Petrography and chemistry of sandstones from the Swiss Molasse Basin: an archive of the Oligocene to Miocene evolution of the Central Alps

Oligocene to Miocene fluvial sandstones from the Swiss Molasse Basin were analysed for sandstone framework composition, heavy minerals, whole-rock geochemistry and detrital chrome spinel chemistry. Samples were taken from the proximal part of the basin close to the Alpine main thrust and are chronostratigraphically calibrated between 31 and 13 Ma. Sandstone composition allows the identification of different source rocks, and their variation in time and space place constraints on the Oligocene to Miocene evolution of the Central Alps. In the eastern part of the basin, sandstones document a normal unroofing sequence with the downcutting from Austroalpine sedimentary cover into Austroalpine crystalline rocks and, slightly later at approximate to 21 Ma, into Penninic ophiolites. In the central part, downcutting into crystalline basement rocks occurred at approximate to 25 Ma, and the removal of the sedimentary cover was much more advanced than in the east. This may be interpreted as a first signal from the doming of the Lepontine area. At approximate to 20 Ma, extensional tectonics in the hinterland led to the first exposure of low-grade metamorphic rocks from the footwall of the Simplon Fault in the Central Alps. Erosion of these rocks persisted up to the youngest sediments at approximate to 13 Ma. In the western part of the basin, a contribution from granitoid and (ultra)mafic rocks is documented as early as approximate to 28 Ma. The source for the (ultra)mafic detritus is Penninic ophiolites from the Piemonte zone of the western Alps, which were already exposed at the surface at that time.