Trace fossils in seep-impacted sediments as a tool to decipher the origin of fine-grained intervals of the Marnoso-arenacea turbidite succession (Miocene, northern Apennines, Italy)

The Marnoso-arenacea turbidite succession outcropping in northern Italy is one of the most studied deep-water systems worldwide due to its excellent exposure. Despite the well-established sedimentology of the arenaceous succession, the origin of fine-grained intervals within the Marnoso-arenacea Fm is reconsidered in the light of methane-rich fluid expulsion, in particular as seep-carbonates are present. Fine-grained intervals are marked by limited vertical and lateral extent, in the order of 102 m and 103 m respectively, and are often associated with small-scale sediment deformation. In this study, we conduct a detailed sedimentological and stratigraphic investigation of the well-exposed Prati Piani fine-grained interval (Serravallian in age) with particular focus on the characterization of trace fossils and seep-carbonates. Trace fossils interpreted as Thalassinoides are widespread throughout the interval and form a complex network testifying long-lasting burrowing activity of decapod crustaceans. Seep-carbonates crop out at different stratigraphic levels and form meter-sized blocks with δ13C from−33.2‰ to−18.2‰. The occurrence of aragonite cements and fossil chemosymbiotic fauna indicates episodic carbonate precipitation close to the paleo-seafloor. Although crustaceans have been frequently reported grazing microbial mats atmodern seeps and leaving a 13C-depleted isotopic signature in trace fossils, in our study we found only one slightly depleted δ13C value (−9.5‰,) allowing us to exclude preferential authigenesis. Moreover, burrows are filled by detrital fine-grained sediments and hemipelagic micrite that may have diluted the methane-related signature. The association between in situ seep-carbonates and well-preserved trace fossils is of use to interpret the interval as formed by fine-grained sedimentation draping a thrust-related anticline. In this model, the stratigraphically- and laterally-confined deformed zones affecting the Prati Piani interval reflect in-situ soft-sediment deformation and small-scale slumping induced by fluid seepage along the ridge, consistent with present-day observations from several continental margins worldwide. © 2021