Facies dolomíticas de sistemas lacustres miocenos en las cuencas del Duero y de Madridrasgos indicativos de su origen microbiano

Résumé

This work is centred on the dolomite facies from lower middle Miocene sedimentary successions of the Madrid and Duero basins, Central Spain. The general sedimentary context for dolomite bearing formations is well established in mudflat-(saline) lake systems developed in closed basins. Traditionally, early diagenetic dolomitization processes of carbonate precursors have been invoked to explain the dolomite formation in those sedimentary environments. However, the sedimentary, isotopic, mineralogical and petrographic results support that dolomite was formed as a primary product linked to microbial activity. The epicellular precipitation of dolomite on coccoid bacteria accounted for the formation of diagnostic hollow crystals. High intracrystalline porosity rates, and poor crystallized morphologies are also typical textures of the dolomite crystals. In spite of the lithostratigraphic and the spatial variability of the study successions, the dolomite features show scare variability, which confirms that microbial imprints can be preserved in the geological record, and validates its use as biomarkers.