Neogene cold seep system reconstruction in the 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 and by a pavement facies consisting of early carbonate-cemented bioclastic and siliciclastic sediments. These latter are commonly colonized by chemosynthetic and non-chemosynthetic macrofauna which, however cannot be taxonomically defined since the majority of the shells and skeletons are dissolved or recrystallized. 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, whereas the crystalline layers are made of microsparitic and sparitic crusts made of prismatic zoned calcite crystals. The pavement facies shows a great variability since it is typified by laminated microbial boundstones, bioclastic bearing micrite, foraminiferal oozes and hybrid arenites. The foraminiferal assemblage, exclusively composed by planktonic forms suggests a deep-water setting occasionally affected by siliciclastic sedimentary flows. The pavement facies also shows common brecciation features, suggesting the establishment of post-depositional overpressure conditions due to the early cementation of the conduits, which triggered localized breakage and poorly energetic outbursts of the pressurized hydrocarbons. Stable isotopes analysis of the different facies reveals overall negative δ13C values (-6.82 to -37.39 ‰) and relatively positive δ18O values (-0.04 to 3.39 ‰), indicating the presence of a complex mixture of methane with other hydrocarbons, consumed by a consortium of sulphate-reducing bacteria and methanotrophic archaea via anaerobic oxidation of methane, and the possible destabilization of gas hydrates and/or dehydration of clay minerals.