Magnetostratigraphic and radiometric constraints on salt formation in the Qaidam Basin, NE Tibetan Plateau

Abstract

The Qaidam Basin is the largest Cenozoic intermontane basin within the northeast (NE) Tibetan Plateau. It contains large amounts of nonmarine evaporite deposits formed during the Pliocene–Quaternary. Even at present, extensive salt deposits dominated by halite and potash are formed by solar-driven concentration of brine water in the basin interior, making it the most important industrial base for potash exploitation in China. The formation of salt required an arid climatic, appropriate hydrological and tectonic setting through geologic times and will do so in the future. Studying the salt formation in the Qaidam Basin will enhance our understanding of processes driven by saline lake evolution, regional climate change, and tectonic movements, not only for the setting of the Tibetan Plateau. Reliable dating is crucial for assessing the time of salt formation in Qaidam Basin and the accumulation process, yet no comprehensive scientific studies have been reported on this important issue until now. In this paper, we critically review and compile magnetostratigraphic and radiometric studies of the salt-bearing strata within seven depressions of the basin. We find that the ages of salt formation are very different in these depressions: for the Dalangtan, Yiliping, Chahansilatu, and Kunteyi depressions, first salt deposits occurred at >3.90 ± 0.02 Ma, 2.88 ± 0.04 Ma, 2.24 ± 0.01 Ma and 1.18 ± 0.02 Ma, respectively. For the Mahai, Gasikule, and Qarhan, the ages of earliest salt formation are much younger i.e., 302 ± 56 ka, 608 ± 38 ka, and 54–24 ka, respectively. However, the result from Mahai has to be considered with caution.The variability of ages suggests an older salt-forming stage in the center of the western basin and a younger salt accumulation period along the basin margin. In a regional view, previous results from stratigraphy, sedimentology, geomorphology, and tectonic history allow us to conclude that the salt formation in the Qaidam Basin was probably controlled by pulsed intrabasinal tectonic movements and the distribution and migration of paleo-rivers during the Pliocene–Quaternary. However, the climatic shift towards drier and colder conditions in the Asian interior during this time also promoted salt formation.