Chronostratigraphy of the Pliocene at Legoli (Valdera-Volterra Basin; Tuscany, Italy)

The shallow and deep water sediments and fossils of Tuscany have been studied for centuries, but uncertainties still exist regarding the chronostratigraphy and the tectonic regime controlling the basin development. In an overall extensional tectonic regime, related to the opening of the Tyrrhenian Sea, Neogene-Quaternary Tuscan basins have been interpreted either as bowl-shaped basins evolving into graben bounded by normal faults, or as thrust-top basins (broken foreland basins). To better understand the dynamics of Tuscan basin infill, we present an integrated stratigraphic study of a succession exposed at the Belvedere waste facility near Legoli (municipality of Peccioli, Pisa, Italy), in the Valdera-Volterra basin (VVB). The section consists of clays and sandy clays interrupted by two sandy intervals and topped by a mainly sandy succession. We reconstruct depositional dynamics through sedimentary facies analysis and palaeoecology, and we use biostratigraphy and palaeomagnetism for chronostratigraphic assignment of the succession. The bio-magnetostratigraphic data indicate that the section falls at the Zanclean/Piacenzian transition, encompassing the Gilbert reversed chron (C2Ar) to the Gauss (C2An.3n) normal chron transition dated 3.6 Ma. Our study constrains the sedimentary infill of the VVB at Legoli between the latter part of the Zanclean and the early Piacenzian, at the onset of the first signs of a Northern Hemisphere glaciation. The section shows a shallow tilting towards the WNW, tilting that decreases towards the top of the section, pointing to tectonic control on deposition at a time of increased sediment accommodation due to basin subsidence. Our results indicate that this sector of the VVB, interpreted as a thrust-top basin under a crustal shortening acme during the Messinian, underwent tectonic subsidence controlled by normal faults during the Piacenzian.