Idrografia pleistocenica della bassa valle del F. Tevere (Italia centrale)

Abstract

A critical reading of both published and unpublished geological reports has brought to the elaboration of a scheme of evolution of the River Tiber hydrographic features downstream of the confluence with Aniene river during middle Pleistocene. Boreholes drilled at Giannottola - a locality between Aprilia and Cisterna, in central Latium - crossed a palaeo-riverbed with thalweg below the elevation -110, cut into the ”Grey Tuff” which is the product of the first stage of activity of Latium Volcano, and into the underlying clay of Emilian age. The relative sea level that is supposed around the elevation -150, is attributable to the Nomentana Regression which dates back to 0.43-0.40 Ma. The palaeo-riverbed is f illed by mainly gravelly alluvial deposits up to the elevation -30; above such elevation up to the elevation 0, the alluvial deposits are covered by Old Lava whose age is about 0.4 Ma. The ”Puzzolana Formation”, ”Lithic Tuff” and ”Puzzolanella Formation” complete the infilling of the palaeovalley which has caused a relief inversion. Minor gravelly riverbeds are shaped at various elevations into the ”Puzzolana Formation”. In the area of Rome, 3 different palaeo-riverbeds of the Tiber have been identified over a length of about 20 km (i.e., Palaeo-Tiber 1, 2, and 3) which are likely coeval of Cassia (0.9 Ma), Flaminia (0.6 Ma) and Nomentana (0.43-0.40 Ma) regressions. Palaeo-Tiber 3 can be correlated both altimetrically and planimetrically to the palaeo-riverbed found at Giannottola. Paludal deposits, that may contain intercalations of minor gravelly riverbeds (authochtonous Maremman age) and which overlie the Palaeo-Tiber 2 and 3 alluvial deposits in the Rome zone, have formed when the river course turned into a swamp because of the obstruction built across the valley by volcanic deposits. Therefore, the deposits of Maremman age are coeval of Old Lava and of the oldest portions of Puzzolana Formation, having being deposited in a time interval between 0.40 Ma (Old Lava) and 0.36 Ma (Villa Senni Tuff). The first evidence of the Tiber present riverbed can be referred to the flood following the obstruction of the Palaeo-Tiber 3 valley. Such primary riverbed has subsequently been deepened and modeled by the Ostia Regression (0.25 Ma) and the various regressions occurred in Upper Pleistocene, especially by the Wurmian Regression with sea level at -110 (0.02 Ma). The finding of Palaeo-Tiber 3 alluvial deposits preceding the emission of Old Lava by boreholes drilled in the Pontina Plain is hindered by their presence at greater depth because of: i) the volcanic deposits which are superposed to the palaeovalley on the W; and ii) a subsiding area on the E including the SW slope of Lepini Mountains and the NE portion of the Pontina Plain. Such subsiding stage is characterised by the deposition of infralittoral silt about 200 m thick covered by some 100-m thick paludal deposits. The only evidence of the palaeoriver is the alignment of valleys in the Calabrian-Pliocenic bedrock as shown by geophysical data and a borehole near the mouth of Sisto river which can probably be correlated to the Giannottola boreholes. The Pontina Plain has not an actual hydrographic net after the infilling of the Palaeo-Tiber 3 valley with volcanic deposits. It is thus possible only to identify a few mainly gravelly accumulations formed by short-lived and wandering water flows at progressively greater elevations. Gravelly accumulations were found: i) in the subsiding zone on the NNE of Borgo Vodice where they are intercalated with the infralittoral silt probably because of the gradual superposition of the subsidence on to the left slope of the Palaeo-Tiber 3 valley; ii) in the non-subsiding area along the 3 main alignments S.Virginia-Borgo Carso (8 km long), BorgoPiave-Latina (9 km long), and Valmontorio-Mt Circeo (40 km long). A few elements point to the probable existence of a Palaeo-Aniene, some 10 km on the S of the present riverbed, and of a Palaeo-Sacro flowing from Sgurgola to the NW which, likely, flowed into Palaeo-Aniene.