Chronostratigraphic revision of some Pliocene basino in Tuscany at the Zanclean/Piacenzian boundary.

The data on Tuscan Pliocene basins (which were acquired during recent years) are revised and re-evaluated here in the light of the formal subdivision into three stages (Zanclean, Piacenzian and Gelasian) of the Pliocene Series. Particularly, data on the sediments close to the Zanclean/Piacenzian boundary are considered. These sediments registered important tectonic changes which can be accurately placed both chronostratigraphically and chronologically. The Pliocene sedimentation in Tuscany started at the Messinian/ Zanclean boundary (5.33 Ma) and ended in the uppermost Piacenzian (about 2.4 Ma). Some areas were characterised by a continuous sedimentation, while in other areas two depositional cycles were realised. In the second case the youngest sediments of the first cycle are referred to the Reticulofenestra pseudoumbilica LO (3.89 Ma)- Globorotalia puncticulata LCO (3.59 Ma) interval, and the basal levels of the second cycle closely postdate the Globorotalia crassaformis gr. FCO (3.1 Ma). Therefore, a sedimentary hiatus of about 500 ky separates the two cycles. The end of the first cycle and the beginning of the second were caused by events which also determined sedimentary and environmental changes in the continuous succession.