The "Villafranchian" sequence in the Castelnuovo Don Bosco area (Asti)

Abstract

The study of the "Villafranchian" sediments in the Castelnuovo Don Bosco area (hereafter CDB) (northern ex-tension of the type-area of Villafranca d'Asti; see Carraro, 1996) including a detailed geological mapping, suggests a complex strati-graphical and structural setting. The CDB sequence is in fact characterized by an uneven thickness and a discontinuous distribution contrasting with the lateral continuity and costantly high thickness of the type-area. The CDB Villafranchian succession is bound by buried scarps and, more generally, fills shallow depressions incised in the marine, pre-Villafranchian sediments. The Villafranchian succession of the type-area is only gently folded, whereas deformation at CDB was stronger and resulted in some major discontinuities (associated with minor structures) morphologically evidenced by the above cited scarps and by local chaotic sediments, due to synsedi-mentary reworking. At CDB, as in the type-area, it was possible to identify two depositional complexes (Lower Complex and Upper Complex) separated by erosional surfaces of large extent underling a slight unconformity. The Lower Complex, widespread and homogeneous, can be referred to a deltaic environment. A Middle Pliocene age is indicated by the abundant fossil content, mainly consisting of continental molluscs, plant remains and some fragments of terrestrial vertebrates. The Upper Complex, less wide spread and more heterogeneous, can be referred to a fluvial environment. Its very rare fossils dubiously suggest a Lower Pleistocene age. The interposed, erosional discontinuity of regional extent was given different names (C. Guerrina Surface and C. Viarengo Surface) in different areas; it corresponds to a prolonged time span probably covering part of the Middle Pliocene, the Upper Pliocene and part of the Lower Pleistocene. The local distribution of the "Villafranchian" sequence, and the presence of sharp, subvertical contacts occurring with the pre-Villafran-chian marine sediments, as well as within the sequence itself, indicate the existence of E-W and N10°E-N50°E trending faults. Evidence such as "soft sediment deformations", fractures and sinsedimentary reworking phenomena is proof that the faults were active during and after sedimentation.