This paper reports an occurrence of varved gypsum-organic sediments from a recent shallow-water hypersaline pool in Ras-Mohamad, Southern Sinai. Primary deposition and diagenetic features in this deposit were followed by studying the mineralogy, the depositional textures, the interstitial brine composition, and the concentrations of ions coprecipitated with gypsum. During the summer, when evaporation is intense, gypsum is deposited on the bottom of the pool. During the winter evaporation is milder and the brine became undersaturated with respect to gypsum. At these lower salinities a carpet of blue-green algae floors the pool. The sediments of the pool are composed of alternating layers of gypsum mush and algal mats. The composition of the interstitial brine is essentially that of evaporated sea water, but the concentrations of Ca and SO 4 in the upper layers are anomalously high. The calculated degrees of supersaturation with respect to gypsum are as high as about 2. The supersaturation was relieved in the lower layers by early diagenetic growth of gypsum. Some of the diagenetic gypsum has grown by displacement within the algal layers. This growth eventually destroyed the fabric of the algal layer, which was ultimately erased. The diagenetic growth of gypsum is reflected also in the concentrations of cations coprecipitated with gypsum. The coprecipitated-ion concentrations decrease with depth, and this is interpreted as due to the decrease in the average growth rate of the crystals.

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