Review of knowledges about the Neogene stratigraphy of Southern Tuscany: the Volterra Basin

Neogene Tuscan deposits record a variety of depositional systems whose evolution resulted from the complex interplay between regional tectonies and eustatism. In Tuscany, after the Alpine collisional phases, Late Miocene extensional tectonics gave rise to morpho-structural depressions limited by emerging topographic highs, still partially recognizable in the present-day opography. All the Neogene Tuscan basins westward of the Orsaro-Chianti-Cetona Ridge, experienced alternating marine and continental settings, which resulted in the deposition of thick sedimentary "neoautochthonous"successions. They were studied since the end of the 19th century (Geological Map Italy, I ed.) and during the 1960s and 1970s5, for the realization of the Geological Map of Italy, Il edition. The whole Neogene succession is typically well exposed in the Volterra Basin; there, new fieldwork carried out in the framework of the National CARG and the Tuscan Region Cartographic projects, led to reconsider and to improve both the stratigraphy and the tectono-sedimentary evolution of the Neogene succession. After a first phase of post-orogenic, coastal marine sedimentation, typified by the Late Serravallian-Early Tortonian Ponsano Sandstone, the Volterra Basin experienced a Late Tortonian continental depositio, represented by marginal fluvio-lacustrine conglomerates grading basinward to freshwater fines. Early Messinian deposits comprise brackish marls with gypsum and marls with serpulids. In the basin margin they are unconformably overlain by patch reef facies and conglomerates, whereas basinward they are covered by marine fines. The following Messinian salinity crisis resulted in the deposition of gypsum strata, which are separated by an unconformity from the overlying, brackish to continental-lacustrine c1ays, marls, gypsum, sands and conglomerates, typically referred to the Lac-Mer depositional phase. A dramatic change occurred at the Miocene-Pliocene transition, when a sudden marine transgression reached the Orsaro-Chianti-Cetona Ridge and the Lac-Mer deposition came to an end. Shelf to coastal settings dominated up to the late Middle Pliocene regression, which led to the emersion of the whole Tuscany. Such a regression resulted from the overall uplift related to the post-collisional extensional tectonics.