C and higher ⁸⁷Sr/⁸⁶Sr 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 δ¹³C-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.