SULFUR ISOTOPE DISTRIBUTION OF TRIASSIC EVAPORITE AND ITS GEOLOGICAL SIGNIFICANCE IN SICHUAN BASIN

Abstract: The evaporite of the sedimentary basin of the upper Yangtze Platform has a wide distribution, which covers an area of 4 x 5km² particularly developes in Sichuan Basin. The evaporites occured from Feixianguan and Jialinjiang Formation of Lower Triassic, Leikoupo Formation of Middle Triassic. The thikness of anhydrite bed is 100-500m. This paper provides a great deal of data of δ³⁴S, and established a δ³⁴S column of Triassic evaporite in Sichuan Basin by analysing samples from deep bore. Stratigraphic column of δ³⁴S value indicates that δ³⁴S value of anhydrite in the fourth member of Leikoupo Formation (namely T₂1⁴) rages 14.5-18.9‰, commonly 16.5‰, T₂I³19.7-23.2‰, average 21.78‰, which is close to the isotopic value of sea sulphate. In T₂l¹27.1-29.5‰.In the Fifth member of Jialinjiang For- mation (namely T₁j⁵) δ³⁴S value 26.1- 28.6‰, T₁j⁴ 28.0-28.5‰ commonly. But the δ³⁴S value of T₁j² 32.4-43.9‰, shows an abnomal high value. As mentioned above, the sulfur isotope data are imoprtant for geologic research. (1) In contemporary beds or a certain sedimentary basin. δ³⁴S value varies very limited. So sulfur isotope may serve to stratigraphic division and correlation. (2) δ³⁴S value in the Triassic column decreases gradually from 34.9‰ at the lower to 14.5‰ at the upper. (3) According to the view of modern sedimentary the deviation of sulfur isotope between evaporite and sea sulphate varies less than 1.6‰. The evaporation experiments on sea water indicate that in chloride sedimentary stage (such as carnallite etc.) the brine is bare of δ³⁴S evidently. This represents the Salinization trend of the ancient saline lake and suggests that T₂l⁴might be the higher deposition stage of salt-mineral. (4) In the upper and middle part of Jialinjiang Formation δ³⁴S value fix at about 28% constantly. In an anhydrite bed. δ³⁴S is poor at some place. While a new sedimentary cycle start, δ³⁴S value shows a new circulation from higher to lower. It may be proved that the dissolved sulphate of the ancient salt lake was supplied constantly. This may gives support to the hypothesis of shallow salt lake model. In other warde, in the lake of Early Triassic, the evaporative sedimentation of Sichuan Basin was a shalow water enviroment. (5) Comparing with sulfur isotopic data worldwide, δ³⁴S value of the marine Triassic evaporite of Sichuan Basin has some character of the high abnomal value, and it is necessary to modify the sulfur isotopic curve prevailing all over th wold based on the data provided in the present paper.