Stratigraphic architecture of Plio-Pleistocene Savigliano and Alessandria Basins (Western Po Plain).

This contribution presents geological interpretations inferred from the sequential analysis of Plio-Pleistocene successions of the Western Po Plain, carried out in a research program, whose principal aim is to define the geometry of deep fresh-water acquifers of the Piedmont Region (CLEMENTE et alii, in this volume). The integration of surface, deep well (ENI) and updated seismic data (MOSCA, 2006) has allowed to propose a revised stratigraphic scheme for Messinian-Pleistocene successions of Savigliano (SB) and Alessandria (AB) basins, that are bounded by the Monferrato and Torino Hill to the N, by the Tertiary Piedmont Basin s.s. (TPB) to the S and by the Alps to the W. Six stratigraphic sequences have been recognized, bounded by unconformities and correlative conformities, related to local and regional events that have controlled the physiography of the basin. The Late Messinian sequence (LM) is floored by an erosional surface associated with an angular unconformity that, in marginal settings, cut primary evaporites. The LM consists of resedimented evaporites and terrigenous sediments that reach a maximum thickness of about 600 m in depocentral zones, now buried under the thick Plio-Quaternary cover; this sequence is followed by Early-Pliocene marine deposits of eP1 sequence, whose base is a regional conformable surface (Miocene-Pliocene boundary), that corresponds to a sharp transition to open-marine environments. The eP2 sequence (Early-Middle Pliocene) testifies the beginning of a regressive stage, characterized at regional scale by the development of N-NE ward prograding systems, consisting of deltaic and shelf depositional settings, towards the S, and of slope to basinal environments towards the N. LP (Late-Pliocene) and Ple (Pleistocene) sequences are related to the ongoing regressive trend and represent the transition from marginal-marine (LP) to continental (Ple) depositional environments. These units reach an overall thickness of about 900 m in depocentral areas (SB and AB) and in marginal settings are split by an angular unconformity, associated to a temporal gap, spanning in age from Late Pliocene to Early Pleistocene (CARRARO et alii, 1996). The above described sequences are unconformably followed by The LQ (Pleistocene-Olocene) sequence and only consists of continental deposits; the proximal portion (near the Alpine chain) of LQ is composed of alluvial fan deposits, that eastward show a lateral facies change to alluvial plain deposits. In conclusion, the Early Pliocene-Quaternary evolution of the study areas has been strongly controlled by tectonic and it can be summarized in the following three evolutionary stages: Early Pliocene: the area experiences fast subsidence and basin enlargement; Middle Pliocene: this stage is marked by uplift and severe erosion of Western Alps, NW-ward tilting of the southern region of the TPB, S-ward tilting of the Monferrato-Torino Hill domains and by the onset of subsidence in the interposed areas (future SB and AB). Late Pliocene-Olocene: the Pliocene basin looses its continuity and is fragmented into two new strongly subsident depozones (SB and AB), surrounded by uplifting areas of Torino Hill, Monferrato and TPB.