柴达木盆地西部新生代气候与地形演变

CENOZOIC CLIMATIC AND TOPOGRAPHICAL CHANGES IN THE WESTERN QAIDAM BASIN

从孢粉植物分异及演变,干旱碎屑及膏盐沉积分布等方面,对柴达木盆地西部新生代气候与地形的演变进行了探讨。结果表明,盆地西部新生代两个极端干燥气候期(膏盐发育期)分别出现在始新世至渐新世及上新世至第四纪。前者与老第三纪行星环流控制下的副热带干燥带有关,而后者与青藏高原的隆升有关。早第三纪盆地西部及周围地区的地势不象以前所认为的那样低平。在第三纪,昆仑山比祁连山低。晚新生代盆地向北推移了7-11个纬距。盆地新生代环境的演化与纬度的增高、高度的增大、地球气候的变冷、青藏高原隆升及地形变化有关。

A Cenozoic sporo pollen sequence is constructed based on sporo pollen analysis of 6000-m-long cores and cuttings. The sporo pollen sequence shows that two extraordinary arid periods during the Cenozoic occurred from the middle Eocene to the end of Oligocene and from Pliocene to Pleistocene. The arid plant pollen make up 40% -60% and 70% -90% respectively in the two periods. They correspond to two periods of lake salinization. The first arid period could be related to the subtropical arid zone controlled by Paleogene the plant circulation system. The second resulted from the uplift of the Qinghai-Tibet plateau, which blocked the entrance of the moisture from the Indian Ocean into the basin. There has existed a trend of decline of subtropical plants since the Eocene. This trend resulted from the northward migration of the basin and the global cooling of the climate in the Late Cenozoic. At the end of the Pliocene, the subtropical vegetation was rapidly replaced by temperate vegetation, which was associated with the rapid uplift of the basin. In the Paleogene, the vertical vegetational zoning had been very conspicuous. There existen on the western Qaidam Basin not only subtropical broad-leaved trees, subtropical conifer trees and temperate broad-leaved trees, but also temperate conifer trees and subalpine dark conifer trees, This suggests that the topographical relief was not as small as though before in the western Qaidam Basin and its surroundings in the Paleogene. The relief could reach 2 km, and some peaks could be as high as 3 km above sea level. In the Tertiary, the Eboliang region (north of the basin) had higher percentages of angiosperm tree (Arbor) pollen and mountainous conifer tree pollen and a lower percentage of arid plant sporo-pollen than the Mangyai region (south of the basin). This indicates that the southern side was drier and had less aboundant conifer-trees than the northern side of the western Qaidam Basin during the Tertiary Period, and suggests that the Kunlun Mountains and Qinghai-Tibet plateau were lower than the Qilian Mountains in the Tertiary and that the moist air from the Indian Ocean could go acfgss the plateau and Kunlun Mountains to the south slope of the Qilian Mountains to form precipitation there.The distribution of arid clastic sediments shows that the Paleogene subtropical arid zone has been twisted because of the northward migration of western China since the Neogene. From the amount of deformation of the arid zone, it may be estimated that western China has migrated at least 800 -1200 km northward in the Late Cenozoic.