Tectonic and sedimentary evolution of the Upper Valdarno Basin: new insights from the lacustrine S. Barbara Basin

We describe stratigraphic, structural and kinematic data from the sediments of the Upper Pliocene Santa Barbara Basin and from its substratum. The results shed light on the relationships between tectonics and sedimentation in the larger Late Pliocene-Middle Pleistocene Upper Valdarno Basin of which the Santa Barbara Basin is considered a precursor. The sediments filling up the Santa Barbara Basin are made up of alluvial to deltaic and lacustrine deposits, grouped in the Castelnuovo dei Sabbioni (CSB) Synthem, related to Late Pliocene. This synthem was deposited in a tectonic depression reasonably delimited to the East by a west-dipping normal fault system and delimited to the North and to the South by left-lateral trans-tensional shear zones, which controlled the main directions of the alluvial drainage. During Early Pleistocene, a new master normal fault system (Trappola fault system) developed further to the East, determining the widening of the previous tectonic depression, now delimited to the North and to the South by the regional Piombino-Faenza and Arbia-Val Marecchia transfer zones, respectively. In this new tectonic depression, with a dominant axial drainage direction, alluvial, fluvio-aeolian and fluvial sediments (Montevarchi Synthem, VRC) deposited during Early Pleistocene. The VRC Synthem, being located in the hanging-wall of the Trappola normal fault system, is slightly tilted to the NE. Finally, during Early-Middle Pleistocene, axial fluvial deposits (Torrente Ciuffenna Synthem, UFF), sealed the previously formed brittle structures. Our kinematic and structural data allow us to confirm the interpretation that the Santa Barbara Basin is the precursor of the Upper Valdarno Basin and that both basins developed in structural depressions formed by the interplay between normal and transfer faults, framed in the extensional tectonics which characterizes Tuscany since Miocene.