Geochemical Characteristics And Paleoenvironment Paleoclimate Significance Of Mudstone In SHANG GAN CHAI GOU Formation,Northwestern Margin Of Qaidam Basin

During the Cenozoic era,the global climate became colder,the Tibet Plateau uplifted and the North Tethys regressed,which profoundly changed the earth's cli Mate and environment.The SHANG GAN CHAI GOU Formation in the northwest Margin of Qaidam Basin is widely exposed in the basin.The study of this Fomation is of great significance to reveal the uplift of the Qinghai Tibet Plateau,climate change in Northwest China and the aridity in Asia.Based on the field geological survey and 26 mudstone samples from Leng Hu No.4 well of SHANG GAN CHAI GOU Formation,the main and trace elements and rare earth elements REE were tested.At the same time,the paleosalinity,paleoclimate and paleoenvironment of SHANG GAN CHAI GOU Formation were restored by combining the lithologic characteristics of mudstone and previous research results of paleontology,palynology and Sedimentology,The results are as follows:(1)The ratio of Sr/Ba of paleosalinity index is less than 1 except LS?4?19,which is 1.18>1,indicating that most of the samples are from freshwater environment;The Th/U ratios of all samples are greater than 2,which is consistent with the terrestrial freshwater environment reflected by Sr/Ba ratio.(2)The maximum U/Th ratio of redox index was less than 0.75;The maximum value of V/Cr ratio is less than 2.0;The maximum Ni/Co ratio is less than 5.0;The maximum V/Sc ratio was less than 9.The four redox indexes indicate that the SHANG GAN CHAI GOU Formation is an oxidation environment,which is consistent with the redox state reflected by the brown color of mudstone samples.(3)The paleoclimate index Sr/Cu ratio indicates that the warm and humid climate periods are24.0?27.5 Ma and 29.5?31.0 Ma,respectively;The dry climate periods are 22.0?24.0 Ma and27.5?29.5 Ma,respectively.The curves of Sr,Sr/Cu,Rb/Sr,Fe/Mn are mirror symmetry,which reflect the consistency and synchronization of paleoclimate change.The temperature and humidity of cli Mate are basically consistent with the trend of salinity change,that is,the salinity is fresh water environment when the climate is humid,and the salinity is salt water environment when the climate is dry.Sedimentological and paleontological characteristics also show that the SHANG GAN CHAI GOU Formation is in a relatively warm and humid climate.The warm humid climate and fresh water environment respond to the climate background of global warming in this period.The appearance of periodic arid climate may be affected by the uplift of Qinghai Tibet Plateau in the early stage,and mainly related to the regression of North Tethys in the late stage.It is speculated that the Mid Hi Malayan movement led to the uplift of the Leng Hu tectonic belt,which led to the subsidence of the lake basin,the northeastward migration of the lake shoreline,and the decline of the lake water level,forming an oxidizing environment.