GYPSUM-ANHYDRITE

Ass'rEAct: Halite pseudomorphs after gypsum crystals preserve depositional textures and record the early diagenetic history of anhydrite beds in cyclic Permian evaporites of the Texas Panhandle. The well-documented sedimentologic setting and good pres-ervation of fabrics permit identification of a sequence of diagenetic processes in gypsum. Gypsum (rather than anhydrite) was the dominant sulfate precipitated, even at salinities near halite saturation. Large gypsum crystals grew vertically on floors of brine pools, surrounded by a matrix of autochthonous transported gypsum sand and silt. All gypsum has been diagenetically altered to anhydfite _+ halite, but the diagenetic fabrics vary depending on position within regressive vaporite cycles. Because the composition and texture of the primary sediment were similar throughout the anhydrile bed, these different diagenetic fabrics are interpreted as the result of changes in the processes of replacement of gypsum by anhydfite and halite as the shallow burial environment became increasingly saline. In the lower parts of anhydrite beds, diagenesis n low-salinity brines favored obliteration of primary sedimentary structures by formation of nodules. In the middle parts of beds, introduction of high-salinity brine into the shallow burial environment favored alteration of gypsum to anbydfite and preservation of pseudomorphs after large gypsum crystals. At the top of anhydrite beds, introduction of halite-saturated brines into gypsum sediments on the floor of the brine pool resulted in pervasive replacement of gypsum by halite. Petrographic relationships indicate that most or all of the primary gypsum was altered to anhydrite in the very shallow (< 2 m) burial environment under the influence of saline diagenetic brines