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Taphonomy and Paleoecology of Shallow Marine Macrofossil Assemblages in a Collisional Setting (Late Pliocene–Early Pleistocene, Western Emilia, Italy)

¹ Dipartimento di Scienze della Terra, Università di Firenze, via La Pira 4, 50121 Firenze, Italy

The late Pliocene to early Pleistocene siliciclastic succession of western Emilia (northern Italy) forms part of the infill of the Po Plain-Adriatic foredeep, a collisional basin with active thrusting during deposition, and is characterized by 5-to-20-m-thick fining-upward sedimentary cycles. Late Pliocene cycles were deposited in an inner-to-middle shelf setting at the end of a long-term uplift-denudation cycle controlled by tectonics. Cycles are characterized by shell beds of biogenic origin, particularly abundant in the middle of each cycle. The stacking pattern of paleocommunities, with particular reference to their paleodepths, reflects that each cycle is the product of sea-level variation. In particular, early transgressive deposits are marked by a Nuculana community, indicating a shelly-sand bottom of the inner shelf; late transgressive deposits and the condensed section are marked by a shelly bottom Timoclea community developed at times of lowered clastic input; highstand deposits are characterized by a muddy soft bottom Pinna community.

The situation changed near the boundary of the early Pleistocene, when a new uplift-denudation cycle started, renewing sediment availability and increasing the slope of the nearby mountain front. Local climatic conditions 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 the lower reaches of the delta front. These are characterized by densely packed shell beds and the occurrence of echinoderm obrution deposits. Recurring 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 deglaciation following glacial maxima, at times of high input of freshwater and river-borne nutrients and high summer pelagic productivity. Mudstones from the upper part of the cycle, with dispersed remains of the muddy-bottom Venus community, indicate return to full interglacial conditions and to humid, low seasonality climates. Thus, early Pleistocene cycles could be correlative to sapropel-non sapropel cycles typical of the eastern Mediterranean deep water successions. Moreover, the western Emilia fossil and sedimentary records suggest that cyclic deposition was controlled by factors other than eustasy, providing a case study for future sequence stratigraphic modeling in active tectonic settings.

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