Distrbution of gypsum and sulfur isotopes in the Palaeogene strata, western Qaidam Basin, Qinghai

The gypsoliths occur as part of the cap rocks of the oil and gas traps in the Qaidam Basin, Qinghai. In the light of the statistics of gypsum contents and ³⁴S_V-CDT values, the gypsum in the Palaeogene strata mainly occurs in the Shizigou depression and Xiaoliangshan depression near the Altun Mountains. Vertically, the gypsum contents display the cyclic distribution. The ³⁴S_V-CDT values show a rarely wide range of -23.3% to 71.4%. The initial ³⁴S values are very low in the Altun Mountains. The increase of the reducing conditions and the fractionation of bacteria towards the Qaidam Basin leads to the great differences in the ³⁴S_V-CDT values. The main controlling factors include the initial ³⁴S_V-CDT values, supply rates of sulfates, and intensity of bacteria fractionation. It follows that the sulfates are derived from the Altun Mountains, and then enter into the Qaidam Basin through the Shizigou depression. The results of research in this study may be helpful to the study of the gypsum distribution, sedimentary environments of the Palaeogene salt lakes, and sulfate origins in the study area.