Evaporation of marine basins: a review of evaporite formation and Messinian Salinity Crisis

The Mediterranean Sea experienced an extraordinary event at the end of the Miocene, when massive evaporites formed rapidly between 5.97 and 5.33 Ma. This event is referred to as the Messinian Salinity Crisis (MSC), which formed the youngest huge salt mass (106 km3), corresponding to ~ 6% of the total sea salt in the world ocean. Although various scenarios have been proposed to explain the MSC, no general consensus has been achieved. The controversy can be summarized as the crisis being either wet or dry; i.e., either the major portion of the evaporites formed under subaqueous conditions or deep basins dried up to form a Mediterranean desert. Here we review 1) the present-day formation of evaporites in an active salt pond in Sicily, and 2) the basic context of evaporite geology/sedimentology in Sicily, where a deep-sea sedimentary succession that spans the MSC is now preserved on land. Evaporites forming in the present-day saltern in Sicily provide crucial information on how sedimentary structures in gypsum, halite, and potash salts develop through evaporation, which is useful for understanding evaporite depositional environments in the geological past. The Messinian evaporite succession in Sicily is divided into i) the Primary Lower Gypsum unit in peripheral settings, and ii) the Resedimented Lower Gypsum unit, iii) a thick halite unit, and iv) the Upper Gypsum unit that formed in deep basin settings (the Caltanissetta basin). The thick halite appears to have formed simultaneously with or slightly after the Resedimented Lower Gypsum (the second stage of the MSC). The Upper Gypsum unit overlies the Resedimented Lower Gypsum and halite units, which formed during the late stages of the MSC. The Messinian evaporite units are covered by Pliocene hemipelagic sediments; the base of these sediments marks the Zanclean flooding. The lithology of the MSC deposits in Sicily clearly shows that most of the evaporites formed under subaqueous conditions, and only one interval in the halite unit indicates subaerial exposure.