The “I Sodi” section is exposed in the homonymous quarry in the northern sector of the Siena Basin, one of the most extended Neogene-Quaternary post-collisional basins of the northern Apennines. The section is composed almost exclusively of marine mudstone containing a rich fossil assemblage and has been extensively investigated in past and recent times. It represents a key section to define the time interval of marine deposition in the Siena Basin and more generally in the inner northern Apennines, with important structural and stratigraphic implications.

The marine infill of the Siena Basin is traditionally attributed to the Zanclean-Piacenzian (Pliocene) age. However, recently published data provided a more recent age for the “I Sodi” section (Calabrian, Lower Pleistocene) and, consequently, for the Siena Basin. This paper provides new data on this scientific debate, from sedimentological and biostratigraphical investigations. Analyses of planktonic foraminifera and calcareous nannofossils have been carried out in order to better constrain the depositional age of the section. As a result, this section is now dated more accurately to the Piacenzian and possibly to the lowermost Gelasian in its upper part.

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