Palaeoclimate Change Across The Triassic-Jurassic Boundary In The Southern Junggar Basin

The Triassic-Jurassic boundary is one of the most important geological boundaries in the evolutionary history of the earth.The previous studies on the changes of paleoclimate at this boundary around the world proposed that global climate warming and humidity increasing occurred at that time.However,these studies mostly focused on Tethys realm at low latitudinal.Therefore,the evidence from the terrestrial sections in high latitudes of northern hemisphere has yet to be studied to find out the true nature of global climatic change at the boundary.The southern margin of the Junggar Basin has thick,continuous largely nonmarine Cenozoic strata,placed at the high latitude continental region of the northern hemisphere during the Early Mesozoic,and is a key area to study the continental paleoclimate change for the Triassic-Jurassic boundary at high latitude.Therefore,in this thesis,the Haojiagou and Badaowan formations,placed at upper Triassic and lower Jurassic,were studied on whole-rock geochemistry such as major and trace elements,especially Cu-Zn-Mg isotope composition of terrestrial clastic sedimentary rocks combined with previous studies of paleontology and carbon isotopes in the area to reveal the sedimentary environment and paleoclimate of the area during the Late Triassic-Early Jurassic.The main achievements are as follows:1.The sample source of the Triassic-Jurassic boundary at the Haojiagou and Badaowan formations on the southern margin of the Junggar Basin has the characteristics of quartz sedimentary source area,and its parent rock is felsic volcanic rock.The major element composition characteristics show that it has a plate tectonic background on the passive continental margin since the samples underwent intense weathering,while the trace element content characteristics show that its parent rock from a continental island arc environment.It is speculated that the felsic volcanic parent rock was formed in the continental island arc environment and was transported to the passive continental marginal environment for sediment.2.The freshwater reduction sedimentary environment of the fluvial delta facies in the upper part of the Haojiagou Formation and the lower part of the Badaowan Formation during the depositional period with a humid climate is confirmed by some element ratios such as V/?V+Ni?ratio and Ce_anomaly for redox condition,Sr/Ba ratio for salinity,Sr/Cu ratio and Fe/Mn ratio for humidity as well as previous studies on the sedimentary facies of the Haojiagou and Badaowan formations.In addition,the sedimentary environment changed at the end of the late Triassic,especially the Fe/Mn ratio for humidity,characterized by sudden decrease below the boundary,and then showed a trend of increase,indicating a brief relatively drought period at the end of the late Triassic before increasing wetting.3.A minimum value of Mg isotope composition identified at the top of the Haojiagou Formation,and its increasing trend after that,combined with the Zn isotope composition gradually reduce above the boundary as well as their Chemical Index of Alteration?CIA?further revealed an increasing hot and humid climate trend the southern margin of the Junggar Basin after short and arid climate below the boundary.4.The climate of the southern part of the Junggar Basin,placed at high latitudes continental region in the northern hemisphere,tended to be hot and humid at the beginning of the Jurassic,which was consistent with the paleoclimate at that time in other parts of the world,indicating a global climate change at the Triassic-Jurassic boundary.