A new facies model for the Upper Gypsum (Sicily, Italy): chronological and palaeoenvironmental constraints for the Messinian salinity crisis in the Mediterranean

The Upper Gypsum unit of the Caltanissetta basin (Sicily) records the last phase of the Messinian salinity crisis (MSC) comprising the so-called ‘Lago Mare’ event.A new facies analysis study let us to recognize 9-10 depositional cycles consisting of 7 rhythmically interbedded primary gypsum bodies, and 2-3 sandstone bodies separated by marly terrigenous horizons showing a laterally persistent vertical organization: a basal thin gypsum bed is overlain by a cluster of 5 thicker gypsum bodies; a marly interval containing 2 distinct sandstone horizons separate this cluster from the overlying uppermost 7th gypsum body. The terrigenous Arenazzolo Formation, in turn followed by the lower Pliocene Trubi Formation, conformably overlies the Upper Gypsum unit.The rhythmic alternation of the sandy marls and gypsum/sandstone bodies records the response of shelfal to deltaic systems to precession-driven arid-wet climate fluctuations causing cyclical changes of both base-level and water concentration. At a basinal scale the Upper Gypsum unit unconformably overlies a clastic evaporite unit containing carbonate breccia (the so called “Calcare di Base”) and/or clastic gypsum. Toward the basin centres, where the basal contact becomes conformable, a primary gypsum cumulate horizon is present. We interpret this layer as a possible lateral equivalent of the halite unit present only in the deepest depocenters.Based on astronomical calibration of the depositional cycles, the Upper Gypsum unit spans between 5.33 Ma and 5.55 Ma. This new age calibration allows to constrain the deposition of halite between 5.6 Ma (top of the Lower Evaporites) and 5.55 (base of the Upper Evaporites) in correspondence of isotopic stages TG12 and/or TG14.