Salt rhythmite formation and organic matter enrichment in the Qianjiang Formation, Jianghan Basin, China: Constraints from alternating dry and wet climates

Abstract

Jianghan Basin is the largest terrestrial saline lake basin in China, with the largest salinization area in the Paleogene. Research on salt rhythmite formations is crucial for petroleum exploration in salt lake basins. With alternating dry and wet climates, the difference in the organic matter (OM) provenance and enrichment remain controversial. In this study, petrology, element geochemistry, and molecular geochemical analyses were performed for the Qianjiang Formation to determine the depositional environment and provenance of its OM. The response of OM enrichment in the saline lake to the alternating dry and wet climates was reassessed. Three lithofacies associations (LAs) were designated: (1) LA1 was rich in argillaceous dolomite and argillaceous limestone with high total organic carbon (TOC); (2) LA3 was interbedded argillaceous glauberite and argillaceous dolomite with low TOC; (3) LA2 was a mixture of the above minerals with moderate TOC values. LA1 was deposited during desalination with a reducing environment and water stratification. The ∑C21-/∑C22+, C29/C30 hopane and maceral indicate the OM of LA1 derive from algae. Compared with LA1, LA2 was deposited with higher salinity and less algae. LA3 was deposited during intense evaporation, high salinity, and a disturbed environment. The OM sources of LA3 include higher plants and low aquatic organisms. Dry and wet climates control changes in organic enrichment. Hypersalinity is unfavorable to algae but beneficial to preservation. The stratified deep water is conducive to the accumulation of OM. The desalination is a crucial period for organic matter enrichment in the saline lake.