Compositional evolution of the Macigno Fm. of southern Tuscany along a transect from the Tuscan coast to the Chianti Hills

The Macigno deep-sea turbidite unit was part of the diachronically migrating foredeep system that characterized the collisional phases of the Apennine orogen at the Oligocene-Miocene transition. Detrital modes of the Macigno sandstones along a transect transversal to the chain in southern Tuscany show homogeneity in the framework composition but differences in the composition of finegrained lithics. Three petrofacies characterize this portion of the foredeep system: the «petrofacies A», present in the innermost successions (Baratti area), shows considerable amounts of carbonate (CE) and volcanic (Lv) lithics; the «petrofacies B», occurring in the central section of the studied transect (Sassetta area) shows low CE and high Lv content; the «petrofacies C», seen in the external and eastern sections (Poggio Ritrovoli and Chianti area), has low CE and Lv contents. The modal analysis indicates that the occurrence of volcanic and carbonate grains tends to decrease from the internal to the external portions of the Macigno. The source of the «petrofacies A» sandstones was an area characterized by crystalline and volcanic terranes, associated with carbonate covers; they could be represented by the Corsica-Sardinia Massif. In contrast, the source of the «second » and «third» petrofacies sandstones is thought to be mainly in basement domains such as the Western-Central Alps crystalline terranes.