Gypsum lithofacies and stratigraphic architecture of a Messinian marginal basin (Piedmont Basin, NW Italy)

During the Messinian salinity crisis (5.97–5.33 Ma) the Mediterranean was transformed into the youngest salt giant in Earth history. In marginal Mediterranean subbasins, the first stage of the crisis is recorded by up to 16 lithological cycles made up of shale/gypsum couplets (Primary Lower Gypsum unit, PLG), making a downslope transition to organic-rich shales. Gypsum comprises a wide array of lithofacies, including bottom-grown gypsum, cumulate deposits (i.e., resulting from gypsum nucleation in the water column and subsequent deposition on the seafloor) and the branching selenite. The latter appeared synchronously in many sections of the Mediterranean in the 6th PLG cycle (i.e., at about 5.87 Ma). To investigate the still enigmatic stratigraphic architecture of the lateral lithofacies transition from the shallower basin margin to the depocenter and the paleoenvironmental modifications driving the development of the branching selenite lithofacies, we carried out sedimentological, petrographic and geochemical analyses of the different types of gypsum lithofacies exposed along a margin-todepocenter transect in the Piedmont Basin (NW Italy), corresponding to the northernmost Mediterranean subbasin. The results of this study suggest that the lateral and vertical distribution of gypsum lithofacies mostly reflects the position of an oxycline with respect to the seafloor. Bottom grown gypsum (cycles PLG1–PLG5) was deposited on a temporarily oxygenated seafloor above the oxycline in the marginal part of the basin. In the case of cumulate deposits (laminar gypsum and gypsiferous mudstones, cycles PLG6–PLG15), gypsum nucleated within the water column at the oxycline and was eventually deposited on an anoxic seafloor. The branching selenite lithofacies corresponds to a syngenetic product formed below the seafloor at the expenses of cumulate gypsum. Deposition of cumulate gypsum and formation of the branching selenite were controlled by the balance between the rate of sulfate reduction in bottom waters and nucleation of gypsum in the water column. The appearance of cumulate gypsum and the branching selenite from the 6th PLG cycle upwards reflects the intensification of water column stratification in response to increased isolation of the basin.