Microbial-mediated pre-salt carbonate deposition during the Messinian salinity crisis (Calcare di Base fm., Southern Italy)

A multi-scale analysis of sedimentary carbonate facies and post-sedimentary diagenetic features of the Calcare di Base Formation, the precursor to evaporites in Upper Messinian successions of Northern Calabria and Central Sicily, has revealed their microbial bio-mediated origin. Massive to laminated microbial boundstones represent the most common sedimentary facies forming flat to low relief cm to m scale stromatolitic and thrombolitic bodies. The fabric of the micrite varies from peloidal to aphanitic, and almost always preserves filamentous bacteria which characterized the original microbial mat. The mat was dominated by sulphur-oxidizing bacteria belonging to the Thiotrichaceae, but there is evidence for a more complex community with sulphate- and/or nitrate-reducing bacteria, all being responsible for the mediation of the carbonate precipitation. Microbial boundstones are rich in pseudomorphs of Ca-sulphate and halite, which formed during the deposition of the microbial carbonate. Layers of primary gypsum are interbedded locally with carbonates suggesting the presence of restricted marine conditions. The stable O and C isotopic composition of the carbonates, that vary from dolomite to aragonite and calcite, suggests a complex interplay between arid to humid climatic changes, expressed cyclic interbedding of the carbonate with marl-marlstone. Later diagenetic events mainly consist of phreatic meteoric recrystallization and cementation. Although considered as diachronous, the microbial carbonates can be mapped out over a distance of more than 500 km across southern Italy; this indicates near-constant environmental conditions upon the central Mediterranean shelf at the beginning of the salinity crisis. Deposition of the extensive subaqueous microbial deposit that largely comprises the Calcare di Base is envisaged to have taken place across a shallow to moderate depth platform with local slopes into deeper water areas, where some resedimentation occurred.