LE UNITÀ TORBIDICHE DELLA MARNOSO-ARENACEA NELL'ALTA VALLE TIBERINA

Abstract

The Miocene turbidite units outcropping in the Upper valley of the Tiber River in the area between Monte S. Maria Tiberina and the Apennines divide, interpreted in the literature as tectonic units (M. Nero Unit; Castellaccio Unit, etc.; TEN HAAF & V AN WAMEL, 1979), constitute on the con­trary two successions of the Marnoso-arenacea. The former consists of calcareous skeletal-siliciclastic turbidites (member A) ----> turbidite arkose (member B) --+ siliciclastic turbidites with the Contessa Layer (member C); it outcrops from Sansepolcro to the Passo della Calla. The latter, outcropp­ing on the right of the Ti ber River, consists of calcareous skeletal-siliciclastic turbidites --+ arkose-type turbidites (Celle Sandstone) --+ siliciclastic turbidites of the Marnoso-arenacea. Both the mixed calcareous skeletal-siliciclastic turbidites and the arkose turbidites represent during the Burdigalian­Langhian two secondary supplies from SW the umbro-roma­gnolo basin, with subsequent deflection mainly toward NW into just opposite to the flowing direction of the siliciclastic turbidites with NW alpine provenance that fed the basin itself. Such supplies formed two small fans on the western edge 􀀛f the basin in the area corresponding to Monte S. Maria Tiberina. The fan connected to the arkose supply fed a nar­row depression stretching the Apenninic trend about 55 Km and 4 Km in width, that acted as an erosional-depositional deep-sea channel, whose filling consists of a clastic wedge thinning out towards NW. The calcareous skeletal detritus is likely to be derived from a ridge that separated the Umbria domain from the Tuscan domain, whereas the arkose supply is likely to be derived from a granitic source pertaining to an Alpine Chain fragment located in the Tyrrhenian area.