Insights into the genesis of thick halite in the Kuqa Depression (Tarim Basin, China): New mineralogical, lithological and geochemical evidence

The Palaeogene Kumugeliemu Group in the Kuqa Depression of the Tarim Basin of western China is characterised by a thick and massive sequence of evaporites, with a maximum thickness of up to 1000 m. The formation mechanism of such thick evaporite deposits remains unclear. In this study, sedimentological and geochemical research methods were employed to investigate the sedimentary characteristics and sources of halite. Sedimentary fabrics in halite, mudstone, sandstone, gypsum and carbonate indicate that halite was formed in a shallow water environment. Results of X-ray diffraction of halite and mudstone show that there is no tachyhydrite in halite and there is no pyrite in mudstone, which differs in the characteristics of deep-water salt and hydrothermal salt, respectively, indicating that halite was not formed in a deep-water environment. Rare earth elements indicate that sea water was the main source material for salt deposition. The colour of mudstone, Sr/Cu ratio and Rb/Sr ratio indicate that palaeoclimate was characterised by dry and hot conditions. The distribution characteristics of lithologic succession and the thickness of halite in Mbr 2 of the Kumugleiemu Group indicate that the halite was formed in restricted environments. According to above evidence, a ‘multistage marine transgression’ salt formation model was established, which suggests that, in a restricted paleogeographic environment, multiple transgressions brought sea water that, under dry and hot climate conditions, underwent repeated evaporation and concentration to form thick evaporite deposits.