Two Volcanic tephra layers within the upper Messinian succession of Tuscany (inner northern Appennine Italy): 40Ar -39Ar dating and implications on evaporitic and postevaporitic stratigraphy

Two volcanic tephra layers have been found intercalated in the upper Messinian sediments of the Migliarino and Pignano sections, located in the Fine and Volterra-Chiusdino neogenic-quaternary basins, respectively (inner Northern Apennines). The Migliarino section (90 m thick), about 2 km SE of Rosignano Marittimo town, consists of 13 beds of alabaster gypsum alternating with mudstones. Lugli et al. (2010) consider the first eleven beds as primary gypsum deposited during the evaporitic Messinian phase. The tephra layer is intercalated to laminated clay between the 10th and 11th gypsum beds. The Pignano section (130 m thick) is located about 7 km W of the Volterra town and consists of alternations of conglomerates, clayey sands and clays with rare secondary gypsum. The volcanic tephra is located at about 85 m from the base of the section and it is characterized by laminated clay covered by sand deposited in a distal delta-front environment. The igneous fraction of the two tephra layers consists of crystal fragments of prevailing fresh, zoned oligoclase and sanidine, and subordinate quartz and brown mica. This latter phase is replaced to variable extent by chloritic products and Fe-Ti oxihydroxides. In both tephra layers the igneous crystal fragments have sharp edges. In the Migliarino tephra layer the igneous crystals are dispersed in an argillaceous yellowish matrix plus brown iron hydroxides. The Pignano tephra layer has a topmost part constituted by finer-grained crystal fragments and by abundant Y-shaped dark and clear fragments, likely representing altered volcanic glass shards, in a dominant argillaceous matrix. The lower portion is made of alternating crystal rich and argillaceous 1-5 mm-thick layers. The nature and composition of the mineral phases occurring in the Migliarino and Pignano tephra layers point to a highly evolved nature of the original magma. Sanidine single-crystal laser fusion 40Ar-39Ar analyses of the two volcaniclastic levels give weighted average ages of 5.52±0.01 Ma for Migliarino and 5.52±0.01 Ma for Pignano [age monitor: TCR sanidine, 28.34 (Renne et al., 1998); decay constants of Steiger & Jäger (1977)]. The comparison of these levels with others in the Northern Apennines, impose new constraints for the chronology of the primary evaporitic Messinian succession.