| The Xining Basin locates in the transitional zone of the eastern humid monsoon region, the northwest arid inland and the Tibet Plateau Paramos Area. This location is sensitive to global climate and environment changes. The long successive fine-grained lacustrine strata deposited in the Basin hold the information of the global climate change and paleoenvironmental evolution of the Tibetan Plateau and its adjacent basins since Eocene to Miocene period.The Xiejia section in Xining Basin is characterized by alternation of distal alluvial fan and shallow playa lake deposited. In this paper, we investigated that sedimentary lithology and collected the geochemical samples for further elemental analysis in the section (aged from 47-19 Ma). The high-resolution records of soluble salts and whole rock elements were obtained through analyzing 424 chemistry samples. Further, we explored the sedimentary facies controls on the distribution of soluble salts and whole rock geochemistry characteristics. The results revealed the relationship between global change and paleoenvironmental evolution of Xining Basin and areas around Tibetan Plateau during 49-17 Ma.Firstly, based on soluble salts analysis of the lacustrine mudstones/gypsum deposition in the section, some conclusions are obtained as follows:1. The gypsum layer and mudstone layer are corresponding to the high value and low value of Ca2+, SO42- and Sr2+, respectively. More specific, the lower part of this section with widely spread gypsum layers has low concentrations of Na+ and Cl- but high concentrations of Ca2+, SO42-and Sr2+; whereas the upper part with dominating red mudstone has slightly high concentrations of Na+ and Cl- and low concentrations of Ca2+, SO42- and Sr2+.2. The difference of soluble salts content shown above reflected the precipitation order of carbonate-sulfate-halide happened successively controlled by solubility, which clearly indicates a dry trend occurred at about 33 Ma.3. The study of sedimentary facies showed that gypsum and red mudstones layers are corresponding to shallow playa lake and distal alluvial fan, respectively. The evolution from distal alluvial fan and shallow playa lake alternations in the lower part of this section to distal alluvial fan in the upper part indicates a regional aridification process.Secondly, based on whole rock elements analysis of the sediments, we received some recognition as follows:1. The pedigree charts of elements cluster analysis were obtained through carrying cluster analysis for whole rock chemical elements. Three types of element groups are divided. The first group contains Al, K, Fe, V, Ti and Ni, indicating that those elements are stable in the weathering process and carried into lake as terrigenous clastic material. The tightly related Ca and Sr in the second group represent the chemical composition of the salt lake water. The third group includes Ba, Mg, Mn and Na, indicate other phases (e.g., carbonate and Fe-Mn oxides) except for above two subdivisions.2. R factor analysis showed that two main factors influence sediments chemical compositions, both of which account for cumulative variance contribution of 81.62%. There obviously exists positive correlation between the elements in each group of in the sediment of Xining Basin. The results indicated that elements in each group have a good symbiotic relationship and have certain origination. Both factors reflect the terrigenous clastic input as well as the lake evaporite deposition dominated by gypsum.Overall, based on analysis of the soluble salt and whole rock elements and comparison with sedimentary facie, palynology and organic geochemical proxies we concluded that the paleoclimate of Xining Basin experienced drying process at 33 Ma, which is the positive response of global cooling and early uplift of NE Tibetan Plateau. |
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