Reconstructing the palaeoenvironment of a deep onshore basin of the Late Miocene Mediterranean salt giant (Belice basin, Italy): Insights from hemipelagic deposits interbedded with clastic evaporites

Large and deep marine evaporitic basins (salt giants) are common in the geological record, but the reconstruction of the palaeoenvironment is challenging. The sedimentary products consist of clastic evaporites (mass transport and turbiditic deposits), commonly interbedded with evaporitic cumulates or non-evaporitic fine-grained deposits representing the background hemipelagic sedimentation. The study of the hemipelagic component can provide crucial palaeoenvironmental information on these basins. This study examines the non-evaporitic, hemipelagic sediments interbedded with gypsum turbidites from the Belice Basin (Italy), a deep onshore basin of the Late Miocene Mediterranean salt giant. The studied succession belongs to the Resedimented Lower Gypsum unit, formed during the second phase of the Messinian Salinity Crisis (5.60–5.55 Ma) and consists of laminated diatomaceous and organic-rich shales, dolomitic and aragonitic mudstones. Sedimentological, petrographic, and geochemical analyses indicate that the hemipelagites accumulated in a rather deep basin, with high primary productivity in superficial waters. The palaeoenvironmental conditions in the water column and at the seafloor were governed by the balance between the inflows of freshwater and marine water. Intervals of reduced continental runoff and enhanced marine ingression, which induced water column mixing and seafloor oxygenation, are recorded by the diatomaceous shales. The other lithologies record prolonged stratification of the water column, induced by riverine runoff and basin restriction. Seafloor anoxia and the input of terrestrial and marine organic matter favoured the formation of dolomite and pyrite following bacterial sulphate reduction. This study provides insights into the chemical, physical, and biological conditions in a deep basin during the Messinian Salinity Crisis.