Stratigraphic architecture and deep hydrostratigraphy in the Pliocene to Holocene deposits of the Western Po Plain.

INTRODUCTION The Western Po Plain represents the most important and large fresh-water reserve of the Piemonte region. This area is characterized by km-scale accumulations of Pliocene to Holocene deposits in two basins, namely the Savigliano (SB) and the Alessandria (AB) basins, separated by the interposed Asti region of reduced thicknesses. The Savigliano and Alessandria basins are bounded by the Monferrato and Torino Hill to the north, the Tertiary Piedmont Basin to the south and the Alps to the west and SW (fig. 1). Until now, the first 200 m of the Pliocene to Holocene deposits were largely used for hydrogeologic purposes giving a rich litho-stratigraphic dataset. By the contrast, data about the stratigraphic and hydrostratigraphic settings beneath this depth, reached only by a few exploratory wells, are quite fragmentary. This contribution is the result of a still ongoing project, financially supported by the CIPE, that has been assigned by the Regione Piemonte to the CNR-IGG (Sez. Torino) and Earth Sciences Department of Torino University. The aim of this project is to reconstruct a regional geologic model in order to define the geometry of deep fresh-water acquifers within the buried Pliocene to Holocene successions of the Piemonte region. STRATIGRAPHIC ARCHITECTURE The reconstruction of the 3D-stratigraphic architecture and facies distribution of the basin fill represents the first step to define the presence of aquifers in order to propose a hydrogeologic model. The dataset used in this work consists of new sedimentological and micropaleontological data for the outcropping Neogene to Quaternary successions, exploratory well-logs (AGIP, 1972; 1994) and available seismic reflection lines (MOSCA, 2006). The analysis carried out for the scope of this work comprises also the Upper Messinian deposits in order to better define the pre-Pliocene physiographic framework of the basin areas. In the SB and AB, six stratigraphic units have been widespread recognized, bounded by unconformities and correlative conformities. The considered dataset allowed to identify major depositional environments within the identified units. The Late Messinian unit (LM), that is the lowermost unit here discussed, is floored by an erosional surface associated with an angular unconformity that cuts primary Messinian evaporites in marginal settings. The LM consists of resedimented evaporites (both chaotic and stratified) and sand-rich successions that reach a maximum thickness of about 600 m in depocenters located in the SB and AB. The Early Pliocene unit (eP1) consists of marine facies correlated with outcrops of clay-rich successions. It marks the Miocene-Pliocene boundary, that in previous marginal settings corresponds to a sharp transgressive transition to open-marine environments. Major accumulations are on the order of 500-600 m in depocenters roughly located in the central part of the SB and AB. The Early to Middle Pliocene unit (eP2) is characterized by the development of prograding basin-margin systems (seismically imaged by clinoforms wedges) from the southern and south-western sectors. This unit marks the beginning of a regional regressive evolution. Principal depocenters (corresponding to the previous ones) accommodated thickness on the order of 500-600 m. The two overlying units, labelled as Late Pliocene (LP) and Pleistocene p.p. (Ple), are mainly represented by marginal-marine and continental deposits respectively. These units are separated in marginal settings by an angular unconformity, associated to a temporal gap spanning from Late Pliocene to Early Pleistocene (CARRARO et alii, 1996). Their total thickness is about 900 m in depocenters stably fixed in the SB and AB. In the depocenter of AB, well-data suggest marine condition during the Pleistocene p.p.. The Pleistocene p.p. to Holocene unit (PH) widespread consists of continental deposits, <100m thick. Along the Alpine margin this unit is typically represented by alluvial fan successions, progressively evolving into alluvial plain environments toward the east. The deposition of foredescribed units underwent in a regional scenario dominated since the Miocene by the development of north-verging thrust systems (Mosca, 2006). HYDROSTRATIGRAPHY The recognized stratigraphic units (fig. 2a), the gross lateral and vertical arrangement of different depositional systems (fig. 2b) and the 3D distribution of principal lithologies (fig. 2c) have allowed to identify a number of different aquifer complexes and their spatial distributions (fig. 2d). Each complex, adopted as basic hydrogeologic unit, groups more aquifers with similar sedimentological features, i.e. homogeneous facies association, referable to analogous but diachronus depositional environments. Attention has been focused on the distribution of fresh-saltwater interface and its relationships with the hydrogeologic complexes. This interface, that represents the lower limit of freshwater reserves, has been traced throughout both the AB and SB where it physically crosses the different stratigraphic units. CONCLUSIONS As a major result of present study, the distribution of the identified acquifer complexes is independent from the reconstructed chronostratigraphic framework. In addition, the position of the fresh-saltwater interface is largely influenced by the kind and the distribution of the depositional environments while it is totally independent by the depositional ages of the stratigraphic units.