The Lombardian Gonfolite Group in central Brianza (Como and Milano Provinces, Italy): Calcareous nannofossil biostratigraphy and sedimentary record of neo-alpine tectonics

Abstract

The sedimentary succession cropping out in central Brianza, in an area embracing the territories of Besana Brianza, Briosco (Milan Province), Arosio and Inverigo (Como Province), is properly ascribed to the Lombardian Gonfolite Group. It consists of prevailing turbidite sandstones and marls, with conglomerate beds occurring in the central part of the section. A precise biostratigraphic dating of the succession was obtained through the analysis of calcareous nannofossils. The occurrence of reliable index species allowed us to recognise the NN3 to NN6 nannoplankton zones (Burdigalian to Serravallian). Since in the type-areas between Varese and Como the youngest dated units of the Gonfolite Group are Burdigalian in age, the studied succession documents a more recent part in the history of the south-alpine foredeep, that might have been recorded in outcrop elsewhere in Lombardy only by the ill-dated Gurone Sandstone and Bizzozzero Mudstone (Varese area). The deformation of the studied succession seemingly requires a single event of tectonic buckling, that is reliably constrained as late Serravallian to Tortonian (late "Lombardic Phase"). Provenance signals point to roughly contemporaneous exhumation of the Pennidic nappes and active stacking of thrust sheets in the uprising Southern Alps. This phase of tectonic activity took place around the Langhian/Serravallian boundary (early "Lombardic Phase").