Revisione del Villafranchiano nell'area-tipo di Villafranca d'Asti

Abstract

The Villanfranchian stage was proposed by Pareto in 1865, after a number of Vertebrate remains had been found during the excavation of the trench for the Turin-Asti railway in the area between Villanova and Villafranca. After this date numerous new findings took place, in particular during the 1960s with the construction of the Turin-Piacenza motorway, when a number of new pits were opened. The term Villafranchian has not always been used univocally, as shown in Figure 1: different authors have used the term with reference only to a part of the type-series, each time redefining the chronological position of the stage. The AIQUA proposed a revision of this important stage as one of its medium-term objectives, in order to provide researchers with an updated definition. This revision is divided into three phases: (1) survey for a detailed, and hitherto lacking, geological map of the type-area. The map separates outcrops from interpolations; the cartographic data are currently been digitalized and form the nucleus of an ad hoc data-base. On the basis of gathered data an interpretative stratigraphic-structural model is proposed; (2) a critical reevaluation of the large amounts of paleontological material put together in the course of a century of research. Particularly difficult has proved to be the positioning of the numerous vertebrate remains gathered during the last century, for which a precise location is unknown; this has been reconstructed on the basis of historical research. Research has shown that two separate routes were originally planned and built for the railway between Villanova and Villafranca and that finds come from both, together with the numerous sand quarries opened in the vicinity. Paleontological data has also been gathered into a data-base; this contains all information concerning each find, both original and deriving from research, and useful for defining the type, the stratigraphic position, the systematic significance and its current museum location; (3) analysis of data collected during the previous phases by specialists operating in different fields; comparison of the different interpretations; comparison of these interpretations and the interpretative model proposed. The Villafranchian type-area is to be found in the "Rilievi dell'Astigiano", on their western edge with the Poirino Plateau. The Villafranchian succession is formed by terms of the Neoautochtonous Messinian-Quaternary Complex, which covers in discordance a pre-messinian substratum (Piana & Polino, 1994): the latter has an Alpine paleogeographic and structural pertinence in the western sector of its outcropping ("Collina di Torino"), while it has an Appennine pertinence in the eastern sector ("Rilievi del Monferrato"). The two types of substrata are separated by a recently-identified transpressive deformation zone ("Zona di Rio Freddo"), which runs NW-SE. Morphologically, the area is located in a wider region which during the Upper Pleistocene exhibited important variations in the hydro-graphic pattern, due to a significant geodynamic evolution. The area currently coinciding with the Poirino Plateau, which develops immediately to the W of the Villafranchian type-area, originally constituted the continuation of the eastern flank of the Southern Piedmontese Plain. During the Upper Pleistocene the combined effect of the northward migration of the Asti Syncline axis, structure in which the whole neoautochtonous succession appears deformed together with the collector of the Southern Piedmontese Basin which followed the same position, and the fluvial aggradation occuring on the northernmost edge of the Southern Piedmontese Basin, determined a migration of the collector itself. The collector, originally running in an E-W direction, south of the reliefs formed by the "Collina di Torino" and by the "Rilievi del Monferrato", took on its present position (the Po River), north of the same rels. During the same period the Tanaro River, which up to this time had flowed into the Southern Piedmontese Basin collector, progressively migrated NNE after a period of evolution of a marginal structure of the "Rilievi delle Langhe" (the Fossano Living Anticline). This determined a flattening, due to lateral erosion, of the western edge of the hills; migration continued until such time as a pre-existing incision was intercepted, finally tracimating into it. From this time onward, the Tanaro River took on its current course, running from Bra to Asti to Alessandria, and reutilizing in the latter area what had previously been the bed of the Southern Piedmontese Basin collector. The Villafranchian type-area is located in a sector in which the forms and deposits linked to the Southern Piedmontese Basin collector are maintained during the period during which the collector followed the southern edge of the "Collina di Torino" and "Rilievi del Monferrato", namely the Lower to Upper Pleistocene. These deposits are currently dissected to form a series of outliers progressively shifted southward by the recent movement along the T. Traversola Deformational Zone". The current hydrographic net is noticeably conditioned by the complex evolution here outlined only in part. The survey for the geological map was conditioned by the extremely low outcroppings of the type-series. A number of outcrops are distributed in an very irregular fashion, and are rare in the upper portion of the sequence. The latter has in fact always been neglected and with time its presence was progressively forgotten. The Villafranchian succession rests, in substantial sedimentary continuity, on littoral marine sediments ("Sabbie di Asti" p.p. Unit). These are represented by horizontal-parallel laminated sands, with local tidal structures, rich in molluscs and which chronologically refer to the Middle Pliocene. Within the type-series which Pareto proposed as Villafranchian stage (1865), the present survey has identified two main complexes, separated by an important erosional surface (the "Casclna Viarengo" Surface). The Lower Complex is made up of (from bottom to top) two units: the Ferrere Unit, composed of coarse yellow sands, with local shifts to fine gravel lenses, characterized by oblique and prevalently planar lamineas; the thickness varies from 5 to 25 m. Its paleontological content is made up of numerous remains of continental vertebrates, in particular mastodonts, as well as marine molluscs; the majority of proboscideans comes from this unit. In general the Ferrere Unit is interpreted as being one deltaic system in the progradation phase, interfingered with beach sediments. It is apparently assignable to the Middle Pliocene. The relationship with lower-lying marine sediments is one of substantial sedimentary continuity. On the basis of clast lithology (prevalently quartz-ite and "anagenites", i.e. Permian quartzite conglomerates) it is possible to state that these sediments are genetically linked to the Tanaro basin. The most recent term of the Lower Complex is the San Martino Unit; this is a heterogeneous formation, made up of silty-clayey sediments developing in coastal swamps, intercalated and interdigitated by sandy entities resulting from fluvial canal infilling. The relationship with the lower-lying Ferrere Unit is again one of substantial stratigraphic continuity, even if local erosional episodes of limited spatial entity have been found. Within this unit, which varies from 20 to 65 m, heteropic relationships are diffuse and local interdigitations with the lower-lying Ferrere Unit have also been identified. The majority of the well-known Villafranchian paleontoiogical remains come from this unit. In particular it has been possible to prove beyond doubt that the remains of Mastodon arvernensis, Stephanorhinus jeanvireti, Mesopithecus monspessulanum, Leptobos stenometopon, Mauremys sp., Tapirus arvenensis, Sus minor, Cervus pardinensis and Eucladoceros sp. belong to this unit. The rich palynologic assemblage indicates an association composed of alternatingly dominant groups of Taxodias (t. Taxodium), Glyptostrobus, t. Sequoia, Sciadopitys, Nyssa, Myrica, Celestraceae, Hamamelidaceae triculpate (such as Parrotia, Hamamelys and Distylium). The Mediocratic group is also sizeable, while values of Cedrus + Tsuga and montane elements (Picea, Abies, Fagus, Betula) are modest. Other arboreal species (PA) are significant, in particular alder. A number of plant macro-remains and fruit remains have also been identified in these sediments: the most significant species are alternatingly characterized by Taxodias and Glyptostrobus forests. Data does not document a cooling climatic trend towards the top of the sequence, as hypothesized in the past. Swampy levels are rich in mollusc associations: during the geological surveys a total of 19 species of extinct Pulmonates have been identified: both their local stratigraphic position and the correlation with Sardinian, German, and French series indicates a middle Pliocene age. This unit appears to date to the Middle Pliocene, based on its paleontoiogical content, on the substantial continuity with the lower-lying Ferrere Unit and the magnetostratigraphic data (see infra). The Cascina Viarengo Surface appears to have been modelled by a hydrographic net during a prolonged period of time. The local organization of this surface is sometimes sub-planar while at other times it is very irregular. On the whole it has a planar attitude and exhibits slight dips towards the West, in line with the general structural setting. In order to determine the importance and chronological extension of this erosinal unit the pre-consolidation load of the clay levels of the Lower Complex was determined; its average value indicates the removal, by erosion, of some tens of meters-thick sediments.