-L'Unità tettonica Groppo Sovrano -alta Val Bratica (PR)

A detailed geological survey of an area located in the Upper Bratica Valley, allowed to carry out a Geological map (1:10.000). The integration of the field data with a biostratigraphic study on the calcareous nannofossil association from the Groppo Sovrano Unit, led to precise distinction of the succession which includes the Groppo Sovrano sandstones (AGS), and that has its development from Late Campanian to Lower Eocene. The study succession is tectonically interposed between the Canetolo Unit and the Mt. Caio Unit, thus being the uppermost (and then originally most internal) Subligurid Unit. The basalmost part of the Groppo Sovrano Unit outcropping in the Bratica Valley is represented by the Riana Shales formation Late Campanian-Early Maastrichtian in age, which is characterized by metasomatic features evident through silica and oxides. These properties are quite similar to some peculiar lithofacies found in the “basal complex” of the Ligurid Helminthoid Flysch. Upward, bounded by unconformity, a marly-clayly turbiditic succession is assigned to Zone NP12 (Ypresian) by means of calcareous nannofossils. Although of restricted thickness, it shows facies analogies with Paleocene-Middle Eocene Ligurid Flysch (e.g. Mt. Sporno Flysch). These intrabacinal turbidites are interrupted by a 100 meters thick succession, known as Groppo Sovrano Sandstones (AGS), which was fed by an hybrid terrigenous discharge. The marly-clayly turbidites at the base of the AGS (Grammatica Flysch) and those at the top (Rio Canalaccio Flysch) present consistent litofacies of the same age (Zone NP12). The Groppo Sovrano Unit is overlain by a chaotic unit (Rio Rodichiasso Mélange), Lutetian in age (NP15), which probably gives evidences of an important deformation (Ligurian Phase), that made this internal part of the Subligurid basin overthrusted by the most external margin of the Ligurian basin.