L’unità Aveto (Appennino Settentrionale): caratteristiche litostratigrafiche, biostratigrafia, petrografia delle areniti ed assetto strutturale.

The Aveto Unit (northern Apennines): lithostratigraphic and biostratigraphic features, sandstone petrography and structural setting. Due to the presence of andesite fragments in association with low and medium grade metamorphic rocks, the Val d’Aveto Fm represents one of the most interesting clastic deposits of the Northern Apennines. It is part of a tectonic unit belonging to the Sub-Ligurian Domain, originally located between the Ligurian oceanic domain and the Tuscan continental margin. This paper introduces some new data on the tectonic setting, Nannoplancton biostratigraphy, arenites petrography and their interpretation. The deformation in the Aveto Unit is characterized by angle thrust surface bounding horse-like structures internally deformed throughout NE-facing asymmetric folds. The relationships between the internal deformation in the Aveto Unit and the uppermost tectonic contact with the Canetolo and Liguride Units, suggests the development of an imbricate fan later deformed by an out-of sequence thrust during upper-Oligocene/Miocene. A third deformation event produced a large scale antiform-like structure affecting all the tectonic units in the Bobbio window. In the Aveto Fm we recognized three lithostratigraphic members: the lowermost pelitic-arenaceous, the middle conglomeratic and the uppermost arenaceous members. The arenite framework petrography indicates the almost completely lack of andesite fragments in the lowermost member while the conglomeratic and arenaceous members are characterized by an andesite debris-rich assemblage (more than 10% up to 60%). On the basis of the percentage of andesite fragments, we recognized two petrofacies A and B. Petrofacies A indicates a provenance area made up of medium- to low-grade metamorphic, plutonic and sedimentary rocks. This petrofacies is prevalent in the lower member. Starting from the upper part of the lower member, petrographic analyses indicate a large and sudden supply of andesite debris that characterizes the composition of petrofacies B. The calcareous nannofossil assemblages found in the Aveto Fm indicate an Early Oligocene age (NP23 Biozone). According to the sudden occurrence of andesitic fragments in the Aveto Fm, an Early Oligocene paleogeographic scenario can be proposed where the Subligurian sedimentary basin was probably supplied by a source area characterized by a contemporaneous magmatic arc.