Seasonal Evolution of Eocene Saline Lakes in Eastern China: A Case Study of ‘Bamboo-like’ Rock Salt in the Shizhai Depression, Northern Jiangsu Province

Abstract

The phenomenon of ‘bamboo-like’ thin interlayers developed in rock salt is one of the most prominent features of Paleogene salt-bearing strata in eastern China, where centimeter-thick rock salts appear separately, forming rhythmic units. At present, detailed analyses of these rhythms of rock salt are still limited, which directly affects the achievement of comprehensive and in-depth understanding of the developmental laws pertaining to this kind of saline lake. Therefore, we selected the typical rhythmic ‘bamboo-like’ rock salts of the Shizhai Depression in Jiangsu Province as the research subject. Through careful observation of rock salts in hand samples and detailed petrographic and mineralogical analyses, we analyzed the hydrogen and oxygen isotopic compositions, homogenization temperatures and chemical compositions of individual fluid inclusions in halite crystals. Early-stage rhythmic deposition was a product of continental saline lake evolution in winter or spring, late-stage rhythmic deposition being the product of evolution in the summer. The seasonal evolution of the halite sequences was determined and two brine enrichment events were identified. In addition, the quiet saline lake environment with concentrated brine represented by rock salt was more likely to precipitate potassium. This study provides a new reference for the evolution of both Paleogene climate and saline lakes in eastern China.