Abstract: The mudstone of the upper Bayingobi formation in Bayingobi basin is one of the research targets for the deep geological disposal of high-level radioactive waste in China. At present, the combination characteristics of clay minerals and the factors affecting the paleosalinity of the mudstone are less studied. In this paper, the paleosalinity of the upper Bayingobi formation is comprehensively distinguished by the methods of B content, Sr/Ba ratio, Rb/K ratio, Na+K mass fraction and X-ray diffraction. The results are as follows：(1) The upper member of Bayingobi formation is dominated by semi deep lake-deep lake sedimentary environment with thick dark grey mudstone developed. And it mainly ranges from semi deep lake to deep lake facies. (2) The lithologic characteristic of the target formation in the study area is argillaceous and silty. Illite, montmorillonite and mixed layer of illite and montmorillonite are well developed while chlorite and kaolinite are relatively rare. (3) The values of Sr/BA, Rb/K, B/GA and Na+K of the selected samples are similar to the paleosalinity changes restored by Couch formula, and the water is mainly featured by brackish and saline water. (4)With increasing sample depths from TZK-1 and TZK-2 boreholes, the paleosalinity changes regularly, with the characteristics of increase-decrease, which is consistent with the change of illite content. The paleosalinity change is coupled with paleoclimate and redox research in this lithologic section. The results show that: The study of paleosalinity in the target formation is not only conducive to the restoration of paleosedimentary environment in the lake basin, but also helps to reveal the influence of paleosalinity variation on clay mineral assemblage characteristics and evolution. These results may provide a certain basis for the later in-depth study of clay mineral’ s nuclide adsorption performance and nuclide migration in mudstone.

Key words: paleosalinity, clay mineral, Couch formula, Bayinggebi Formation, Bayinggebi Basin