Late Cenozoic Magnetostratigraphy Of Mazatagh Section

On the systematic geological survey, Mazatagh section, a consecutive and well-outercropping section of cenozoic in central Tatim basin was chose for investigation on stratigraphy, paleomagnetism and paleoclimatology. Thermal demagnetizer and cryogenic magnetometer were used to systematically measure the magnetic remanence from room temperature up to 650°C with 50°C step in order to dertimime magnetostratigraphy of the section. Meanwhile, magnetic susceptibility was measured as a proxy of paleoclimate for the section. Magnetic polarity boundary ages are used as the controlling points, while using the linear-interpolated accumulation rate for building up the time scale of the section. Consequently the bottom age of continental facies, the age of eolian accumulation in large scale and the age of large gravel bed were estimated. On that basis, we get some new understanding about the evolution of the Tarim gulf, the age of the Taklimakan desert as well as the uplift of Qinghai-Tibet Plateau in Cenozoic .On the analysis of paleoclimate records, the drought history of Cenozoic in Tarim basin was revealed. The following points mark the main conclusions of this thesis:1. On a systematic thermal demagnetization and remanence measurement using a cryogenic magnetometer, Magnetostratigraphy of Mazatagh section was produced for the Cenozoic continental strata, which shows a consistent polarity structure. Magnetostratigraphy of the section consists of reversed Chron of Matuyama, normal polarity Chron of Gauss and reversed Chron of upper Gilbert. Chron Matuyama is characterized by a long reversed polarity with three normal subchrons, Chron Gauss is characterized by a long normal polarity with two reversed subchrons at lower part of the Chron.and Chron Gilbert can be determined by a long reversed polarity at the top of the Chron. The bottom age of the continental facies is 4.2Ma and the top age of the section is 0.8Ma.2. Magnetostratigraphic study of the section shows the boundary between alluvial plain facies and marine facies lies in revised Chron of Gilbert, the age is estimated to be 4.2Ma. It suggests that the evolution of semienclosed sea in western Tarim ended at 4.2Ma, and the regression of Tethys occoured at 4.2Ma.3. The time when the gravel bed appeared for the first time is about 3.0Ma, and it may suggest the quick uplift of Qinghai-Tibet Plateau in Cenozoic.4. The initiation of large scale accumulation of eolian material is estimated to be 3.4Ma, which is a symbol of a further drying occurred in Late Cenozoic in inland of northwest area, and also a symbol of the appearance of the Taklimakan desert, meanwhile, it may suggests that Qinghai-Tibet plateau had reached a significant height at that time.