STABLE ISOTOPE GEOCHEMISTRY AND GEOLOGICAL INTERPRETATION OF TRIASSIC MARINE CARBONATE ROCKS IN EASTERN SICHUAN BASIN

The carbon isotope values (δ¹³C) of Triassic marine carbonate rocks in the Sichuan Basin range from -8.5 to 5.92‰, generally from -2 to 2‰, in equilibrium with those of sea water. The organic-rich carbonate rocks exhibit lower δ¹³C values, indicating a closed reducing environment. The δ¹³C values tend to increase with shallowing of water body, salinization of sea water and oxidation of the environments. The δ₁₈O values vary between - 19.71 and 2.77‰, usually between -2 and -7‰. The higher δ₁₈O values may suggest a low-temperature, highly salinized and organic-rich environment. The above-mentioned carbon and oxygen isotope values of Triassic marine carbonate rocks may reflect the variations in sea water depth, temperature and salinity, and be employed as a tool to determine marinity of the sediments and lateral facies changes and to understand the evolution of the sedimentary basin.
The δ³⁴S values of Triassic evaporites in the Sichuan Basin range between 14.5 to 34.9‰. The higher δ³⁴S values in this paper may contribute to the modification and refinement of the worldwide sulfur isotope curves for Triassic evaporites, and be used as the basis for the stratigraphic division and correlation. The δ³⁴S values of anhydrite developed in No. 4 Member of the Lower Triassic Jialingjiang Formation and in No. 1 Member of the Middle Triassic Badong Formation vary around 28‰, showing the steady supply of soluble sulphates into ancient saline lakes. This has been for geological interpretation of a shallow saline lake model. The decrease in δ³⁴S values from 34.9 to 14.5‰ is responsible for the sulfur isotopic depletion in the brines and sediments in the ancient saline lakes. It is inferred that the salt minerals formed during high salinization stage may occur in No.4 Member of the Leikoupo Formation.