Genesis of salt deposits in the Jiangling Sag, Jianghan Basin, China: Constraints from H, O, and C isotopes and fluid inclusions of evaporites

Abstract

Due to Yanshan movement and Himalayan movement, a series of faulted basins were formed in the late Cretaceous and Paleogene, including Shashi Formation of Paleocene in Jiangling sag of Jianghan Basin, when a set of salt lake was deposited. Analysis of lacustrine carbonate rocks in Shashi Formation of Paleocene shows that the δ13CPDB values of carbonate rocks range from −4.8 ‰ to −1.0 ‰, with an average value of −3.3 ‰. The δ18OPDB values ranged from −5.4 ‰ to −0.6 ‰, with an average value of −3.2 ‰. The positive correlation between δ13CPDB and δ18OPDB of the Shashi Formation carbonate rocks indicates that they developed in a relatively closed brackish lake system with obvious evaporation. The homogenization temperature of the upper Shashi Formation in Jiangling sag ranges from 15.3 °C to 51.8 °C, mainly concentrated in 25–30 °C. The δ18O isotope of halite inclusions ranges from −0.8‰ to 9.4‰, with an average of 4.9‰. The δD isotope of halite inclusions ranges from −91.3‰ to −65.0‰ with an average of −79.3‰, which are all lower than the Global Meteoric Water Line. The intense evaporation environment is consistent with the overall drought paleoclimate at that time. The quantitative LA-ICP-MS was used to analyze the composition of the primary inclusion of halite, and it was concluded that the original brine composition was Na-K-Mg-Cl-SO4 system with relative low concentration of Ca2+, high concentration of Mg and SO42- ions and K ions. The high content of Li, B, Br and Rb is related to the intense volcanic activity, deep fluid source and hot spring recharge in Jiangling Depression at that time. Based on carbonate carbon and oxygen isotopes, fluid inclusions temperature and composition, it is concluded that during the Shashi Formation period of Paleocene in Jiangling Sag, the lake basin was well sealed, with high salinity and high temperature, and belonged to a salt lake sedimentary environment, which was favorable for the deposit of salt deposits.