The Pliocene deposits in the Central-Eastern Valdelsa Basin (Florence, Italy) revised through facies analysis and unconformity-bounded stratigraphic units

The Valdelsa Basin is one of the widest post-collisional basins developed i n central Tuscany (Italy) since the late Miocene. More than 2,000 m of uppe r Miocene-Quaternary deposits fill the basin and 1,000 m of which, ascribed to the Pliocene, crop out extensively. In previous studies these deposits have been included in poorly defined lithostratigraphic schemes, and also a detailed chronologic Calibration via biostratigraphic analysis has produce d few results due to the paucity of biomarkers. In this study the Pliocene deposits have been subdivided into eleven lithofacies associations, in turn included in five unconformity-bounded stratigraphic units (synthems) spann ing in age from the uppermost Zanclean to most part of Piacenzian. Lithofac ies associations, defined as the recurring vertical alternance of several b asic lithofacies, are in fact recognized based on both the lithologic featu res and the sedimentary/bio- (marine to continental molluscs) facies conten t which formed in specific environments. Several depositional systems activ ated in the basin, from alluvial to fluviodeltaic to paralic and inner shel f systems, in response to change of sediment supply and accommodation space . The Pliocene paleogeography of the portion of the basin considered in thi s study was characterized by the occurrence of a structural high that separ ated two distinct depositional areas in the early stage of the basin fill. A marine (delta front to inner shelf) area developed in the central part (C ertaldo-Castelfiorentino area), while an alluvial plain was forming eastwar d (in the S. Casciano area). During the Piacenzian the basin was characteri zed by the progressive filing of a shallow marine area (central part) due t o the southwestward progradation of fluvio-deltaic systems.