New biostratigraphic data from the Early Pleistocene tyrrhenian palaeocoast(western Umbria, central Italy)

摘要

Plio-quaternary marine deposits are largely documented in western Umbria (central Italy), although they still lack biostratigraphic defi nition. Contrary to published data, Early Pleistocene deposits outcrop more extensively than previously reported in the Orvieto area. A composite biostratigraphic succession, almost continuous from the top of the G. gr. crassaformis Zone to the top of the Gl. cariacoensis Zone, can be reconstructed in offshore clay sections. Nannofossil assemblages and marker events (bmG, tCm, blG, tHs, tlG) from the MNN16a to MNN19e subzones have been documented. Lower shoreface – transition to offshore sections as described, are characterized by poor planktonic assemblages; nevertheless, they are still referable to the same stratigraphic interval. Deposits can be partially inserted into the “Chiani – Tevere” depositional cycle, also documented in this area. Moreover, marine conditions persist in the area from the base of the Gelasian to the top of the Calabrian, and it can be modelled as a peripheral,survival sea-branch, cut-off from the main river supply and from continental infl uence. However, Zanclean to Piacenzian deposits occur in a small area, localized around the town of Orvieto, so the former distinction of superimposed depositional cycles can only be speculative.