Hydrocarbon-derived imprints in olistostromes of the Early Serravallian Marnoso-arenacea Formation, Romagna Apennines (northern Italy)

Evidence of hydrocarbon venting within slumped bodies associated with the siliciclastic, dominantly turbiditic, Marnoso-arenacea Formation (Umbria-Romagna structural domain, Romagna Apennine, northern Italy) is documented with sedimentological, faunal, and geochemical data. Specifically, C-13-depleted carbonate concretions and limestones and clusters of chemosynthetic clams (Vesicomyidae) have been identified in the marls of the Le Caselle Olistostrome and other slumped bodies contained within the Early Serravallian section of the Marnoso-arenacea Fm. Most of the olistostrome marls and limestones are extrabasinal and must have slid from a source area located several kilometers southwest of their present position. Thus, they presumably pertain to the Vicchio Marls Formation of the northeastern (outer) Tuscan structural domain, with possible minor contributions from the epi-ligurian Bismantova Fm. It is suggested that venting of methane in the source area of the olistostromes permitted the establishment of exotic chemosynthetic communities and promoted the precipitation of carbonate concretions and limestones. According to the field evidence, these materials were later subjected to multistep downslope remobilization and were eventually carried into the Marnoso-arenacea basin through gravity mass transport.