Sedimentology of the Cretaceous Maha Sarakham evaporites in the Khorat Plateau of northeastern Thailand

Evaporites of the Cretaceous to early Tertiary Maha Sarakham Formation on t he Khorat Plateau of southeast Asia (Thailand and Laos) are composed of thr ee depositional members that each include evaporitic successions, each over lain by non-marine elastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins are presently separated by t he northwest-trending Phu Phan anticline. The thickness of the formation av erages 250 m but is up to 1.1 km thick in some areas. In both basins it thi ckens towards the basin centre suggesting differential basin subsidence pre ceding or during sedimentation. The stratigraphy, lithological character an d mineralogy of the evaporites and clastics are identical in both basins su ggesting that they were probably connected during deposition. Evaporites in clude thick successions of halite, anhydrite and a considerable accumulatio n of potassic minerals (sylvite and carnallite) but contain some tachyhydri te, and minor amounts of berates. During the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline-pan enviro nment. Many halite beds, however, contain a curious 'sieve-like' fabric mar ked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow-water gypsum. The delta(34)S isotopic values obtained from different types of an hydrite interbedded with halite range from 14.3 parts per thousand to 17.0 parts per thousand (CDT), suggesting a marine origin for this sulphate. Bro mine concentration in the halite of the Lower Member begins around 70 ppm a nd systematically increases upward to 400 ppm below the potash-rich zone, a lso suggesting evaporation of largely marine waters. In the Middle Member t he initial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromine concentration in the Upper M ember exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence of tachyhydrite in association with the potassic salts was probabl y the result of: (1) the large volumes of halite replacement of gypsum, on a bed by bed basis, releasing calcium back into the restricted waters of th e basin; and (2) early hydrothermal input of calcium chloride-rich waters. The berates associated with potash-rich beds likely resulted from erosion a nd influx of water from surrounding granitic terrains; however, hydrotherma l influx is also possible. Interbedded with the evaporites are non-marine r ed beds that are also evaporative, with displacive anhydrite nodules and be ds and considerable amounts of displacive halite. The delta(34)S isotopic v alues of this anhydrite have non-marine values, ranging from 6.4 parts per thousand to 10.9 parts per thousand (CDT). These data indicate that the Kho rat and Sakhon Nakhon basins underwent periods of marine influx due to rela tive world sea-level rise but were sporadically isolated from the world oce an. (C) 1999 Elsevier Science B.V. All rights reserved.