Authigenic carbonates related to chemosynthetic activity in the Miocene foredeep deposits of the northern Apennines.

Methane-derived chemosynthetic carbonates, identified by the peculiar isotopic signature and by paleoecological and sedimentological characters, are present in the middle-late Miocene foredeep deposits of the northern Apennines. They consist of authigenic carbonates and carbonate- cemented detrital sediments associated with chemosymbiotic taxa, and represent an excellent geological record of ancient hydrocarbon seepage processes on the sea floor related to the flourishmment of chemosynthetic communities. Chemoherms in the foredeep are hosted in thick pelitic intervals, deposited in structural highs, which are entirely or partly involved in large-sized slumps. Brecciated textures, as well as structures related to sediment instability processes, are commonly observed in carbonates. Brecciated limestones mark the sites where methane is episodically released explosively, frequently with the offscraping of exotic sediments during the rapid fluid rise along diapiric conduits or fractures. Widespread discharge of pore fluids lowers the shear strength of the sediments and triggers the movement of slumps or debris flows, which are frequently associated with chemoherms. Multiple phases of explosive breakage of micritic carbonates, diapirism, slumping, reworking processes as debris flows or local turbiditic events explain the chaoticization of many deposits associated with Miocene chemoherms of the northern Apennines. We suggest that the growing of structural highs was an effective mechanism for concentrating gas hydrates by gradually moving upward the base of their stability zone with subsequent decomposition, thus contributing to undermine the stability of slopes and inducing sediment deformation and failures.