Unconformity-bounded stratigraphy in the Plio-Pleistocene continental record: New insights from the Alessandria Basin (NW Italy)

Combined stratigraphic and bio-magnetostratigraphic analyses on a fluvial sequence from the southern margin of the Alessandria Basin (at the junction between the Alps and the Apennines) provide evidence for the first documentation of a late Piacenzian-Gelasian continental record in northern Italy. The basin experienced general growth at the margins and ongoing subsidence in the depocentre, during overthrusting onto the Po Foreland Basin. The studied fluvial succession is 25-30m thick and can be divided into three units (MRZ1, MRZ2 and MRZ3) delimited by erosional unconformities (S1, S2 and S3). The MRZ1 and MRZ2 units (shallow to confined braided systems) are chronologically restricted between 2.8 and 2.58Ma based on bio-magnetostratigraphic analyses. The overall stratigraphic framework indicates deposition and erosion during the progressive uplift of the Alps/Apennines system. The frequency of unconformities and sedimentary changes, compared to other coeval successions of other syn-orogenic basins of northern and central Italy, suggests the interaction between high-amplitude global cold stages and tectonic deformation. Tectonic climax occurred in the Gelasian, as revealed by the 450- to 600-ka-long hiatus associated with the S3 unconformity, whereas the subsequent deposition of the MRZ3 unit (meandering alluvial system) including a Gelasian magnetic reversal points to the fading of tectonic activity. The stratigraphic reconstruction provided in this study enriches the current knowledge of the European continental biota: the absence of thermophilous elements in the MRZ1 carpoflora reflects the late Piacenzian cooling phase after 2.8Ma, whereas the persistence of several extra-European taxa in MRZ3 (e.g. Ampelopsis cf. ludwigii, Boehmeria lithuanica, Cryptomeria rhenana) rules out an age younger than 1.4Ma. The freshwater mollusc assemblage of MRZ2 shows the occurrence of Pomatias elegans and Tournouerina belnensis, identified for the first time in the Pliocene of northern Italy.