DIAGENETIC ORIGIN OF BASAL ANHYDRITE IN THE CRETACEOUS MAHA SARAKHAM SALT - KHORAT PLATEAU, NE THAILAND

Development of a diagenetic anhydrite bed at the base of the Cretaceou s Maha Sarakham Saline Formation (the 'Basal Anhydrite' member) of the Khorat Plateau in north-eastern Thailand took place due to leaching a nd/or pressure dissolution of salt at the contact between an underlyin g active sandstone aquifer system and an overlying massive halite-domi nated evaporite sequence. Basal evaporites composed of halite with int ercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing, with anhydrite produced as the ins oluble residue. The result is a 1.1 m thick interval of nodular anhydr ite displaying unique, basin-wide continuity. Observed textures, petro graphic features and chemical data from the anhydrite and associated a uthigenic minerals support the origin of the Basal Anhydrite Member as an accumulation residue from the dissolution of the Maha Sarakham sal ts. Petrographically, the anhydrite in this unit is made up of crystal s that are blocky and recrystallized, sheared, generally elongated and broken, and is bounded at the bottom by organic-rich stylolite surfac es. Authigenic and euhedral dolomite and calcite crystals are associat ed with the anhydrite. Traces of pyrite, galena and chalcopyrite are p resent along the stylolite surfaces suggesting supply of fresh water f rom the underlying sandstone at highly reducing conditions of burial. The delta(34)S Of sulphate in the Basal Anhydrite averages 15 parts pe r thousand, (CDT) and falls within the isotopic composition of the anh ydrite in the Cretaceous Maha Sarakham Formation proper and the Cretac eous values of marine evaporites. Measured delta(18)O in dolomite rang e from -4.37 to -14.26 parts per thousand, (PDB) suggesting a re-equil ibration of dolomite with basinal water depleted in O-18 and possible recrystallization of dolomite under relatively elevated temperatures. The delta(13)C, however, varies from +1.57 to -2.53 parts per thousand , (PDB) suggesting a contribution of carbon from oxidation of organic matter. This basal anhydrite bed, similar to basinwide beds found at t he bottom of many giant evaporite sequences, has always been considere d to be depositional. Here, at the base of the Maha Sarakham Formation , we demonstrate that the anhydrite is diagenetic in origin and was fo rmed by accumulation of original anhydrite by dissolution of interbedd ed halite from waters circulating though the underlying aquifer: it re presents an 'upsidedown' caprock.