RECONSTRUCTING SEEPAGE DYNAMICS OF A MIOCENE SYSTEM THROUGH SEDIMENTOLOGICAL AND GEOCHEMICAL CHARACTERIZATION OF AUTHIGENIC CARBONATES AND HOST SEDIMENTS (CORELLA, NORTHERN APENNINES, ITALY)

New data from facies, stable isotopes (C and O) and CHN analyses on seep-impacted sediments and authigenic carbonates provided insights into dynamics and geochemical background conditions at a Miocene seepage system (Corella outcrops, northern Apennines, Italy). The examined outcrops comprise 4 lenticular carbonate bodies with maximum length of 200 m and thickness up to 12 m. These carbonates strike parallel to the bedding of the enclosing sediments and show lateral pinch out terminations. Chemosynthetic fauna (mainly Lucinids and Vesycomids) are very common and forms local concentrations of disarticulated or articulated shells; veins and conduits crosscut the carbonate bodies. Microfacies show mottled micrite including clotted textures related to bacterial activity and framboids of pyrite (rosette-like features observed at SEM). δ13C values of the carbonates (micrites and sparry cement filling veins) range between -42.32‰ to -26.63‰ and are indicative of AOM. The δ18O range between -5.67‰ to 1.13‰ (average -0.96‰) and is close to the Miocene seawater signature and therefore considered unaffected by significative diagenetic alteration. The host sediments at the top of the carbonates body is depleted in 13C compared to normal marine carbonates and range between -8.40‰ to -4.75‰ and δ18O is comprised between -3.50‰ and 1.68‰. CHN analyses on the enclosing sediments revealed low TOC values in the range 0.21-0.72% with an average of 0.37%, similar to other Miocene foredeep successions of the northern Apennines. The atomic C/N ratio ranges from 8.93 to 14.05 (avg. 10.84) suggesting minor admixture of land-derived and marine organic carbon. We suggest, based on field data, geotectonic setting, and geochemical analysis, that a blind fault, rooted in the underlyingTertiary turbidites, acted as a pathway for deep methane-rich fluids. Advective fluid flow resulted in a shallow, near seafloor sulfate-methane transition zone (SMTZ), resulting in the anaerobic oxidation of methane and the precipitation of massive authigenic carbonate. The presence and preservation of chemosynthetic fauna throughout the authigenic carbonate body indicates much of the advective fluid flow breached the seafloor, resulting in methane seepage that helped sustain these chemosynthetic organisms