Il controllo strutturale nei bacini intermontani Plio-Pleistocenici dell'Appennino settentrionale: l'esempio della successione fluvio-lacustre del Mugello (Firenze)

Abstract

The Mugello basin is a small asymmetric graben, which developed at the end of Pliocene as a consequence of extensional tectonic movements in the northern Apennines. The basin is filled with a fluvio-lacustrine sequence up to 600 m thick, which formed during two main depositional phases. During the first phase, fluvio-lacustrine sediments (lacustrine silty clays) were deposited in the basin. At the basin margins, locally silty clays contain peat horizons interlayered with fan delta gravels and sands. The second phase started when the fluvio-lacustrine basin was almost completely filled and is characterized by alluvial deposits. Sedimentation occurred in alluvial fan and braided/low sinuosity river settings and was influenced by local base-level lowerings due to the erosion of several downbasin thresholds. These episodes led to the formation of a typical terraced alluvial succession. Tectonics is considered to be the main factor controlling the sedimentation pattern at different physical and time scales. The depositional pattern of the whole succession is largely controlled by the structural asimmetry of the basin, characterized by a main fault system running along its south-western margin (footwall). Wider depositional systems developed during both phases along the hanging-wall rather than on the footwall of the basin. During the first phase, the hanging-wall underwent uplift pulses that produced syndepositional deformations marked by several angular unconformities in the fan delta deposits. These deformational pulses controlled the evolution of the depositional processes. An abrupt change in facies associations forming the depositional elements of the lacustrine fan delta, stems for a sharp passage from massive (lower fan deltas) to selective (upper fan deltas) depositional processes. Without excluding a prevailing climatic control (i.e., oscillation from a dryer to a moister regime), this variation can reasonably be associated with the tectonic evolution of the hanging-wall shoulder. The rate of hanging-wall uplift decreases throughout the first phase allowing, in its attenuation phase, the enlargement of the drainage systems feeding the upper fan deltas. A tectonically-induced hydrological variation can be assumed as one of the probable causes for the shift from mass-flow-dominated to streamflood-dominated fan deltas. Deformation structures (for instance, water-escape structures) occur in fan delta deposits probably testifying the tectonic control at the scale of single depositional events.