ABSTRACT

Modern nodules of primary gypsum occur beneath partly vegetated sabkhas west of Alexandria, on the semiarid Mediterranean coast of Egypt. The host sediment is a desert loess. forming a gypsiferous sonochak soil with a thin salt crest. The nodules occur only in areas where the capillary fringe intersects the ground surface, usually where the surface is within 1.5 m of the water table. Nodules occurring a few centimeters beneath the surface consist of white, very fine grained gypsum. Nodules at lower levels are of more coarsely crystalline gypsum. Phreatic groundwater brines, although saturated for calcium sulfate, are only moderately hypersaline, about 60. New evidence suggests that they are mainly of marine origin. Surface evaporatio causes capillary brines above the water table to move upwards through the loess and to increase in salinity. A marked increase in the ^mMg²⁺/^mCa²⁺ ratio of the interstitial brines at the zone of fine-grained nodules demonstrates active precipitation there of calcium sulfate. It is because of the existence of a very high salinity zone (>300) with halite, at the top of the profile, that gypsum precipitation takes place beneath the surface so as to form nodules rather than a surface crust. Na⁺ and Cl^- ions, in addition to Ca²⁺ and S0^2-₂ ions, seem to be necessary for the development of gypsum nodules by the "per ascensum" mechanism.