Depositi continentali plio-pleistocenici nell'area di Monte Giovi. Relazione tra l'evoluzione idrografica e la neotettonica della ValdiSieve (Firenze)

Abstract

The results of a study on the Plio-Pleistocene continental deposits cropping out in the Monte Giovi area (Mugelio-ValdiSieve area, Florence, Italy) are given in this paper. These deposits are interpreted as the record of tectonic events which influenced the hydrographic and physiographic regional evolution. Three continental successions can be identified, i) A fluvio-lacustrine succession of mass flow-dominated fan delta cobbles and gravels and lacustrine silty clays of latest Pliocene-lower Pleistocene age, which crops out only along the Mugello side of Monte Giovi. The top of the succession is covered by alluvial fan gravels, ii) A fluvial terraced succession of gravels, sands and silts which were deposited by braided, low sinuosity rivers and alluvial fans during three main fluvial episodes in late early Pleistocene?- Holocene times, iii) Colluviai sediments (latest Pliocene?-Pleistocene), which consist of chaotic deposits formed by debris fall, debris flow and colluviai slide materials. Travertine was deposited in a quiet environment (palustrine?) by carbonate-rich water from springs probably related to deep faults. Stratigraphic and sedimentological analyses together with geomorphic and structural observations indicate the presence of at least four tecto-sedimentary phases in the area during the latest Pliocene and Pleistocene. The first phase (latest Pliocene?) was characterized by extensional processes that caused the opening of the Mugello asymmetric fluvio-lacustrine basin. During the second phase (lower Pleistocene) the flanks of this basin were greatly uplifted and the depositional pattern of the fluvio-lacustrine succession was deformed. Uplifting might be related to compressive pulses. The third and fourth phases (latest lower/middle Pleistocene) were associated with extensional tectonics that reactivated the Monte Giovi differential uplifting movements, probably controlling fluvial and colluviai depositions. The fluvio-lacustrine and fluvial successions seem to have had a greater bearing on tecto-sedimentary events than the travertine and colluviai deposits. From the depositional point of view similar successions can be recognized also in other basins located in nearby areas (Firenze-Prato-Pistoia, Valdarno Superiors, Arezzo, Casentino). Deposition during the fluvio-lacustrine and fluvial phases was controlled by subsidence and uplift processes acting at different times in the various basins.