Neogene cold-seep microbial carbonates (Crotone Basin - South Italy)

For the first time, the Neogene cold seep carbonate deposits of the Crotone Basin (south Italy) are described. These deposits form a carbonate body reaching a maximum length of 350 m and a thickness 40 m and are characterized by a conduit facies made of authigenic carbonates filling the previously active gas/fluid escape pipes. In addition, a pavement facies is observed, which consists of early carbonate-cemented bioclastic and siliciclastic sediments commonly colonized by a chemosynthetic macrofauna dominated by articulated and in life-position Lucinids bivalves. The conduit facies is characterized by the inward accretion of dark micritic laminae alternating with clear crystalline layers. The micritic laminae show a microbial peloidal to dendrolitic fabric, which commonly incorporates planktonic foraminifera and coprolites. These contrast with the crystalline layers, which are characterized by microspar laminae and sparry crusts made of prismatic zoned calcite crystals. The pavement facies is characterized by laminated microbial boundstones, bioclastic bearing micrite, foraminiferal oozes and hybrid arenites. The foraminiferal assemblage is characterized exclusively by planktonic forms, which - together with the relative proportion of sandy/silty grains - suggests a deep-water setting with occasional siliciclastic coarser sedimentary flows. The pavement facies shows common brecciation features, possibly indicating the establishment of post-depositional overpressure conditions due to gas/fluid injection. Clasts of breccias show overgrowth by primary fibrous to acicular isopachous to fan-shaped calcite cement. Stable Isotopes analysis of all the studied facies reveals negative δ13C values (-6.82 to -37.39 ‰) and relatively positive δ18O values (-0.04 to 3.39 ‰), most probably indicating the presence of a complex mixture of methane with other hydrocarbons and the destabilization of gas hydrates and/or dehydration of clay minerals.