Abstract

Middle Miocene gypsum on the eastern coast of the Gulf of Suez, west-central Sinai, includes massive and bedded selenite. These selenites are interpreted as primary deposits because of their textures and fabrics such as vertically oriented crystals. Primary fluid inclusions in gypsum are used to interpret depositional settings and parent water chemistries. The final melting temperatures of ice in fluid inclusions nearly all fall between 0.0 degrees C and -8.0 degrees C. Fluid inclusions with ice melting temperatures above -1.9 degrees C (melting temperature of ice in seawater) show that some of the gypsum must have formed from a relatively dilute, non-seawater parent water. The nearly total absence of fluid inclusions with final ice melting temperatures below -7.0 degrees C (melting temperature of ice in evaporated seawater at gypsum saturation) indicates that the gypsum did not form from simple evaporative concentration of seawater. Rather, the fluid-inclusion data suggest that gypsum grew from: (1) "recycled" seawater, enriched in CaSO 4 , whereby seawater dissolved surrounding gypsum or anhydrite deposit, or (2) mixed seawater-nonmarine waters. Sulfur and oxygen isotope analyses (delta 34 S values of +21.9 to +23.6 per thousand , CDT and delta 18 O values of +10.1 per thousand to +12.7 per thousand SMOW) suggest a marine sulfate source but do not rule out any of the above parent waters. Given the rift tectonic setting, the most likely interpretation is that the parent waters were seawater with input of marginal nonmarine groundwaters and surface waters, all of which may have recycled CaSO 4 via dissolution of gypsum or anhydrite. Miocene gypsum deposits formed in a subsidiary closed basin that may have been separated from the main Gulf of Suez trough. During periods of low sea level or active faulting, the marginal closed basin may have been cut off from the Gulf of Suez, and thus completely nonmarine. The same basin would have become a marine-fed lagoon if it had been connected to the Gulf of Suez and flooded by seawater during high sea-level stands or breaching of tectonic barriers.

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