ESR DATING OF THE CENOZOIC STRATA AT LAOJUNMIAO SECTION, YUMEN AND ITS SIGNIFICANCE FOR UPLIFT OF TIBETAN PLATEAU

Vast thickness gravel formation developed widely around Tibetan Plateau, it provides the evidences of the uplift strongly process of Tibetan Plateau and it is also the products of uplift. So the study on origin, sediment environment and deposit faces of gravel layers can reveal the process and feature of Tibetan Plateau. According texture and components and glue degree and lithostratigraphy context of gravels, these gravel layers were divided into lower Pleistocene Yumen gravel formation and Middle Pleistocene Jiuquan gravel formation by Prof. Sun Jianchu in 1942. Since then, a lot of works have been carried including magnetic chronology. However, the absolute accumulated age of gravel is not yet identical because of different methods and precision. In this paper, a 1000m thick Cenozoic sediment at Laojunmiao, located at the northern foothill of Qilian Mt., is dating for ESR systematically. 19 block samples were collected for ESR dating. The pretreated samples were irradiated with a 60 Co\|source in different dosage. The irradiated samples were kept untouched for about ten days and then measured with a JES\|FEIXG ESR spectrum with the following measurement conditions: room temperature, X\|band, Microwave power: 0 1mW and 2mW, modulation amplitude: 0 8mT, magnetic field scanning range (334±5)mT. We select Ge and E' centre as the dating signal. The concentrations of radioactive elements, U, Th, K 2O, were determined by laser fluorescence, colorimetric spectrophotometry and atomic absorption techniques, respectively. AD value were obtained by linear regression. The results show that it is linear relationship between age and depth (thickness), and the age is well identical with paleomagnetic age.