The Messinian salinity crisis: a sequence-stratigraphic approach

Facies analysis and sequence-stratigraphic techniques allow to define a chronostratigraphic framework for the Messinian successions of the Northern Apennine and Sicilian-Maghrebian foredeep basins based on the recognition and correlation of a hierarchy of stratal units recording in time and space the evolution of shallow to deep-water evaporitic and siliciclastic depositional systems during the Messinian salinity crisis (MSC). The evolution of Mediterranean depositional systems during the different MSC stages was controlled by a complex interplay between tectonics and orbitally-forced climate changes. Precession-modulated climate changes, likely due to amplification effects in a strongly restricted Mediterranean basin, control the development of elementary depositional sequences (EDS) whose key surfaces are the most prominent stratigraphic features at the outcrop scale. EDS are arranged into higher-rank stratal units which form the systems tracts of two large-scale depositional sequences separated by a sequence boundary corresponding to the subaerial Messinian erosional surface (MES); such surface can be traced downbasin into a correlative conformity at the base of deep-water evaporite-bearing turbidite systems. These large-scale depositional sequences appear mainly controlled by tectonics and its side-effects on the hydrologic balance of the Mediterranean basin, modulating the water exchanges at the Atlantic gateways and modifying the internal physiography. Timing and organization of higher-rank systems tracts appear to be influenced by climate changes at the obliquity and eccentricity scale. The resulting stratigraphic scenario has important implications for the general MSC evolution. Lower Gypsum evaporites actually comprise both primary (PLG) and resedimented (RLG) facies which accumulated in different depocenters and at different times. PLG deposits formed within shallow-water, restricted sub-basins during the HST of the lower depositional sequence between 5.96 and 5.61 Ma; RLG, associated in Sicily with Calcare di Base limestones and Halite, accumulated in deep-water settings during the subsequent base-level fall associated with an important evaporative drawdown culminating at around 5.55 Ma with local salt basin fill and desiccation. The Sicilian Upper Gypsum was deposited in shallow-water settings during the lowstand and transgressive systems tracts marking the gradual refill of the Mediterranean basin with oligohaline to mesohaline waters.The sudden re-opening of the Atlantic gateways marks the MSC end at the Pliocene base (5.33 Ma) and the maximum flooding surface of the upper depositional sequence