Neogene seepage carbonate deposition in the Crotone Basin (South Italy), preliminary results

A large Neogenic “Calcari a Lucina” deposit - i.e. hydrocarbons-rich cold seep carbonate -, reaching 40 m of thickness and several tens of meters of extension, crops out in the Crotone Basin (South Italy). It is composed of authigenic carbonate forming a facies referable to the conduits of gas/fluid, crosscutting early carbonate-cemented bioclastic and siliciclastic sediments definable as pavement facies. Conduits are commonly characterized by the accretion of meters thick thinly laminated and banded calcite and formed by the alternation of dark micritic laminae and clear crystalline layers. The micritic laminae are characterized by an autochthonous microbial peloidal to dendrolitic fabric, incorporating coprolites and planktonic foraminifera. In contrast the crystalline layers are characterized by microspar laminae and centimeters thick sparry crusts composed of prismatic zoned calcite crystals. The pavement facies is typified by foraminiferal bioturbated mudstone/wackestone gradually passing to mixed arenites as the increasing of the siliciclastic grains composed of quartz, plagioclase, phyllosilicates, aphanitic lithic grains, phaneritic rock fragments, and heavy minerals, all immersed into a micritic carbonate matrix, and indicating both sedimentary and crystalline basement source. The foraminiferal assemblage, characterized by almost exclusively by planktonic forms, and the relative proportion of sandy/silty grains, indicate a deep-water setting with occasional siliciclastic coarser sedimentary flows. A common chemosynthetic macrofauna composed of often articulated and in life-position Lucinids, Mytilids and Vesicomids bivalves, associated to tubeworms and gastropods, is commonly present into such sediments. The deposits of the pavement facies are commonly brecciated, possibly because the overpressure conditions established by the gas/fluid injection, and the displacive growth of the conduit deposits that crosscut the sediments, suggesting a very early cementation of the micritic matrix. Clasts of breccias are also later lined by primary fibrous to acicular isopachous to fan-shaped calcite cement. The high diversity of the heavy minerals in the mixed arenite deposits together with the presence of unstable (green amphibole) and moderately stable (epidote, garnet, sillimanite) heavy detrital species, indicates a burial depth comprised between 600 m and 2000 m during which a late diagenetic phase occurred. A pervasive neomorphic recrystallization of the carbonate grains and the precipitation of mosaic equant calcite cements, filling the remnant voids, took place during this phase.