Taphonomy and paleoecology of shallow marine macrofossil assemblages in a collisional setting (late Pliocene-early Pleistocene, western Emilia, Italy)

The late Pliocene to early Pleistocene siliciclastic succession of western Emilia (northern Italy) forms part of the infill of the Po Plain-Adriatic f oredeep, a collisional basin with active thrusting during deposition, and i s characterized by 5-to-20-m-thick fining-upward sedimentary cycles. Late P liocene cycles were deposited in an inner-to-middle shelf setting at the en d of a long-term uplift-denudation cycle controlled by tectonics. Cycles ar e characterized by shell beds of biogenic origin, particularly abundant in the middle of each cycle. The stacking pattern of paleocommunities, with pa rticular reference to their paleodepths, reflects that each cycle is the pr oduct of sea-level variation. In particular, early transgressive deposits a re marked by a Nuculana community, indicating a shelly-sand bottom of the i nner shelf; late transgressive deposits and the condensed section are marke d by a shelly bottom Timoclea community developed at times of lowered clast ic input; highstand deposits are characterized by a muddy soft bottom Pinna community. The situation changed near the boundary of the early Pleistocen e, when a new uplift-denudation cycle started, renewing sediment availabili ty and increasing the slope of the nearby mountain front. Local climatic co nditions periodically triggered the sudden delivery on the shallow shelf of large amounts of sediment through hyperpycnal flows and related turbidity currents generated by catastrophic river floods. Sandy shelfal lobes form t he lower reaches of the delta front. These are characterized by densely pac ked shell beds and the occurrence of echinoderm obrution deposits. Recurrin g sedimentological assemblages from sandstones, characterized by the sandy bottom Spisula and Ditrupa communities, and in situ faunas from intervening mudstones, characterized by the muddy-bottom, hypoxic Arctica (and Corbula ?) community, suggest that the lower part of a cycle formed during deglaci ation following glacial maxima, at times of high input of freshwater and ri ver-borne nutrients and high summer pelagic productivity. Mudstones from th e upper part of the cycle, with dispersed remains of the muddy-bottom Venus community, indicate return to full interglacial conditions and to humid, l ow seasonality climates. Thus, early Pleistocene cycles could be correlativ e to sapropel-non sapropel cycles typical of the eastern Mediterranean deep water successions. Moreover the western Emilia fossil and sedimentary reco rds suggest that cyclic deposition was controlled by factors other than eus tasy, providing a case study for future sequence stratigraphic modeling in active tectonic settings.