I bacini pleistocenici di Anghiari e Sansepolcro nell'alta valle del Fiume Tevere

Abstract

On the Umbria-Tuscany boundary, between Montedoglio and Città di Castello, the alluvial plain of the Tiber River spreads into the tectonic depression of the Tiber river upper valley. Turbiditic complexes of the Tuscan-Umbria sequence form substratum of the "basin", which is filled by fluvio-lacustrine clastic sediments. The lower units of this sequence are dated to the Lower Pleistocene. The area is divided into two minor depressions, drained by the Sovara stream (Anghiari Basin) and the Tiber River (Sansepolcro Basin), respectively. On the basis of geomorphological features, anomalies in the drainage pattern, and of lithological, structural and stratigraphica! considerations, the sequence of paleogeographical events that gave rise to the present landscape can be hypothesized as follows: 1) Extensional tectonic movements in the Lower Pleistocene gave rise to the formation of a wide lacustrine basin, filled by a thick sequence of fluvio-lacustrine sediments (mostly clay and sand, with gravel on the borders of the basin). 2) The extensional tectonic activity continued into the Middle Pieistocene(?) as shown by displacements in fluvio-lacustrine sediments. These are cut by two parallel faults causing the westward tilting of the displaced "block", which formed the Anghiari Ridge and split into two parts the original depression. As a result the original drainage (NE oriented) of the main depression (Sansepolcro Basin) digressed southwestwards producing a partially independent basin (Anghiari Basin) drained by the Sovara stream. 3) Late landscape modifications were governed by the course of the Tiber River forming terraces of uncertain age at various elevations, while marsh remnants were present in the lowest portions of the upper Tiber valley up to historical times.