Sediment instability related to fluid venting in Miocene authigenic carbonate deposits of the northern Apennines (Italy).

Chemosynthetic carbonates, identified by isotopic, paleoecological and sedimentological features, are concentrated in middle-late Miocene satellite and foredeep deposits of the northern Apennines. Chemoherms in the foredeep are hosted in thick pelitic intervals, probably deposited in intrabasinal structural highs, which are entirely or partly involved in large slumps, in many cases associated with extrabasinal slides. Sediment textures in carbonates and in the enclosing foredeep pelitic sediments indicate a link between hydrocarbon-fluid venting, sediment deformation and mobilization, and tectonics. The intensity and style of fluid release phases directly influenced chemoherm typology, and also determined overpressure conditions in low shear strength pelitic sediments, favouring sediment mobilization and influencing slope instability which widely affected the Apennine foredeep. Chemosynthetic carbonates are associated with sites of tectonically-fractured and compressed sediments in the Apennine foredeep-thrust belt system, thus indicanting a relation to the tectonic loading of the Apennine thrust-sheets which favoured fluid expulsion along forerunner faults. Possible gas hydrate contributions to fluid expulsion processes are discussed, based on sediment textures compared to modern vent areas. Finally, sediment instability may have facilitated large amount of fluid escape, thus stopping carbonate precipitation