MIDDLE MIOCENE GYPSUM, GULF OF SUEZ - MARINE OR NONMARINE

Middle Miocene gypsum on the eastern coast of the Gulf of Suez, west-c entral Sinai, includes massive and bedded selenite. These selenites ar e interpreted as primary deposits because of their textures and fabric s such as vertically oriented crystals, Primary fluid inclusions in gy psum are used to interpret depositional settings and parent water chem istries. The final melting temperatures of ice in fluid inclusions nea rly all fall between 0.0 degrees C and -8.0 degrees C. Fluid inclusion s with ice melting temperatures above -1.9 degrees C (melting temperat ure of ice in sea water) 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 da ta suggest that gypsum grew from: (1) ''recycled'' seawater, enriched in CaSO4, whereby seawater dissolved surrounding gypsum or anhydrite d eposits, or (2) mixed seawater nonmarine waters. Sulfur and oxygen iso tope analyses (delta(34)S values of +21.9 to +23.6 parts per thousand CDT and delta(18)O values of +10.1 parts per thousand to +12.7 parts p er thousand SMOW) suggest a marine sulfate source but do not rule out any of the above parent waters, Given the rift tectonic setting, the m ost likely interpretation is that the parent waters were seawater with input of marginal nonmarine groundwaters and surface waters, all of w hich may have recycled CaSO4 via dissolution of gypsum or anhydrite, M iocene gypsum deposits formed in a subsidiary closed basin that may ha ve 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 conne cted to the Gulf of Suez and flooded by seawater during high sea level stands or breaching of tectonic barriers.