The growth of the Chianti Ridge: progressive unconformities and depositional sequences in the S. Barbara Basin (Upper Valdarno, Italy)

The Chianti Ridge, elongated in a NW-SE direction, separates the Upper Valdarno from the Siena Basins. It is made of Macigno and Scaglia Toscana Fms. belonging to the Tuscan Nappe that underlies the Ligurian Units. Inside the Upper Valdarno basin three main synthems have been identified, separated by major unconformities; from the bottom: 1. Castelnuovo synthem, subdivided into Spedalino and Meleto subsynthems, 2. Montevarchi synthem and 3. Bucine synthem. The Spedalino subsynthem, that unconformably lies over the pre-Pliocene bedrock, is made up of thin and laterally discontinuous coarse gravels. The Meleto subsynthem, commonly referred to a lacustrine environment, is made of alluvial plain lithofacies, including channel, crevasse splay and swamp depositional systems where a Taxodium forest developed and peat accumulated. Close to the western margin of the basin these subsynthems are tilted up to 60°. The Montevarchi synthem, mainly made of sands and gravels accumulated inside a wide braidplain, and lies unconformably over the previous units. This synthem is tilted eastward up to 20°. Finally, the Bucine synthem lies unconformably over the previuos synthems generating a fluvial terrace of the Arno River. The two lower units date back to the Early Pliocene. In fact the Meleto clays contain mammal remains that can be found in the Triversa Faunal Unit (Early Villafranchian, late Early - Middle Pliocene) as well as in the Ruscinian (Early Pliocene). They are also negatively magnetized and could correspond to the Gilbert Chron. Palaeomagnetic investigations and the presence of aeolian sediments in the Montevarchi synthem and cold pollen flora allowed its attribution to the Middle and Late Pliocene. The relationships between the different synthems and the dominant unconformities coupled with thermochronological data recently obtained in the Apennine ridge to the east (BALESTRIERI et al., 2003) allow us to establish that the Chianti Ridge, after a major planation, became the eastern edge of a moderate relief (Spedalino subsynthem) that was later transformed into a wide alluvial plain (Meleto subsynthem). Updoming tilted the previous units before the modelling of a second major unconformity that preceed the deposition of the Montevarchi synthem. It is possible that during this period the Valdarno was still connected to the Siena Basin to the west. A third major unconformity followed finally separating the two basins. These deformations occurred almost at sea level. During the Early and Middle Pliocene, the Apennine Ridge to the east did not exist and the Upper Valdarno represented the western part of the peri-Adriatic Basin. The deepening of the drainage network is due to the generalised uplift that in the nearby areas started during the Early Pleistocene. Finally the creation of the present-day Apennine watershed was not gradual but very rapid in the geological time scale and started mostly during the Early Pleistocene.