四川盆地下侏罗统自流井组湖相碳酸盐岩的碳、氧同位素特征及其古湖泊学意义

CARBON AND OXYGEN ISOTOPIC COMPOSITIONS OF LACUSTRINE CARBONATES OF THE EARLY JURASSIC ZILIUJING FORMATION IN THE SICHUAN BASIN AND THEIR PALEOLI MNOLOGICAL SIGNIFICANCE

四川盆地侏罗纪的湖相碳酸盐岩主要分布于早侏罗世自流井组地层之中。在对四川自贡、重庆合川自流井组上部大安寨段中的湖相碳酸盐岩的碳、氧同位素进行分析后,根据其组成特征及其相关性,探讨其碳、氧同位素的古湖泊学意义。两条剖面上碳酸盐岩的碳、氧同位素在组成上存在一定的差异,自贡贡井—高硐桥剖面上,样品的碳、氧同位素均为负值,δ13C值在-4.49‰^-0.85‰之间,1δ8O值-9.14‰^-6.75‰之间,两者具有一定的相关性(γ=0.62);合川炭坝剖面上,样品的碳同位素正负均有,氧同位素均为负值,δ18O值在-8.52‰^-4.56‰之间,δ13C值在-3.46‰~1.62‰之间,两者也具有一定的相关性(γ=0.64)。通过与现代湖泊碳酸盐碳、氧同位素特征的比较,认为四川盆地早侏罗世自贡地区的湖泊为开放型的淡水湖泊,而合川地区为具有一定封闭性的微咸水湖泊。

The lacustrine carbonates mainly distribute in the early Jurassic Ziliujing Formation in the Sichuan basin. Stable carbon and oxygen isotopic analyses of lacustrine carbonates from Daanzhai Member of Ziliujing Formation in Zigong area,Sichuan and Hechang area of Qongqing, have been carried out. Based on the analysis ,paleolimnological significance of isotopic of lacustrine carbonates have been discussed in the paper. For Gongjing and Gaodong section in Zigong, the δ^13C and δ^18O value vary from -4.49‰～-0.85‰ and from -9.14‰～-6.75‰ respectively, which display slight correlation in covariance between cabon and oxygen variations （γ= 0.62 ）; and the δ^13C and δ^18O value vary from -3.46‰～1.62‰ and from -8. 52‰～-4.56‰ in Tanba section in Hechang,which display correlation in covariance between cabon and oxygen variations （γ=0. 64）. Comparison with carbonate and oxygen isotopes from different types of modern lakes, it suggests that the lake in early Jurassic in Zigong area is characteristic of open freshwater, whereas in Hechang area is a slightly closed brackish one.