Estimation of the Ambient Temperatures during the Crystallization of Halite in the Oligocene Salt Deposit in the Shulu Sag, Bohaiwan Basin, China

Abstract

The centimeter-scale halite rhythmites in the first member of the Shahejie Formation in the Shulu Sag of the Bohai Bay Basin are investigated, and the Eocene to early Oligocene paleoenvironmental characteristics of a typical saline lake basin are restored by reconstructing the temperature and compositional information of ancient brines. The obtained homogenization temperatures (Th) of fluid inclusions range from 6.5 to 49.2 °C, with a relative lower Th from transparent halite samples than from gray halite samples. This suggests different temperature conditions and a probable association with seasonal changes. The ion contents of halite fluid inclusions reveal the lake brine is a Na-Mg-K-Ca-Cl type and reached the initial stage of halite deposition. The transparent halite samples plotted within different phase regions than the gray halite samples on plots of ion contents and showed significant change within phase regions. Combined with the observed cm-scale rhythm in the evaporite sequences of the Shulu Sag, these results suggest a shallow water environment and frequent dilution by inflows of fresher water caused by seasonal climate change. The gray halite formed under higher temperatures and increased inflow conditions, and the transparent halite formed under lower temperatures and decreased inflow conditions. Compared with the Jiangling Sag in Hubei Province in southern China, the Shulu Sag may have been less affected by igneous rocks in the Es1 Formation due to the material source, and the concentration of trace elements such as lithium, strontium and boron in the ancient salt lake brine was lower.