The Lower Permian in the Orobic Anticline (Southern Alps, Lombardy): A review based on new stratigraphic and petrographic data

Facies mapping at the 1:10 000 scale of the Lower Permian in the Orobic Ant icline. carried out in the framework of the CARG Project, allowed reconstru ction of the basin architecture and refined subdivision of the stratigraphi c succession with respect to the available framework. Paroxysmal effusion o f benmoreites in the lower volcanic member of the Collio Formation was foll owed by a synsedimentary polymodal volcanic activity (mugearite flows to rh yolite tuffs) during deposition of tl-le overlying arenaceous-volcaniclasti c member. After progradation of the Ponteranica Conglomerate proximal fan-d elta facies, a possible marine transgression is suggested by in the upper a renaceous-pelitic member foraminifers, newly-found in ber of the Collio For mation. Magmatic evolution fits into a palaeogeodynamic scenario of continental wre nching; lack of thyolites in the lower volcanic member points to distinct a nd/or non-coeval volcanic activity at the western edge of the Collio Basin (Orobic Anticline) with respect to the central-eastern sector (Trabuchello- Cabianca to Camuna Anticlines). Sandstone petrography, indicating provenance from overwhelming neovolcanic sources, is fairly uniform both vertically and laterally, and thus suggests high sedimentation rates for the Collio Formation, in agreement with the s hort time span recently assigned to this unit by radiometric data from the Camuna Anticline. Different sandstone composition in the Orobic and Camuna Anticlines is another line of evidence against physical continuity of the C ollio Basin.