L’evoluzione geologica e morfologica quaternaria dell’area adiacente la bassa valle del Fiume Fiora (Viterbo)

Geological and morphological Quaternary evolution of the lower Fiora Valley area (Viterbo)

Abstract

The lower reaches of the Fiora River valley show a tectonically disturbed sequence of Palaeozoic to Pliocene age, unconformably overlaid with marine and continental Quaternary deposits. Fifteen lithostratigraphic units have been identified within the Quaternary section, with a total thickness of some thirty meters. The depositional environment of the Quaternary section was mostly littoral but also deltaic and lagoon. The sedimentary units overlie each other transgressively; a few paleosoil remnants interbedded in the sequence point to relatively long emersion periods. A characteristic feature of the studied area is the flatness of the summit areas, which dip weakly towards the present coastline. They are separated by valleys larger to the west and smaller to the east of the Fiora River, due to the different watershed dimensions. Four esplanades have been identified, which fit the Quaternary geological interpretation of the area on the basis of their bedrock lithology, their mutual morphological and areal relationships as well as, where present, of their pedologic features. The chronologic references utilized for dating the litho and morpho-stratigraphic units are the following: 1) presence of volcanic materials attributable for their features to the Vulsinic volcanic activity; 2) remnants of a pedogenesis which may be observed on the oldest of the esplanades; 3) morphologic, stratigraphic and faunal correlations, with the Quaternary deposits of the northern Latium. The two lowermost units of the Quaternary sequence (“Sabbie e Ghiaie Silicee di Mandria Polidori” and “Argille di Fontanile Secco”), most likely of upper Pliocene (?) – lower Pleistocene age, are tilted and faulted. On the contrary, the thirteen overlying units do not appear to have been affected by disjunctive tectonic movements, except for the “Ghiaie e Tufiti del Tafone” sedimentaty unit, in the upper Tafone Valley. These are in fact disrupted by small direct and reverse faults and, accordingly, the summit esplanades are here much steeper (~5 %) than in the southern part of the lower Fiora River valley where they do not attain a steepness of 1 %. The data so far collected point to a middle-upper Pleistocene evolution mostly characterized by several relative sea level fluctuations which brought about the deposition of at least as many marine, brackish or continental units. These may be divided into two large groups. The first group includes the older units of about 600÷700 000 y. to 200000 y.B.P. in age (“Calcareniti, Ghiaie e Sabbie di Montalto” to “Complesso Continentale di C. Palomhini “). Traces of maximum ingression lines of at least five trangressions belonging to this group are recorded at 20 to 30 m above the present sea level. The second group of units, which are less than about 200 000 y. old, includes the “Complesso Marino e Continentale Superiore ” of “Tyrrhenian” age (Auctt.). At least one erosive surface is recognizable in this complex. The coastline of the older “Tyrrhenian” cycle is at 25 m a.s.l. Such evolutionary pattern whereby erosive and sedimentary events alternate, due to fluctuations of the relative sea level does not entail the intervention of disjunctive tectonic movements in middle-upper Pleistocene times (with the exception of the upper Tafone Valley). This is confirmed also by the lack of appreciable discontinuities in the summit esplanades and by the small difference in height which separate the esplanades from