A high resolution stratigraphic framework for the latest Messinian events in the Mediterranean area

A high-resolution stratigraphic model for the uppermost Messinian deposits of the Mediterranean basin is here proposed. The model provides new constraints for calibrating the time and space distribution of palaeoenvironmental proxies of the final phase of the Messinian salinity crisis (MSC), characterized, after the main phase of massive evaporite precipitation, by the progressive and generalized establishment of brackish to freshwater aquatic environments throughout the Mediterranean basin (‘Lago Mare event’). The corresponding stratigraphic unit, bounded by the intra-Messinian unconformity (MES) at the base (~5.6 Ma) and the M/P boundary at top (5.33 Ma) is split into two sub-units by a minor unconformity marking a sharp facies change. The lower sub-unit (p-ev1) is localized in deepest and/or strongly subsiding basins and is commonly characterized by ‘regressive’ depositional trends. It records the transition from hyper- to hypohaline conditions over a short time span characterized by an acceleration of tectonic processes in many Mediterranean geodynamic contexts. The upper sub-unit (p-ev2) is more basin-wide distributed and records the generalized periodic activation of catastrophic flood-dominated fluvio-deltaic depositional systems, indicating important modifications in the drainage areas and/or in fluvial runoff. In this sub-unit four to five precessional cycles are usually recognized below the Miocene/Pliocene boundary, allowing basin-wide correlations and suggesting to place its basal age at around 5.42 Ma. While p-ev1 records the local and ephemeral development of Lago Mare environments with low-diverse, pioneer taxa, p-ev2 is marked by the diffusion of conspicuous brackish to freshwater fossil assemblages of increasing diversity and complexity upwards. The change is best approximated by the boundary between the two sub-units. No clear evidence of normal marine conditions established before the Zanclean flooding have been recognized. Converging sedimentary evolution and paleontologic record suggest that the last MSC phase was characterized by a dilution of the surface waters of a large, residual Mediterranean water body with strongly reduced ocean connections. These conditions likely resulted from a positive feedback loop between processes (increase of fluvial runoff, generalized subsidence, base-level rise, enlargement of shelf areas) promoting the establishment of progressively larger, more stable intra-basinal connections and water exchanges with the Paratethyan basins, up to the final full reopening of the Atlantic gateways.