Depositional environments of the Plio-Pleistocene Upper Valdarno Basin (Tuscany, Italy)

The Upper Valdarno Basin is located about 35 km SE of Florence between the Chianti Mountains and the Pratomagno Ridge. The basin fill is made of four synthems named as Castelnuovo dei Sabbioni, Montevarchi, Fosso Salceto and Torrente Ciuffenna synthems. The Castelnuovo dei Sabbioni Synthem (Late Pliocene) consists of coarse-grained, stream gravels grading upwards into sheet-like, alluvial sand. These sands are overlain by a muddy lacustrine unit bearing, at its base, two well-developed lignitiferous seams accumulated in a coastal marsh setting. The lacustrine mud grades upwards into deltaic sand accumulated in a shallow-water delta under repeated lake-level oscillations. The Montevarchi Synthem (Late Pliocene to Early Pleistocene) consists of two portions separated by an unconformity surface passing basinward into a correlative conformity. The lower portion of the Montevarchi Synthem is made of alluvial fan gravel and sand passing upwards into fluvio-aeolian sandsheet deposits, consisting of aeolian-reworked, alluvial sand bearing isolated channels. Fluvio-aeolian sandsheet deposits are covered by mollusc-rich, alluvial sand which makes lateral transition into lacustrine muddy deposits. The upper portion of the Montevarchi Synthem consists of fluvial and alluvial fan deposits. Fluvial deposits occupy the axial part of the basin and are referred to sandy channels wandering through a muddy floodplain hosting shallow lakes and swamps. Alluvial fan deposits occur along the basin margins and consist of proximal gravels grading downfan into gravelly sand and a variety of sandy facies. Floodplain lakes deposits are well-developed in the middle part of the upper Montevarchi Synthem and in the Palazzolo sub-basin (Fosso Salceto Synthem), where they are overlain by alluvial-fan gravels. The Torrente Ciuffenna Synthem (Early to Middle Pleistocene) consists of fluvial sediments in axial part of the basin and alluvial fans deposits along the basin margins. The axial fluvial deposits were accumulated by the paleoArno River and consist of gravel and overlying sand. The basal gravels were deposited by low-sinuosity channels, whereas sandy deposits were formed by moderate to high-sinuous channels. The alluvial fan deposits consist of proximal gravels passing downfan into gravelly sand and sandy facies.