The Miocene/Pliocene boundary in the Mediterranean area: New insights from a high-resolution micropalaeontological and cyclostratigraphical study (Cava Serredi section, Central Italy)

Tuscany (Central Italy) is a key area for studying the Messinian Salinity Crisis and overlying Lower Pliocene deposits due to the high number and excellence of the outcrops. Although the Miocene/Pliocene transition is well exposed and suitable for detailed analyses, a high-resolution integrated stratigraphic study of such a crucial time span for the geological history of the Mediterranean has never been attempted. In the Fine Basin (Central Tuscany), the Cava Serredi section is one of the best exposures of the upper Messinian–Lower Pliocene transition of the Northern Apennines. The Miocene/Pliocene boundary corresponds to a surface separating thinly laminated lacustrine (“Lago-Mare”) marls from homogeneous circalittoral to upper bathyal marly clays. Detailed quantitative micropalaeontological analyses have been performed on 111 samples from the 22 mthick Lower Pliocene succession. They resulted in the improvement of the biostratigraphic resolution of the basal Zanclean interval due to the recognition of several chronologically constrained bioevents typical of the Mediterranean region and based on benthic and planktonic foraminifera and calcareous nannofossils. Moreover, to unravel cyclical patterns of deposition, and given that the investigated succession shows no evident lithological pattern, micropalaeontological analyses have been coupled with calcium carbonate content variations. The latter revealed more-regular fluctuations, which are especially evident if the exclusive contribution of the planktonic foraminifer fraction (PCaCO3) is considered. Spectral analysis of the CaCO3 curve indicates that sedimentation occurred under the control of Earth's precession. This datum is also confirmed by the cyclical patterns of Neogloboquadrina acostaensis and Sphenolithus spp. among foraminifers and nannofossils, respectively. According to the proposed cyclostratigraphic and astrochronological reconstruction, the Lower Pliocene interval of the Cava Serredi section consists of eight precession-controlled cycles ranging from the i-cycle 510 (5.33 Ma) to the i-cycle 494 (5.16 Ma). Finally, the comparison with other astronomically calibrated sections, including the Zanclean GSSP (Eraclea Minoa, Sicily), allowed us to prove the completeness of the basal Zanclean interval of the study section and to state that Pliocene re-flooding after the Messinian Salinity Crisis occurred synchronously with respect to other deeper sectors of the Mediterranean Basin.