Il 'Villafranchiano' nel sottosuolo della pianura cuneese

Abstract

This paper describes an informal stratigraphic unit made-up of continental and coastal marine sediments. These deposits constitute the framework of the southern portion of the Cuneo Plain and develop under an alluvial cover generally referred to the Quaternary. Because of the lack of significant outcrops the stratigraphic analysis of these deposits has been carried out using also a great number of geognostic and geophisicai cores. The unit is bounded by two unconformity surfaces; the first, at the base, separates the unit from deposits spanning in time from Serravallian to Lower Pliocene the other one, at the top, separates it from terraced quaternary deposits. The lower boundary is marked by a sharp angular unconformity, by anomalous facies sequences and, locally, by a surface of subaerial exposure. On the contrary, the upper boundary is characterized by less marked angular unconformity and by some surfaces of erosion dating from Pleistocene to Holocene. Under the quaternary deposits ranging in thickness between 60 and 10 meters, the unit is made-up of apparently conformable lithofacies which had been previously ascribed to the "Villafranchian" and "Astian" periods. The shape of the unit is markedly affected by the two boundary surfaces as it appears as the infilling of a trough roughly directed N-S. Westwards and southwards the unit overlies the pre-cenozoic basement, while eastwards it overlies pre-pliocenic deposits which are variously eroded. In the area Trinita-Magliano Alpi-Morozzo the unit is from 10 to 15 m thick while more westwards, near the Stura River, it becomes thicker and thicker up to more than 200 meters. To the West of Stura River it is not possible to define the lower boundary of the unit on the basis of data so far available. The integration of stratigraphic and sedimentological data achieved from outcropping sections has allowed us to delineate a first rough paieogeographic sketch related to the last depositional phases of the unit. It is therefore possible to recognize one sector, roughly coinciding with the present piedmont zone, made-up of a thick sequence of wheatered gravels alternating with a few meter thick strata of whithish and reddish clay. These deposits can be ascribed to an alluvial fan environment that developed under humid and warm climatic conditions. Toward E-NE (Fossano zone) these deposits are progressively replaced by sandy-gravels and subordinately muddy sediments of alluvial plain and braided river environment. Within these deposits generally known as "Villafranchiano" an aboundant fluvio-lacustrine fauna and many continental vertebrate fragments have been found. In the Salmour and Magliano Alpi zone the continental deposits are replaced by deltaic and beach deposits known as "Astiano". In this zone there are areas (Salmour) characterized by thick sand deposits and areas where there are smaller amounts of muddy deposits very rich in brackish water fossils. All the described deposits should be considered as laterally equivalent and coeval facies associations deposited during the Lower and Upper Pliocene under humid and warm climatic conditions. Thus, they probably formed long before the climatic cooling identified in the literature as occurring in Upper Pliocene-Lower Pleistocene times.