Evolution Of Middle And Late Miocene Environment For The Dongwenquan Basin In The East Kunlun Mountains

The thesis studied lacustrine sedimentary strata in the Dongwenquan basin in the East Kunlun Mountains with multidisciplinar cross theories and methods involving geomorphology, sedimentology, structural geology, environmental geochemistry, sporopollen analysis, micropaleontology and so on. A typical Miocene section there was in detail studied in which fossil records to indicate changes of paleovegetation and paleoclimate were obtained. The paleotemperature and paleoelevation of the basin were also estimated through the content of carbon isotope. Thereby the paleoenvironmental evolution of the basin was briefly analyzed. Finally, the relationship between the Miocene environmental evolution of the basin and the uplift of the Tibet Plateau was approached by regional correlation of contemporaneous lacustrine sediments.The main results and conclusions obtained by this study are as follows:1. The stratigraphic sequence of the middle and upper parts of the Wudaoliang Formation of Miocene age was established by historical analysis of activities of the East Kunlun strike-slip fault, Cenozoic mapping of Dongwenquan pull-apart basin, and measuring of a lacustrine facies statigraphic section in Keteng Habucihaguole gully. It is revealed that the lacustrine facies strata of the Wudaoliang Formation should deposit in Dongwenquan pull-apart basin formed by the East Kunlun strike-slip fault after disintegration of the Tibet Plateau's planation surface. This provides new data for the study of the uplift and climate change of the Tibet Plateau and the division and correlation of Late Cenozoic strata.2. The paleoclimate and paleoenvironment of the middle-late of Miocene were reconstructed by statistical analysis of Ostracoda and sporopollen in lacustrine sediments of the Wudaoliang Formation. The Ostracoda fossils show that lacustrine sediments of the Wudaoliang Formation should obviously form in a lake sedimentary environment. The sporopollen assemblage shows an evolution of Gymnospermae to Angiosperm to Pinus family to Angiosperm, dominated by Angiosperm. The vegetation type gradually changed from medium height mountain coniferous forests to cool dry grasslands. All of these suggest a climate fluctuation: from warm wet to dry warm to wet cool to dry cool.3. In addition to the above paleoclimate reconstruction based on Ostracoda and sporopollen, analyses of stable carbon(δ¹³C/‰)and oxygen isotopes(δ¹⁸O/‰)also show that there were a period change of paleotemprature in the middle-late Miocene in the East Kunlun Mountains region: paleotemperature dropped from22.17°C in the middle of Miocene to 17.16°C at some point of the late of Miocene and then rose to 20.24°C before the end of Miocene, approaching to the average temperature of today's June in the south of the Tibet Plateau. On the other hand, the content change of stable carbon and oxygen isotopes would correspond to a process in which ancient lakes appeared, expanded and withered.4. The paleoclimate change was related both to the uplift of the Tibet Plateau and to the formation of the East Kunlun Mountains. According to the relation formula between carbon isotope and elevation, it was estimated that the East Kunlun region in the middle-late Miocene uplifted to an elevation of about 2650m, much lower than the contemporaneous elevation in the south of the Tibet Plateau but approaching to the elevation of the Qaidam Basin. The climate of this period overly trended to cold and dry. The East Kunlun Mountains formed in the early of Quaternary.