The early Messinian Velona basin (Siena, central Italy): palaeoenvironmental and palaeobiogeographical reconstructions

A multi-disciplinary study of the basal lacustrine succession of the Velona basin (Tuscany, central Italy) provides informations about the paleoenvironmental and paleogeographical evolution of a post-collisional basin located in the Mediterranean area. Detailed sedimentological, paleontological (molluscs, ostracods, mammals, charophytes and pollen) and geochemical analyses on biogenic carbonates were carried out on a 66-m-thick muddy-sandy succession. Integrating the vertical distribution of selected species of ostracods and molluscs with the range of the mammal assemblage it has been possible to correlate the basal succession of the Velona basin to the Early Messinian (preevaporitic) corresponding to the lower part of the MN 13 zone. The basal portion of the Velona filling deposits corresponds to an unstable shallow lacustrine/marshy environment, rich in aquatic vegetation, surrounded by swamps dominated by Taxodiaceae and uplands dominated by warm temperate deciduous forests. Waters were generally brackish (oligo/mesohaline), alternating with hypohaline episodes. Salinity changes recorded by the faunal assemblages do not parallel with changes in the geochemical signatures in ostracod shells and charophyte gyrogonites. Geochemistry of biogenic carbonates points to an athalassic origin of the water salinity probably linked to the dissolution of the Triassic evaporites of the basement. Thus, the absence of connections between the Velona basin and the Mediterranean Sea during Early Messinian is confirmed. Ostracods, molluscs and mammals show a central European paleobiogeographical affinity, showing that, at the time of deposition of the Velona filling deposit, the endemic Tusco-Sardinian paleobioprovince was already disrupted and the northwards connections were noticeable. On the other hand, ostracods do not show any paleobiogeographic affinity with the ostracod assemblages of the Paratethyan realm, suggesting that no links between Mediterranean and Paratethys were present at that time.