Petrography and sulfur isotope geochemistry of middle Miocene evaporites on the active continental margin, Ras Banas Peninsula, Red Sea coast, Egypt

Abstract

The present study aims to investigate the paleoenvironmental conditions and relative sea-level change during the middle Miocene evaporite deposition at the Ras Banas Peninsula, Red Sea Coast, Egypt. The tectono-sedimentological evolution of the evaporite suite may provide a geological relationship with the surrounding structures of the peninsula. To address this objective, the study examines the petrology and sulfur isotope geochemistry of the middle Miocene sulfate evaporites (approximately 100 meters in thickness) of the Abu Dabbab Formation at the Ras Banas Peninsula, located along the Red Sea continental margin. The Abu Dabbab Formation can be subdivided into three distinct sedimentary facies, from base to top: (1) lower supratidal gypsiferous claystone, (2) middle lagoonal sandy and clayey gypsum, and (3) upper subaqueous marine laminated gypsum. Petrographic analysis reveals the presence of both primary and secondary evaporite facies, with the mineralogical composition dominated by gypsum, alongside minor amounts of anhydrite and dolomite. Geochemical data indicate that the average sulfur isotopic compositions (δ³⁴S) for the lower, middle, and upper facies are 23.62⁰/₀₀, 23.30⁰/₀₀, and 23.83 ⁰/₀₀ VCDT, respectively. These findings suggest that the Abu Dabbab evaporites were deposited in a tectonically active basin that received inputs of meteoric water and fine clastics, influenced by seasonal shifts between arid and wet conditions. The occurrence of stratabound mineralization and isotopically light sulfur suggests that algal biological activity played a role in reducing sulfate to iron sulfide and native sulfur. The rapid lateral and vertical facies changes observed in this active continental margin are primarily attributed to tectonic processes and the associated paleoenvironmental dynamics.