LITHOFACIES CHARACTERISTICS, DEPOSITIONAL ENVIRONMENT AND KARSTIFICATION OF THE LATE MIOCENE (MESSINIAN) GYPSUM DEPOSITS IN THE NORTHERN WESTERN DESERT, EGYPT MAHMOUD A. M. AREF Geology Department, Faculty of Science, Cairo University, Giza, Egypt The gypsum sequence outcropping in an exploited gypsum quarry in AI-Barqan area, northern Western Desert, Egypt, is composed of primary gypsum that retains primary textural and structural features indicative of the very shallow marginal marine environment. Three gypsum layers are distinguished. The lower layer consists of stromatolitic gypsum composed of graded, rippled gypsum laminae intercalated with calcitized microbial laminae. The middle layer consists dominantly of skeletal gypsum and remains of grass-like gypsum. Small (<3 cm) selenitic gypsum twins are the main component of this layer, in addition to reworked gypsarenite. The upper layer is composed of coarse, crystalline selenitic gypsum that grades southward into grass-like gypsum, which contains numerous truncation surfaces. The facies continuum, stromatolitic gypsum - grass-like and skeletal gypsum - grass-like and coarse selenitic gypsum, indicates increasing salinity of the brine with time and deepening of the marginal marine depositional basin from few centimeters up to 20-meter water depth. The upper gypsum layer is marked by an erosional surface and karstic features. The latter is represented by vertical dissolution conduits and cavities (restricted to the middle gypsum layer) filled with claystone and sandy claystone. Other karstic features are horizontal dissolution pipes and cavities; the latter are filled with coarse lenticular gypsum. The stratigraphic setting of the gypsum sequence, comparison with other Messinian evaporites. and the presence of an erosional surface and karstic features on top of the gypsum sequence indicate a most probable Messinian age for the studied gypsum deposit.