The geological history of Messinian (upper miocene) evaporites in the Central Jordan Valley (Israel) and how strontium and sulfur isotopes relate to their origin

Abstract

Evaporites, comprising gypsum, anhydrite, halite and dolomite are described from the Messinian Bira Formation outcrops and from two boreholes (Newe Ur-2 and Zemah-1) in the Central Jordan Valley, Israel. Strontium and sulfur isotopic compositions of the evaporite minerals, and their Sr/Ca and Br/Cl ratios are used to interpret their enviromments of deposition and processes of formation and diagenesis. The brines from which the evaporites precipitated originated from seawater. The processes caused by mixing of surface water, seawater and subsurface brines, resulted in dolomitization and also sulfur reduction. The surface water and subsurface brines reacted with the rocks they drained, including Cretaceous and Eocene carbonates and Neogene basalts. The gypsum deposits in the Central Jordan Valley are interpreted to have formed as a result of evaporation of the magnesium-rich Lake Bira water which became oversaturated with respect to calcite and gypsum. The gypsum was deposited in stratified, relatively closed basins, where a partial reduction of the sulfur resulted in high δ34S of the precipitated gypsum. Gypsum and early diagenetic dolomite formed from the same water bodies. The Bira evaporites in Newe Ur-2 borehole, precipitated from mixtures of sea- and fresh waters with basaltic contribution. The samples from the Lower Gabbro and Halite Unit in the Zemah-1 borehole were deposited from evaporated seawater, which leached basaltic rocks, in closed basins; the Middle Halite Unit formed from seawater, whiles the brines that deposited the Upper Halite Unit leached also basalt rocks.