Il Pleistocene inferiore nel ramo sud-occidentale del Bacino Tiberino (Umbria): aspetti litostratigrafici e biostratigrafici

Abstract

Several outcrops located in the southwestern branch of the Tiber Basin, where the S. Maria di Ciciliano Unit crops out, have been studied from the lithological, sedimentological and neotectonic points of view; paleontological and biostratigraphical analyses have also been carried out. Evidence of an alluvial plain environment with fluvial belt and flood plain fades are present in the Unit. A marshy environment evolving into a fluvio-lacustrine one is testified by a malacofauna and ostracofauna assemblage. Malacofauna collected in the S. Maria di Ciciliano Unit is characterised by primitive prosobranch molluscs which became extinct during the Early Pleistocene (T. (N.) groyanus, V. belluccii, Viviparus sp., M. affinis, and M. (G.) zitteli). Various molluscal assemblages are present in the Villa S. Faustino succession that shows a paleoenvironmental evolution varying from muddy humid subaerial conditions to a marshy environment. Near the top of the Villa S. Faustino section the dominance of M. (G.) zitteli is indicative of an oligo-mesohaline (<6‰) water composition. On the top of the succession a fresh flowing water faunal association is present. The malacofauna (Unio valves) of one of the studied sections (Colle S. Andrea) indicates an evolution from a shallow lacustrine to a fluviatile environment. At Villa S. Faustino, ostracofauna is characterised by two associations, one dominated by Cyprideis torosa and the other by Candona angulata, which suggests alternating chemical compositions of water, from oligo-mesohaline (NaCl governed) through freshwater to oligohaline (NaCl governed). On the whole, a marshy environment with abundant vegetation slowly evolves towards a system dominated by flowing waters (abundance of Ilyocypris bradyi). Sand overlays the silty-clay succession, deposited by highenergy waters; ostracods and molluscs are absent. Several mammal bones have been collected within the S. Maria di Ciciliano Unit at Villa S. Faustino, Colle Violino, Colle S. Andrea and Casale le Grotte; the following species have been recognised: Megantereon cultridens, Pachycrocuta cfr. brevirostris, Stephanorhinus etruscus, Equus stenonis, Sus strozzii, Pseudodama nestii, Pseudodama sp., Eucladoceros sp., Leptobos cfr. vallisami, Leptobos sp. (medium size) and Castor sp. Mammalofauna supports an Early Pleistocene age for the S. Maria di Ciciliano Unit, which can probably be correlated with the Tasso Faunal Unit.