Facies analysis of four superimposed Transgressive-Regressive sequences formed during the two last Iinterglacial-glacial cycles (central Tuscany, Italy.)

In this paper we present the results of a detailed facies analysis carried out on a set of 115 late Quaternary-Holocene stratigraphic sections from the northern Tyrrhenian coast (central Tuscany, Italy). The stratigraphic leitmotif is the cyclic alternation of coastal-marine and continental deposits grouped in four transgressive/regressive sequences (T-R1-4). The marine-coastal deposits constitute the transgressive base of each T-R sequence; to the top they are replaced by continental deposits that characterize the top of each T-R sequence. Three OSL dates provide the chronological constraints of the T-Rs. Marine deposits related to the last interglacial period (Marine Isotope Stage 5) constitute T-R1, which represents the base of T-R2. T-R2 consists of coastal dune deposits related to the last MIS 5 phase (MIS 5a). T-R3 consists of coastal dunes and marine deposits related to MIS 3. T-R4 (Holocene age) is occasionally preserved within the transgressive tract. The present-day altitude of MIS 5e (T-R1) sea-level markers suggests the occurrence of uplifting tectonic movements within the study area. Conversely, the evidence of the stratigraphic overlap of MIS 3 coastal dune deposits above the MIS 5 beachface sediments implies the activation of subsidence processes that are necessary to compensate the post-MIS 5 eustatic fall rate. As a consequence, the uplift processes responsible of the present-day outcrop elevations of MIS 5 sea-level markers occurred after MIS 3, pointing out a far more complex local tectonic history. This approach, based on facies analysis of the whole sedimentary succession, emphasizes the need of a consistent number of dates in order to better constrain the framework of the spatial-temporal evolution. In the study area, this would enable to improve and/ or validate the temporal evolution of the local tectonics. As the context seems to be more complex than the one derived from the sole use of MIS 5e sea-level markers, we encourage the application of this methodological approach to different study areas.