Il Pliocene ed il Pleistocene inferiore del bacino del Fiume Tevere nell’Umbria meridionale

The Pliocene and the Lower Pleistocene of the Tevere Basin in Southern Umbria

Abstract

The purpose of this study is to talk over sedimentation, sedimentary environments and their evolution during Pliocene and Lower Pleistocene. Moreover, sedimentary deposits are described in order to show their horizontal and vertical relationships. Sedimentary sequences in southern Umbria (Central Italy) are grouped mainly by lithostratigraphic features, but there is evidence of good agreement betweeen lithologic variation in time and biostratigraphic and geodynamic evolution. In this area it is possible to define two main sedimentary basins: marine the former and lacustrine and fluviolacustrine the latter. The two are separated by the Apennines and in both of them it is possible to distinguish two sedimentary cycles. The lower is Pliocene in age, Early Pleistocene the upper. During Pliocene the marine sequence is made up by mud («argille di Fabro»), sand («sabbie a Flabellipecten») and conglomerate («conglomerato di Città della Pieve») at the top. By the detailed sedimentological analysis of each lithologic unit it is possible to ascertain that the sequence is prograding from the shelf (mud), to the shore face and the foreshore (sand) and, at the top, to the backshore (conglomerate). Biofacies analysis confirms this environmental evolution during Lower and Middle Pliocene. At the same time, the continental basin sedimentation is characterized by lacustrine mud («argille grigie inferiori»), Middle Pliocene in age; there is no field evidence of communication between the two basins. Everywhere the Lower Pleistocene sediments overlay the Middle Pliocene deposits with angular unconformity; the sedimentation gap corresponding to Upper Pliocene is attributed to the tectonically controlled erosive phase said Acquatraversa (2.07- 1.8 m.y.). The Lower Pleistocene marine sequence consists of muddy sand («argille sabbiose del Chiani-Tevere») whose sedimentation is partly in marine, partly in brackish water. The sequence is regressive at the Pliocene one, but regression is less intensive and probably controlled by eusthatic changes. The sediment features are locally controlled by coast morphology too. Also in the continental basin, presumably a broad lake with coves, small deltas and swamps, muddy sand («complesso argilloso-sabbioso») is prevailing and its age is Late Villafranchian. Field (lateral variation) and biofacies (widespreading of transition species) evidences confirm that during Lower Pleistocene the two basins were communicating. The sea level fall beging before the first appearance of Hyalinea balthica (1.4 m.y.) and above the partly eroded marine and continetal deposits somewhere swamp and peaty muddy sediments («torbe e limi») are laid down. During marine regression a travertine facies («travertini antichi») older in the continental than in the marine basin, is set up. Time elapsed between the beginning of the travertine deposition in the two areas corresponds to the chronological interval during which the erosive phases underlying the swamp and peaty sediments took place. This phase corresponds to Aulla (1.6 – 1.3 m.y.) and is controlled mainly by Tectonic and is characterized by a climatic deterioration, as the disappearance of warm-temperate molluscan-faunas points out. In the lacustrine domain there is a lateral variation from the travertine to clastic sediments («complesso detritico superiore») whose sequence is characterized mainly by sand at the base and conglomerate at the top. The coarsening upward of these sediments probably indicate a deeper land denudation and a stronger stream flow energy probably due to a more intense tectonic activity. The sudden increasing of rivers load caused the lacustrine basins filling up and the setup of fluviatile facies with new and quite different fresh water faunas. The conglomerate lithofacies and the travertine top level presumibly correspond to the start of the Cassio erosive phase which probably begun about 1 m.y. ago.