The geological history of pliocene-pleistocene evaporites in Mount Sedom (Israel) and how strontium and sulfur isotopes relate to their origin

Abstract

Evaporites, comprising of gypsum, anhydrite and halite are described from the Pliocene Sedom Formation, the Caprocks units and the Pleistocene ‘Amora Formation in Mount Sedom, Southern Jordan Valley, Israel. Strontium and sulfur isotopic compositions of the evaporite minerals, and their Sr/Ca and Br/Cl ratios were used to interpret their environments of deposition and processes of formation and diagenesis. Some of the evaporites of the Sedom Formation were deposited from evaporated seawater. Others were deposited from a mixture of seawater and brines. The brines were composed of seawater which penetrated the carbonate rocks of the Rift margins, participated in dolomitization processes and, when hydrologic conditions allowed, seeped out into the Sedom basin and were mixed with evaporated seawater. These processes yielded non-homogeneous fluid masses of mixtures, as indicated by their wide range of87Sr/86Sr ratios (0.70824–0.70905) as compared to the narrow Sr/Ca ratios of the derived evaporites. Their marine origin is indicated by their δ34S values which are around 20‰ The evaporites of the ‘Amora Formation were precipitated from Ca-chloride brines only, which were originally evaporated seawater trapped in the Rift walls in the “Sedom Formation times”, returning to the Sedom basin after its disconnection from the sea. The high Sr content and Sr/Ca ratios indicate that the anhydrites existing today in the Sedom and ‘Amora formations were originally deposited from the evaporated seawater as gypsum which was later recrystallized at depth, at high pressures and temperatures. The caprocks are residual rocks of marine origin, formed by the dissolution of the exposed rock-salt units in the Sedom Formation. They represent mainly the gypsum (or anhydrite) beds intercalated in the rock-salt units of the Sedom Formation in Mount Sedom rather than Ca-sulfates disseminated in the halites.