The Upper Valdarno Basin (Northern Apennines, Italy): from “Azzaroli & Lazzeri (1977)” to the present understanding of a pivotal continental succession in the Pliocene-Pleistocene of Europe

This paper reviews studies of the Upper Valdarno Basin, in order to highlight the unique stratigraphic and palaeontological characteristics of the area. Over recent decades, the stratigraphy of the basin has been meticulously refined, establishing it as one of the most well-documented basins in the Northern Apennines. The sedimentary fill of the basin consists of three unconformity-bounded units: Castelnuovo dei Sabbioni (CSB), Montevarchi (VRC), and Torrente Ciuffenna (UFF) Synthems, with the Palazzolo sub-basin comprising the Fosso Salceto (OLC) and Torrente Ciuffenna (UFF) Synthems. Formed during the Late Pliocene, the basin underwent significant geological changes, transitioning from deposition of fluvial gravels to lacustrine conditions around 3.1 Ma, followed by deltaic sand accumulation and subsequent tectonic uplift events. The Early Pleistocene saw the formation of floodplain lakes and swamps, leading to the accumulation of organic-rich sediments. The vertebrate fossil record of the Upper Valdarno Basin is distinguished by exceptional preservation of the material and is of historical significance, with documentation dating back to the Renaissance. The efforts and finds both by fossil hunters and palaeontologists have contributed to the importance of the collection in defining numerous Pliocene and Pleistocene species and genera, as well as in confirming biochronological subdivisions of the continental timescale. The palaeontological studies initiated by Augusto Azzaroli in the post-war period further enriched the documentation of the fossil collection of the basin, underlining its status as a crucial site in the understanding of Pliocene and Pleistocene vertebrate faunas