Seep-carbonates from the Oltrepo Pavese area (Northern Apennines, middle-late Miocene)

Accurate isotopic and facies analyses have been carried out on a marly-calcareous lens cropping out in the Voghera Apennine which contains a particular macrofauna. The data collected show that these carbonates can be interpreted as sedimentary products related to the expulsion of methane-rich fluids and cold-vent processes, analogous to modern sea-floor seep-carbonates. The facies distribution and cyclicity are indicative of different stages in seep-carbonate growth and, presumably, of variations in fluid flow intensity. Field observations have demonstrated the primary position of the carbonate lens, which is located on top of a lenticular level of resedimented sandstones and arenaceous siltstones (Serravalle Sandstones), partially corresponding in facies and age to the Termina Formation deposits of the Emilia Apennines. Stratigraphic data confirm that in the epi-Ligurian Sequence of the Northern Apennines sedimentary and/or tectonic conditions prone to the precipitation of seep-carbonates occurred only during the sedimentation of the basal part of the Termina Fm. (late Serravallian-early Tortonian). The close links between tectonic, sedimentary instability and deposition of seep-carbonates suggest that chaotic deposits of the Termina Fm., such as sedimentary melanges or debris flow deposits, could be reinterpreted in the light of mud diapirism.