PALEOCLIMATIC CHANGES RECORDED BY MIOCENE SEDIMENTS IN THE TIEJIANGGOU SECTION OF THE SUBEI BASIN,NORTHEASTERN TIBETAN PLATEAU
The timing and driving mechanisms of inland aridification in Asia remain controversial,largely due to a lack of high-resolution paleoclimate records that can be systematically compared in arid regions,and the Subei Basin,located in the northeastern margin of the Tibetan Plateau,preserves thick and continuous Miocene fluvial-lacustrine sedimentary strata,making it an ideal area for studying the relationship between inland aridification in Asia and the uplift of the Tibetan Plateau.Here,we selected the Tiejianggou section(39° 29′N,94°41′E)of the Subei Basin,where bio-magnetostratigraphic studies have been conducted.The Tiejianggou section(ca.22.8～9.0 Ma,2800 m)is divided into four units from bottom to top:the bottom is composed of conglomerate,glutenite and sandstone with mudstone and siltstone;the lower part is red fine-grained mudstone;the middle part is mainly composed of fine-grained mudstone or siltstone and sandstone;and the upper part is composed of conglomerate and thin siltstone interlayers,gradually coarsening into massive conglomerate.Diffuse reflectance spectroscopy(DRS),simple carbonate tester and Mastersizer 2000 laser organic matter particle size analyzer were used to obtain the color,carbonate content and particle size change sequence of 280 samples.According to the change characteristics,the sequence can be divided into three stages:(1)During ca.22.8～17.0 Ma,the redness,Hm and Hm/Gt gradually increased,with averages of 34.67,0.24 and 3.78,respectively;the yellowness,Gt and brightness gradually decreased,with average values of 11.68,0.07 and 26.12,respectively;CaCO3 gradually decreased upwards and slightly increased in the upper part,with an average value of 13.78.Among them,the frequency distribution curve of sediments during ca.22.8～21.4 Ma showed asymmetric bimodal distribution characteristics.The transport mode was dominated by suspension transport,with a small amount of rolling and jumping,and sorting was poor.The frequency distribution curve of sediments during ca.21.4～17.0 Ma showed an obvious unimodal distribution,dominated by suspension transport,with fewer saltation components and better sorting.In this stage,the average particle size first became coarser and then finer upward;the hydrodynamic conditions were weak,and the sedimentary environment transitioned from low-energy fan delta to lake-shallow lake facies.(2)During ca.17～14 Ma,the redness,Hm and Hm/Gt remained relatively high,with average values of 36.77,0.25 and 4.87,respectively,while the yellowness and Gt remained relatively low,with average values of 1 1.56 and 0.05,respectively.Brightness and CaCO3 gradually decreased,with average values of 21.42 and 13.07,respectively.The average particle size again became coarser upward;the frequency distribution curve showed a bimodal distribution,with the transport mode dominated by saltation and suspension,and sorting was poor.The sedimentary environment's energy was high,characterized by low-energy fluvial facies.(3)During ca.14～9 Ma,the redness,Hm and Hm/Gt gradually decreased,with average values of 35.97,0.24 and 4.52,respectively.The yellowness,Gt and brightness exhibited a weak upward trend,with an average values of 11.62,0.06 and 22.71,respectively.CaCO3 exhibited a weak downward trend,with an average value of 11.09.The average particle size obviously coarsened upward.The frequency distribution curve was dominated by bimodal distribution.The transport mode was dominated by saltation and suspension,with a small amount of rolling.The sorting was poor,and the hydrodynamic force was enhanced,indicating a gradual transformation of the sedimentary environment into high-energy fan delta facies.Combined with the research results of the Qaidam Basin and adjacent basins,it is shown that the Miocene climate of the Subei Basin underwent three stages of evolution:relatively mild during ca.22.8～17.0 Ma,warm and humid during ca.17～14 Ma,and gradually cold and dry during ca.14～9 Ma.Combining these findings with previous research,we propose that the obvious coarsening of grain size parameters in the profile after 14 Ma was mostly due to tectonic uplift in this region.The climate change in the Subei Basin during the Early to Middle Miocene(ca.22.8～14.0 Ma)was mainly influenced by global climate change,while the gradually cold and dry climate during the Middle to Late Miocene(ca.14～9Ma)was likely affected by both global cooling and the uplift of the Tibetan Plateau.
万方数据
维普
