GEOLOGICAL ENVIRONMENTS SINCE PLIOCENE AND ACCUMULATION PROCESS OF SALINE DEPOSIT IN WEST CHAIDAMU BASIN, QINGHAI, CHINA
Chen Zhenou (1st team of Bureau of Geology and Mineral Resources, Xining, China. 810000) Tong Guobang Zhang Junpai Yu Shuteng Li Yinluo (Institute of Hydrogeology and Engineering Geology, MGMR, Zhengding, Hebei, China. 050803)
Studies has been carried out of sediments and sporopollen fossils since late Pliocene in Dalangtan and Chahansilatu regions of the western Chaidamu Basin, and 9 sporopollen zones, 9 vegetation periods, 30 climatic fluctuations and 3 accumulation processes of saline deposits have been established with the methods of paleomagnetic and isotopic dating for the sporopollen fossils from five boreholes. Climatic fluctuations were advantageous to transportation and accumulation of saline deposits. 1. In late Pliocene, climate tended to become cold and dry. Vegetation was represented by sparse woods with coniferous broad leaved trees-steppe landscape. Lake water became concentrated. And saline deposits began to deposit. 2. From late early Pleistocene to late middle Pleistocene, climate was extremely dry. There appeared Ephedra fruticeta-desert steepe vegetation. Lake water concentrated much faster. And a great number of saline deposits were deposited. 3. In early late Pleistocene, climate turned relatively humid. Vegetation was shown by pine forest with deciduous broadleaved trees-steepe landscape. Lake water desalinated gradually. And mud sediments developed in the lake. 4. During late late Pleistocene, climate changed to be dry and cold again. Desert appeared in the area. Lake dried up. And saline deposits well developed with accumulation of sylvine. That was the process of birth, development and end of a saline lake. Low boundary of Quaternary, located between sporopollen zones Ⅲ and Ⅱ, is corresponding to the paleomagnetic M/G boundary. Bottom boundary of middle Pleistocene is between sporopollen zones Ⅵ and V, corresponding to the B/M boundary. Bottom boundary of late Pleistocene is located between sporopollen zones Ⅷ and Ⅶ, 0.15-0.17ma B. P. by U-isotopic dating.