Multiproxy approach to the stydy of Croce della Moggiona cold seep-carbonates (Northern Apennines)

Seep-carbonates are described in both the present day marine environments and in the sedimentary record of ancient basins. They are the result of microbially-mediated processes that occur on the seafloor where hydrocarbon-rich fluids are seeping. These carbonate bodies have received considerable attention in the international scientific community because they may provide a clue to investigate the origin and the composition of the fluids from which they have precipitated. In the Northern Apennine foredeep, seep-carbonates are concentrated in pelitic successions from different settings. Seep carbonates occur in large turbiditic bodies (Mt. Cervarola and Marnoso-arenacea Formations) and in slope hemipelagites (Vicchio and Verghereto Marls, and Ghioli di letto mudstones).In this study we present evidence for a long time of hydrocarbon seepage during the Middle Miocene in the form of newly discovered occurrences of carbonate bodies and concretions. About ninety authigenic carbonate bodies occur in the examined Fosso Riconi outcrop. Carbonates are irregularly scattered, both laterally and vertically, and have irregular shapes and geometries, varying from stratiform to amygdaloid, lenticular bodies and pinnacles. The thickness vary from 50 dm to 7-8 m, with lateral extension ranging from 1 to 3 m. Carbonate lithologies consist of marly limestones and calcareous marls. The lateral conctact with host sediments varies from sharp to transitional and interfingering. The marly limestone, calcareous marl lithologies are typically associated with abundant fossil remains, mainly consisting of thick recrystallized closed shells and moulds of lucinid and vesicomyid-like clams and rare and small gastropod shells. Lucinids and vesicomyids are infaunal ad semi-infaunal bivalves living in cold-seep areas and sustained by mutual symbiosis with sulphide-oxidizing bacteria. The authigenic carbonate mineralogy is dominated by low Mg-calcite, ankerite, and dolomite. Petrographic observations show complex facies relationships, as indicative of different stages in seep-carbonates growth. Stable isotope analyses of these nodules show depleted δ13C values which are consistent with the formation of carbonates in a cold-seep setting. Thus, the Fosso Riconi carbonates exhibit numerous characteristics (mineralogy, sedimentary fabrics, fossil assemblages) found at modern and ancient hydrocarbon seeps.The objectives of this contribution are to investigate the nature and source of the fluids associated with Fosso Riconi carbonate precipitation and to delineate an evolutionary model of the formation and development of these seep-carbonates.We suggest that examined seep-carbonates may be a consequence of episodes of varying fluid-venting rates.