Stratigraphy and strontium geochemistry of Messinian evaporite-bearing successions of the southern Apennines foredeep, Italy: implications for the Mediterranean “salinity crisis” and regional palaeogeography

An integrated study, comprising detailed mapping, stratigraphy, evaporite facies analysis, strontium content, and (87)Sr/(86)Sr ratio data, is presented for two Messinian evaporite-bearing units cropping out in Irpinia-Daunia Mountains (southern Apennines, Italy). These units record the tectonic and palaeogeographic evolution of the Messinian southern Apennine foreland basin system during the abrupt salinity crisis which occurred in the Mediterranean region. The Daunia unit comprises the Monte Castello evaporites, formed by euxinic diatomitic marls, evaporitic limestones, and shallow-water gypsum, having a (87)Sr/(86)Sr average value of 0.70898 +/- 0.00005, close to the Sr isotopic value of the coeval seawater. These marginal evaporites are capped by an erosional unconformity and continental elastic deposits. The Vallone del Toro unit is made up of thin-bedded marls, claystones, calcilutites, and diatomite layers with small gypsum crystals, laminated gypsum, and gypsarenite, testifying deposition in relatively deep-water settings. Gypsum has variable Sr isotopic data with an average value of 0.70899 +/- 0.00005 for the "Argilliti policrome del Torrente Calaggio'' formation and an average value of 0.70863 +/- 0.00014 for the "Argilliti di Mezzana di Forte" formation, reflecting major riverine freshwater input. The different patterns of lithofacies and Sr isotopic composition of gypsum discriminate two distinctive evaporite units, reflecting the tectonic and palaeogeographic controls within the Apennines mountain belt and the Mediterranean. Data for evaporites of the southern Apennines testify the presence of basins in the central and eastern Mediterranean Sea, which never desiccated during the entire salinity crisis and evolved to widespread Lago-Mare conditions in the latest Messinian.