Abstract:

The sulphur isotope of anhydrite and glauberite in Member 4 of Palaeocene Shashi Formation in Jiangling depression changes between 25.2‰-32.6‰, which is much higher than the sulphur isotope of synchronism sea water (20‰). Through the study of strontium isotope in evaporate in Member 4 of Palaeocene Shashi Formation and the comparison analysis of Mg, Ca, Sr, Ba content and Sr/Ba, Mg/Ca ratio, the authors propose that lake basin of Jiangling depression is closed continental salt lake facies. Primary glauberite, typically warm facies saline minerals, is widely developed in this formation. The Fe₂O₃/FeO ratio represents drought climate. This feature reveals that Palaeocene Shashi Formation of south Jiangling depression in Jianghan basin is drought and warm paleoclimate. In drought and warm paleoclimate condition, because of the low supply rate of sulphate deposites, the sedimentary environment at that time is closed to sulfate. Through the study of main trace element, including Fe, Mn, Al, Mg, Ca, Ni, V, Co, Cu and Fe/Mn, (Fe+Al)/(Ca+Mg), w(V+Ni+Mn), V/(V+Ni), Ni/Co,Cu/Zn ratio, it reveals that the environment of Member 4 of Shashi Formation is anoxic and reducing environment under wave base of lake basin, which is relative deep layered lake for years. A lot of anaerobic bacteria lived in this anoxic sedimentary environment. Under hypergene condition (≤50 ℃), anaerobic bacteria reduced SO^2-₄ to H₂S, which reacted with metal ion to form sulfide or escaped from the system. In addition, the continental environment at that time is relative closed to sulfate. This cyclicprocess caused the high sulphur isotope value of anhydrite and glauberite, which was also the most important sulfur isotope fractionation process in the study area.

Key words: anaerobic bacteria, Jiangling depression, sulfate, sulfur isotopic, reduction, sedimentary environment, Paleocene