STRATIGRAPHIC AND TECTONIC NOTES ON THE VILLAFRANCA D’ASTI SUCCESSION IN TYPE-AREA AND CASTELNUOVO DON BOSCO SECTOR (ASTI RELIEFS, PIEDMONT).

Abstract

Interest in the sedimentary succession in the Asti Reliefs fostered research since the beginning of the science of geology, before the designation by Pareto (1865) of the “Piano Villafranchiano” and its type area at Villafranca d’Asti. The villafranchian sediments have been the subject of many recent investigations, which focused largely on the palaeontology of vertebrates and plant remains. Geological surveys in the type area and in the Castelnuovo don Bosco sector revealed that the sedimentary succession is stratigraphically and tectonically more complex than was previously supposed. The succession exhibits an important hiatus represented by an angular unconformity that allows for the differentiation of two complexes containing different facies. The Lower Complex (LC)(delta facies), which contains abundant fossils (vertebrates, pollen and plant remains, molluscs), is referred to the Piacenzian. The Upper Complex (UC) (fluvial facies), which is very sparsely fossiliferous, is uncertainly referred to the Calabrian. New surveys in the type area of Villafranca d’Asti involve an integration of the stratigraphic and paleopedologic framework. These studies focused on a paleosol, including bodies of colluvial sediments, that lie on the unconformity which separates the two complexes. All the sediments display significant deformation as indicated by the overall geometry of the succession, the presence of soft sediment deformation, the localized presence of deposits produced by gravitational reworking of the deltaic sediments and finally by the presence of deformational zones (T. Traversola Deformation Zone, C. Fagliaverde Fault Zone and Castelnuovo Fault Zone) and faults (Gariglio Fault, Serra Fault and Agagliate Fault). New data regarding the T. Traversola Deformation Zone suggest its southern extension, confirming the strike-slip nature. The critical revision of mammal-fauna and micro- and macrobotanical evidence permitted to discuss the chronological and stratigraphical context of such sedimentary succession.