The Magnetic Geochronology Research On The Late Miocene Sedments In Xining Basin And Its Environmental Significance

The Xining Basin is a Meso-Cenozoic faulted basins,located in the transition zone of the northeastern margin of the Qinghai Tibet Plateau and the Loess Plateau.The basement of the basin is composed of Proterozoic gneiss.With uplift process of the Qinghai-Tibet Plateau,thick Cenozoic strata deposition in the Xining Basin,which become a natural place to record the relationship between rise of the Qinghai-Tibet Plateau and climate change.This paper discusses the relationship between climate change and tectonic uplift in late Miocene in Northeast Qinghai-Tibet Plateau,base on the research of lacustrine strata sedimentology,250m thick,of upper member of Xianshui River formation in Huzhu area in Xining Basin.The author conducted a study of rock magnetism and magnetic stratigraphy through paleomagnetics to build a magnetic polarity sequence of late Miocene strata in Huzhu area.In meanwhile,the author dated the ancient animal fossils in strata,analyzed paleoclimate alternative indexes of magnetic susceptibility,granularity,etc.There are five main conclusions after the studies:1.According to the research on paleomagnetism samples of section of Banyan and Hongyazi that collected in lacustrine deposit in Huzhu area by the method of rock magnetism including isothermal remanent magnetism curve,reverse field demagnetizm curve,thermal demagnetization,etc.,magnetic minerals in lacustrine deposit in Huzhu area include black iron ore,maghemite,hematite,goethite,etc.And the magnetic records mainly exist in black iron ore and maghemite,which are in low coercive force.2.The section of Banyan is 46m think,Hongyazi is 250m think.A magnetostratigraphic study conducted on the two sections shows that the formation in Hongyazi area records 12 normal polarity sections(N1?N12)and 13 negative polarity sections(R0?R12),which match well with C3Bn?C5An.1n in the standard paleomagnetie scale:the formation in Banyan area records 5 normal polarity sections(N1?N5)and 5 negative polarity sections(R1?R5),which match well with C3Bn?C4n.2n in the standard paleomagnetie scale.In the same time,magnetic polarity sequences of Banyan section and Hongyazi section can be match well.The sedimentation rate of the top and bottom section predicts that the age of top of Banyan section is around 7.049 Ma,the bottom is around 8.215 Ma and whole section is1.166 Ma;the age of top of Hongyazi section is around 6.923 Ma,the bottom is around 12.149 Ma and whole section is 5.226 Ma.The conclusion above has extended late Miocene Magnetic chronology's record range of magnetic polarity formation in Xining Basin,on the basis of previous studies.3.A large amount of mammal fossil has been found on the bottom of Banyan section.After restoration and identification by Dr.Yuan Wang and researcher Changzhu Jin,who work for Institute of Vertebrate Paleontology and Paleoanthropology(IVPP),the Chinese Academy of Sciences,these fossils are Parelasmotherium sp.and Hipparion dongxiangense,early late Miocene.For the first time,large mammal fossils are found in Xining Basin.After calculation,paleontologists fossil layer located at the bottom of Xining Basin dates about 8?8.1 Ma,presents the time of their existence.The discovery not only fills the research vacancy of such fossil in Xining Basin,but also provides new evidence of the researches of Environment Evolution and stratigraphic division in basin on northeastern margin of the Qinghai-Tibetan Plateau,evolutionary process of Hipparion,etc.4.According to Hongyazi section magnetic susceptibility and grain size changing feature,climate in Late Cenozoic in Xining Basin can be divided into four stages.Stage ?:12.149?11.53 Ma,the temperature drops,meaningwhile,the magnetic susceptibility curve appeared a peak value fluctuation,grain size curve showed a continuous decreasing trend;Stage?:11.53?8.24Ma,the magnetic susceptibility curve fluctuation is more stable and smaller size,showed the particle size increased and the value of smaller amplitude,plenty of rain and water flow on behalf of the relatively warm and humid climate;Stage?:8.24?7.07Ma,magnetic susceptibility values were significantly increased untile 7Ma,the grain size characteristics of reaction near 8Ma began to decrease,hydrodynamic condition had weaken,area of lake had reduced,climate had become dry;Stage?:7.07?6.92Ma,magnetic susceptibility values decreased,while grain size curves of this stage performance is not obvious,similar to the variation characteristics and the preceding stage,this stage temperature had rebounded.5.Study on deposition rate profile Hongyazi show that a tectonic uplift event at about 8Ma happened in Xining basin.Magnetic susceptibility,particle size and fossil evidence show that the main climate of semiarid,what is a environmential response for the uplift of the Tibetan plateau about 8Ma.6.The hipparion fossils show that in late around 8.1Ma Pliocene on the northeastern margin of the Tibet Plateau with an average elevation of not more than 1000m,which had not yet reached the current maximum height.Influencing by the remote collision of the Tibet Plateau uplift,the Xining basin uplift at least 1600?2100m of the magnitude since the late Miocene.