Revisione del significato dei depositi "Villafranchiani" in Piemonte

Abstract

: The paper presents the geological map of the "Villafranchian" deposits of Piedmont apart from those of the Viliafranca d'Asti type-area. The term "Villafranchian" is utilized in the paper to indicate regressive successions with a typical aggradational character (i.e. with no internal terracing phenomena), which ended the cenozoic sedimentation and preceded the typical terraced succession of plenipleistocenic age. The work referred to in this paper Is still in progress, and the results described are restricted to the previous outliers (under revision) and to new recently discovered outliers that had never been reported. The most interesting result is related to the characteristics of the upper boundary of 'Villafranchian" deposits, which is in all cases an erosional boundary; this fact indicates that the boundary does not represent the original top of the deposits (which might have been at an higher elevation such as it is in some cases). Geological, palynological and paleocarpological studies showed that these'Villafranchian" deposits are not attributable to a single wide fluvio-lacustrine basin. On the contrary, they represent remains of sedimentary successions linked to distinct basins and differrent depositions environments, although there are strong fades analogies. In these successions it is possible to recognize statigraphic lacunae of various extent with no mutual correlations. The successions are distributed on a very wide altimetrical interval and their palynologic assemblages indicate different climatic conditions and chronology. Their age ranges from the Early Pliocene to the Middle Pleistocene, which signifies that sedimentation conditions for the "villafranchian" lithofacies are not exclusive of a definite time interval. Differently from the type-area Villafranchian succession, vertebrates remains are almost absent in these deposits. The distribution of these deposits indicates that the main alpine valleys had already been modeled before the Pliocene because were filled by Pliocene marine sediments and, on a later time, by the'Villafranchian" sediments; starting from the middle Pleistocene, the alpine valleys have progressively been cut up to the present shape. The 'Villafranchian" deposits described in this paper as a whole are distributed at elevations higher than it was previously believed. The valleys, where the 'Villafranchian" deposits previously taken as the highest in elevation outcrop (the Sesia and Sessera Valleys; cfr. Sacco, 1888), are located in areas of mild recent uplifting: some of the newly found outliers - those characterized by smaller extent - are preserved at higher elevations; the almost complete removal of the rest of their stratigraphic complexes is to relate to a stronger uplifting which favoured a deeper erosion. In some localities (Chisone Valley; Collo, this volume) 'Villafranchian" deposits are highly deformed.