Transition from marine to hypersaline conditions in the Messinian Tripoli Formation from the marginal areas of the central Sicilian Basin

Abstract

Abstract Three sections of the early Messinian Tripoli Formation from the northern and southern margins of the central Sicilian Basin (Serra Pirciata, Torrente Vaccarizzo, and Marianopoli) have been studied with the aim to reconstruct the sedimentary and environmental changes which occurred during the transition between marine conditions and the evaporitic events of the Salinity Crisis recorded in the overlying Calcare di Base Formation. A detailed biostratigraphic and cyclostratigraphic study provided the opportunity of cycle-by-cycle correlations between the marginal sections and the reference section of Falconara. The main paleoenvironmental changes are recorded by: (1) the evolution of calcareous microfossils towards low diversity and their complete disappearance; (2) the composition of the carbonate fraction which commonly changes from calcite, mostly related to calcareous microfossils, to authigenic carbonate phases consisting of either calcite, dolomite, and/or aragonite; (3) the appearance of shallow water deposits and evaporite pseudomorphs; (4) the variation of the stable isotope composition of the carbonate fraction, indicative of large fluctuations of the freshwater dilution/evaporation balance, with a general trend towards hypersaline conditions. Both mineralogical and isotope data indicate that the dolomite precipitated generally from concentrated pore waters while other carbonates formed in the water column submitted to large fluctuations of salinity. Except locally, the increase in salinity did not reach concentrations high enough to precipitate significant volumes of evaporites as during the deposition of the overlying Calcare di Base. The transition from normal marine to hypersaline conditions is recorded diachroneously in the three sections, respectively, in cycle 34 (6.32 Ma) at Serra Pirciata, cycle 42 (6.15 Ma) at Torrente Vaccarrizzo, and cycle 44 (6.12 Ma) at Marianopoli. These changes represent the hydrological and sedimentary response to the fragmentation of a large marine basin into different subbasins, which ended in the more or less complete closure recorded by the deposition of the overlying evaporite-rich Calcare di Base Fm (true onset of the evaporitic event). Two hypotheses not exclusive may explain this change, either the regional consequence of the compressive tectonics or the incipient drop of the global sea level which led to the Salinity Crisis. Our results confirm that the Salinity Crisis was preceded by evaporative episodes in the evolution towards hypersaline conditions which occurred diachroneously as a response to the diversity of paleogeographical settings even if the paroxysmal phase seems to have affected the basin at the same time.