Primary chemoherms in the Mt. Cervarola Formation (Bologna Apennines): a tool in understanding the evolution of the Northern Apennines during the Miocene.

In the upper portion of the Mt. Cervarola Formation in the Bologna Apennines, between the Silla and Setta streams, a decametric level made up of marls and mudstones intertongued with numerous carbonate bodies crops out. Based on preliminary biostratigraphic data, the age of these deposits is Langhian. A number of distinctive geometrical, sedimentological and compositional characteristcs allow the carbonate bodies to be interpreted as primary chemoherms, related to methane venting. They consist of micritic, marly calcareous to calcarenitic lenses, strongly brecciated and very rich in densely packed lucinid bivalves. The dimensions vary from a few decimeters to several meters and the thickness ranges from 20-30 cm to 4-5 m. Carbonate lenses are horizontally and vertically scattered and grade into the surrounding pelitic sediments with transitional lithologies in between. The methanogenic carbonates consist of authigenic micrite associated with neoformed pyrite; the micritic groundmass is commonly accompanied by abundant bioclastic debris and scarse fine-grained detritus. Brecciated structures are common, represented by monomictic and polymictic breccias, dense networks of non systematic carbonate-filled veins and fractures, and extensive vuggy fabrics. Other features, such as micritic doughnuts, nodular and cylindrical to encircling concretions and pipe-like structures, are interpreted as fluid-flow conduits. Chaotic structures are rare, consisting of small slumps involving marly and carbonate deposits.The recognition of primary chemoherms in the Mt. Cervarola Formation indicates that synsedimentary mobilization (cannibalism) was frequent not only in slope pelites but also in foredeep deposits. In fact, lucinid-bearing deposits occur in the Mt. Cervarola turbidites in various areas, such as Bobbio and Segavecchia, and in minor basin successions (Porretta, Suviana). They are not chemosynthetic and are reworked, consisting of lucinid clasts occurring at the base of resedimented arenite beds. A minor reworking is evident in the problematic Barigazzo outcrop, made up of limestone blocks with biogenic debris and disarticulated bivalve shells. Lucinid clasts and blocks come from different sources, such as the Mt. Cervarola primary chemoherms, intraformational argillaceous levels including chemoherms but no more preserved, and adjacent slope pelites (Vicchio Marls). The primary chemoherms in the Mt. Cervarola Fm from the Bologna Apennines are remarkably similar to the time-equivalent (Langhian) deposits of the inner Marnoso-arenacea Fm cropping out on the right side of the Sillaro line. In both cases chemosynthetic carbonates are enclosed in argillaceous lithofacies, probably draping over folds and thrust top. They may indicate interruption of normal turbidite deposition and could be used as indicators of tectonic activity, as confirmed by the associated chaotic structures and slumps. These observations are crucial to more accurate paleogeographic reconstructions of foredeep and minor basins (Porretta-Suviana) during the middle Miocene.Finally, the relationships among methane-derived authigenic carbonates, venting of methane rich fluids and tectonic activity are testified by the peculiar position of the Mt. Cervarola primary chemoherms. They are located in proximity of the tectonic contact separating the Mt. Cervarola Fm from the Sestola-Vidiciatico Unit, and are aligned along the overturned limb of an anticline. In this context, the expulsion of fluids was probably induced by tectonic compression and the chemoherms could be considered as syntectonic deposits. This has broad implications for constraining the age and the evolution of the structural setting.