青海柴达木盆地西部上新世以来的地质环境与成盐期

GEOLOGICAL ENVIRONMENTS SINCE PLIOCENE AND ACCUMULATION PROCESS OF SALINE DEPOSIT IN WEST CHAIDAMU BASIN, QINGHAI, CHINA

对柴达木盆地西部大浪滩及察汗斯拉图地区的上新世晚期以来的沉积物及其孢粉化石的研究,结合古地磁、同位素年代测定,建立了九个孢粉带、九个植被演变期、三十次气候波动及三个成盐阶段。分析了本区主要成盐时期的古气候特征。 第四纪下限位于Ⅲ/Ⅱ孢粉带之间,对应于古地磁的M/G界面。中更新世底界位于Ⅵ/Ⅴ孢粉带之间,对应于古地磁的B/M界面。晚更新世底界位于Ⅷ/Ⅶ孢粉带之间,铀系测年为距今15—17万年。

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.