A unique saline lake sequence in the eastern Tethyan Ocean in responses to the Palaeocene-Eocene thermal maximum: A case study in the Kuqa Depression, Tarim Basin, NW China

A unique lithologic sequence with the order of sandstone, mudstone, halite, gypsum, dolomite, gypsum, and mudstone is present in the Palaeogene Kumugeliemu Group of the Kuqa Depression. Compared to common saline lake sequences, the sequence in this study shows a reverse cycle of rapid salinization and then desalination. To analyse the relationship between this unique lithologic sequence and the Palaeocene - Eocene thermal maximum (PETM), mudstone and evaporite cuttings were selected for X-ray diffraction, carbon and oxygen isotope, trace element, and rare earth element testing. Based on the dolostone; excursions of carbon isotopes; detrital index values; halite, gypsum, calcite, illite, and kaolinite contents; Ba/Th, Cu/Th, Zn/Th, and Ni/Th ratios, a record of the PETM is determined, i.e., in the zone between sub-member 2 and sub-member 1 of member 1 of the Kumugeliemu Group. Based on multiple comprehensive indicators, we conclude that a very arid climate dominated the carbon isotope excursion in the eastern Tethyan Ocean, which created this unique lithologic succession. This lithologic sequence represents three climate evolution stages. In stage A, due to the arid climate, the salinity rapidly increased and then decreased, with a rapid drop and gradual rise in the sea/lake level, and a lithologic succession that features halite, gypsum, and dolostone formed. In stage B, as heat and aridity peaked, the sea/lake levels dropped abruptly, the salinity quickly increased, and a lithologic succession featuring gypsum and mudstone formed. In stage C, the hot climate retreated, and mudstone with thin sandstone was deposited and covered the gypsum.