Large-scale methane venting and clathrite-like structures recorded in the late Miocene foredeep deposits of the northern Apennines (Marnoso-arenacea Fm, Italy)

Ancient methane seep deposits exposed in the Apennine mountains of northern Italy are key to reconstruct the geodynamic evolution of the Miocene foredeep basin prior to the Messinian Salinity Crisis. Here, we report on a new Miocene outcrop of seep-carbonates in the Romagna region which showed indications of paleo-gas hydrate destabilization. We combined carbonate facies analyses, petrography of thin sections, scanning-electron microscopy and stable isotope analyses to interpret the origin of these authigenic carbonates and biogeochemical conditions at the time of their formation. These deposits show vuggy, brecciated and massive-with-lucinids facies and are characterized by light carbon isotope signatures, as negative as −47.8‰. The oxygen isotope compositions of these carbonates are out of equilibrium with coeval bottom seawater, exhibiting anomalously-heavy values. We calculated the stability of paleo-gas hydrates, which revealed that gas hydrates type I (pure methane) were stable at depths>480 m, in agreement with the paleo-bathymetric domain of this outcrop. This area was geologically active in the Late Miocene and affected by continuous bathymetric changes caused by the eastwardly-migrating accretionary wedge and foredeep basin. We propose that gas hydrate destabilization and pro- longed anaerobic oxidation of methane in the shallow sediment produced the studied carbonate deposits.