Evoluzione paleogeografica del territorio umbro alia confluenza Tevere-Nestore (bacini di «S. Fortunato e di Riparvella»)

Paleogeographic evolution of the Umbrian territory at the confluence of The Nestore-Tevere rivers (S. Fortunato basin and Ripalvella basin)

Abstract

The surface deposits of Tiber valley, among Perugia, Castiglione della Valle and Todi, has been attribued geologically to fluvial-lacustrine facies of «Lago Tiberino» (Villafranchian AUCTT). However, the study of this area shows the presence, at W of the large Tiberine depression, of a stratigraphic sequence not well relating to the typical one of «Tiberino» lake. In this paper such deposits are ascribed to two different pleistocenic sedimentation basins: «S. Fortunato» at North and «Ripalvella» at South. The geomorphological evolution of the area, appeared strongly controlled by extensional tectonic linked to a fault system oriented both NW-SE and NE-SW. The basins of «S. Fortunato» and «Ripalvella» are bounded, westward, by normal faults. The basins are located in a lowered narrow area, almost synclinalic, elongated and parallel to the large depression of Tiber river, and divided from this by a little anticlinalic relief. Later the evolution of the area goes on with the relative lifting of a trapezium-shaped block (including the edge between Perugia and Nestore river) and with contemporaneous sinking, eastward, of an area inside «Tiberino» basin (Marsciano). The local hydrographic net fits in with these movements: Nestore river Joins the Tiber river (which is developing) towards the lowered area, shaping its course on the faults of the trapezium-shaped block. The Faena creek cuts the anticlinal structure which limits, to the E «Ripalvella basin», causing its draining and its deposit erosion. The whole territory, including the actual Tiber river’s valley, tilts eastward. This event is witnessed by the present Tiber which flows leaned against the hydrographic left and leaves, on the right, meander bends and terraces. The arrangement of these terraces allows to express some hypothesis about the age of the most recent tectonic phases.