Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 209 – 222
*Geology Department, Chulalongkorn University. Bangkok 10500. Thailand
**Institute for Thermal Spring Research, Okayama University, Misasa, Totori-ken 682-02, Japan
Abstract: The sulphur and oxygen isotopes of 10 anhydrite samples from 5 anhydrite beds of the Maha Sarakham Formation of Thailand (Basal Anhydrite, Basal Cap Anhydrite, Middle Anhydrite L, Middle Anhydrite U and Upper Anhydrite U) were investigated. This probably is the first time a Southeast Asian evaporite deposit has been studied. The isotopic compositions of both sulphur and oxygen of these anhydrites show a narrow variation (e.g. 14.5 to 16.5‰ with an average of 15.4‰, for the δ34S value and 11.0 to 14.2‰ with an average of 12.‰ for the δ18O value) reflecting a fairly homogeneous source. There is no systematic change in both δ34Sand δ18O values with respect to the stratigraphic position. There was also no distinct difference in those values between the cap anhydrite and the normal anhydrite. The isotopic and geologic data suggest that the δ34S and δ18O values of the anhydrite represent the original isotopic compositions of sulphate mineral precipitated from the restricted Khorat sea and also those in equilibrium with the world ocean sulphate during that geologic time period. Input and output of sulphur and oxygen through the sedimentary sulphur cycle, namely, reduction of seawater sulphate, crystallization of sulphate minerals, dissolution of pre-existing evaporites and oxidative weathering of old sulphides, were negligible in the restricted Khorat sea. Using the fairly well-established sulphur isotope age curve for the world sulphates is the late Mesozoic and Cenozoic and the average δ34S value of the Maha Sarakham anhydrite together with the reported age of the underlying Khok Kruat Formation, the age of the Maha Sarakham evaporites in the Bamnet Narong area can best be estimated to be around Cenomain (lower part of Upper Cretaceous). The oxygen isotopic compositions of these anhydrites are slightly different from those of the best estimated values of word evaporites during this age. The new isotopic data should be added to the list for future revision of the curve because of the scarcity of previous data base in the construction of the curve.