A pelitic interval enclosing primary chemoherms in the M. Cervarola Formation (Northern Apennines): evidence for synsedimentary tectonics during the Miocene.

A decametric pelitic interval made up of marls and mudstones enclosing numerous carbonate bodies is present in the upper portion, Langhian in age, of the M. Cervarola Formation in the Bologna Apennines, between the Silla and Setta streams. A number of distinctive geometrical, sedimentological and compositional characteristics allow the carbonate bodies to be interpreted as authigenic carbonate buildups (primary chemoherms), related to methane venting. Chemoherms are always located in proximity of the tectonic contact separating the M. Cervarola Fm from the Sestola-Vidiciatico Unit. In this context, the expulsion of fluids was probably induced by tectonic compression and the chemoherms could be considered as syntectonic deposits related to structural highs. Syntectonics highs affected the early-middle Miocene sedimentation of foredeep turbidite units (M. Cervarola and Marnoso-arenacea formations) with pelitic turbidites overlapping structural reliefs, probably related with blind thrusts-fold drapes. Chemoherms within the pelitic intervals could be used as indicators of tectonic activity, as confirmed by the associated chaotic structures and slumps. These observations, and the comparison with the resedimented chemoherms occurring in other outrops of the M. Cervarola Fm. as well as in minor basins (Porretta-Suviana), are crucial to more accurate paleogeographic reconstructions of the middle Miocene foredeep.