Evoluzione tettonico-sedimentaria neogenica lungo una trasversale ai bacini di Volterra e della Val d’Elsa

The tectono-sedimentary Neogene evolution of a key area of Southern Tuscany has been analyzed. The area is characterized by two extensional basins (the Volterra Basin to the west and Val d'Elsa Basin to the east) separated by a morphological and structural high (the "Dorsale Medio Toscana"). The analysis has been carried out along a cross section from the southwest edge of the Volterra Basin (Mazzolla-Castellina in Chianti) to the eastern edge of the Val d'Elsa Basin (Chianti Mountain). Six paleo-sections were reconstructed ranging in age from the Serravallian to the Recent. Biostratigraphic and paleoenvironmental information obtained from micropaleontological analyses has allowed the evaluaionn of fault activities in the two basins at the various stages of evolution. During Serravallian-early Tortonian the whole area was part of a single sedimentary basin: the Ponsano Basin. That is, there is no evidence to suggest that at that time the "Dorsale Medio Toscana" was a morphological high. During late Tortoian the "Dorsale Medio Toscana" developed and separated the basins of Volterra and Val d'Era. The separation was not complete, however, as indicated by identlcal ostracod assemblages in the two basins. The basins contain fluvial and lacustrine sediments. Rapid rise and partial emersion of the "Dorsale Medio Toscana" and possible presence of relatively steep slopes are suggested by conglomeratic debris flow deposits interbedded with lacustrine sediments. Furthermore, pebbles of conglomeratic layers of fluvio-lacustnne sequence were totally originated from the rocks of ligurian units and from the Ponsano Formation that formed the ridge bedrock. During early Messinian the "Dorsale Medio Toscana" was fully emerged, and separated completely the two basins. In fact, Messinian marine facies have been found in the Volterra Basin but not in the Val d'Elsa Basin. During late Messinian ("lago mare" phase) a connection was re-established between the two basins as indicated by identical microfaunistical assemblages. This connection must have developed elsewhere, because along the studied cross-section the "Dorsale Medio Toscana" remained a morphological high. During Pliocene the two basins behaved differently. The Volterra Basin experienced a continuous high rate of sedimentation, whereas the Val d'Era Basin experienced discontinuous, variable sedimentation. Furthermore during the early Mlddle Pliocene, subsidence affected the area studied and much of the "Dorsale Medio Toscana" was submerged except in the Spicchiaiola-Pignano region. Afterwards still in the Middle Pliocene, the whole area was affected by regional uplift, which led to a major marine regression everywhere in Southern Tuscany.