A fossil cold seep system in the Crotone Basin (South Italy)

Within the Crotone Basin, a Neogene cold seep carbonate body reaching a thickness of 40 m and a maximum length of 350 m and generally ascribed as “Calcari a Lucina”, crops out. It is dominantly made by a conduit facies made of authigenic carbonates filling the previously active gas/fluid escape pipes and a pavement facies made of early carbonate-cemented bioclastic and siliciclastic sediments colonized by a chemosynthetic macrofauna. This latter is generally characterized by articulated and in life-position lucinid, mytilid and vesicomid bivalves associated to tubeworms and gastropods. Particularly, the conduit facies is generally characterized by the inward accretion of dark micritic laminae alternated to clear crystalline layers. The micritic laminae show a microbial peloidal to dendrolitic fabric which incorporate planktonic foraminifera and coprolites. Whereas the crystalline layers are characterized by microspar laminae and sparry crusts made of prismatic zoned calcite crystals. The pavement facies is instead characterized by foraminiferal bioturbated mudstone/wackestones gradually passing to mixed carbonate-siliciclastic arenites. The foraminiferal assemblage is characterized exclusively by planktonic forms which, together with the relative proportion of sandy/silty grains suggest a deep-water setting with occasional siliciclastic coarser sedimentary flows. Moreover, in the pavement facies, brecciation is very common, possibly indicating the overpressure conditions established by the gas/fluid injection. Clasts of breccias are also later lined by primary fibrous to acicular isopachous to fan-shaped calcite cement. Stable Isotopes analysis in all the studied facies revealed negative values of δ13C (-12 to -38‰) and an enrichment in δ18O (0.83 to 3.4‰) reflecting the destabilization of gas hydrates probably made of thermogenic methane.