Girotti, O.

The Middle Valley of the Tiber River (MVT) corresponds to the “Paglia-Tevere Graben”, a “neoautochthonous” basin developed since the latest Early Pliocene. The basin is in part linked with the intrapenninic Tiberino and Rieti Basins to the east, and with the Roman Basin to the south. The filling is mostly made up of Plio-Pleistocene marine deposits, unconformably overlaying the meso-cenozoic substratum. Two outcropping 3rd order depositional sequences have been recognised: 1) the older is composed of Lower Pliocene-earliest Upper Pliocene shelfal clays and sands (G. puncticulata and G. aemiliana Chronozones) and rarer continental deposits; 2) the younger is late Gelasian-Santernian in age (G. inflata and G. cariacoensis-B. elegans marginata Chronozones) and mostly corresponds to the “Chiani-Tevere formation”, composed of neritic clayey-sandy sediments interfingered with fluvial-deltaic gravelly-sandy deposits. The correlation between marine and non-marine deposits is based both on physical-stratigraphic observations and data (lithostratigraphy, facies analysis, sequence stratigraphy) and on their integration with biostratigraphic (foraminifera) and magnetostratigraphic data and with the numerical values of the 87Sr/86Sr ratio measured on mollusc shells, for the marine deposits, and with biochronological data (fresh- and brackish-water molluscs and ostracods, mammals) for the non-marine and transitional deposits. As for the younger successions from the latest Early Pleistocene to Holocene, which are characterised by travertines and gravelly fluvial terraced deposits, the correlation with the marine successions of the Roman Basin is indirect and mostly based on biochronology and on relations between sedimentary and volcanic units.

New data on Pliocene-Quaternary stratigraphy and on the geological evolution of the Middle Valley of the Tiber River (M.V.T.) are here presented. It is also presented the Geological Map of the Middle Valley of the Tiber River (two sheets), at scale 1:40,000. Two main tectono-sedimentary phases are recognised: the first one, Middle Pliocene-Early Pleistocene in age, was dominated by subsidence; the second, spanning from Early Pleistocene to Holocene, was dominated by the Apennine Chain’s uplift and volcanism. Two 3rd order sedimentary cycles characterise the first phase. The 1st cycle corresponds to the Tenaglie-Fosso San Martino Unit (Piacentian-early Gelasian) and is composed of coastal and marine shelf deposits. The 2nd cycle is composed of: the shallow marine and transitional Chiani-Tevere Formation (Late Gelasian-Santernian); the laterally continuous Poggio Mirteto Formation, of the fluvial-deltaic environment; the regressive, fluvial-lacustrine Giove in Teverina Formation. Fourth order progradational-transgressive cycles are recorded within the Chiani-Tevere Formation. The second phase comprises the products of the Mt Cimino Volcanic Complex (late Early Pleistocene) and the terraced fluvial deposits of the Civita Castellana Unit (late Early Pleistocene-Middle Pleistocene), Graffignano Unit (Middle Pleistocene), Rio Fratta Unit (late Middle Pleistocene), Sipicciano Unit (Late Pleistocene) and of the modern Tiber River plain. The fluvial units are covered by volcanics from the Vulsini Mts, Vico and Sabatini Mts Districts (Middle-Late Pleistocene), by the travertines of the Grotte Santo Stefano Unit (Middle Pleistocene), Fiano Unit (Late Pleistocene) and “Travertini recenti” (Holocene). The palaeogeography during the first phase was characterised by the transverse drainage of prograding fluvial- deltaic systems, which flowed from ENE to the centre of the M.V.T. basin. During the second phase the modern Tiber river developed with axial drainage. Type sections for the Chiani-Tevere and Poggio Mirteto Formations and for the Civita Castellana, Graffignano, Grotte Santo Stefano, Rio Fratta and Sipicciano Units are proposed.