The Environment And Evolution Of Zhada Basin, Tibet

Uplifting and environment change of the Tibetan Plateau are afocus topic in the earth sciences. Accompanying the uplifting of the Tibetau Plateau ,many sedimentary basins in the inner plateau were formed. The sediments of these basins are the records of uplifting and evolution of the plateau in the different stages. As a result of the integrated research, the strata in the Zhada basin is subdivided into two formations, ie. Tuolin Formation (Fm.) and Xiangzi Fm., and the former can be subdivided into three members The first member of Tuoling Fm. consists of conglomerate with sandstone, occurring with though and tabular cross-beddings, which are consided to be as the alluvial fan deposit; the second member consists of pelitic silstone and pebbled sandstone interbedded with sandstone, which are consided as channel deposit; the third member is composed of flat-bedding-occurred pelitic siltstone and siltpelite, which is belong to fan delta-lacustrine deposit. Xiangzi Fm. is made up with red and yellow conglomerate, and sandy conglomerate interbedded with sandstone, which was alluvial-pluvial fan deposit. The first member of Tuolin Fm. have been dated by ESR (electron spin resonance) at 5.1Ma, the third member of Tuolin Fm. have at 3.3Ma. Both these ages belong to Pliocene. Xiangzi Fm. have been dated by ESR at 2.58Ma as Eopleistocene. These data show that the basin formed in Pliocene and closed in Eoplestocene. There are much sporomorph in the second and third member of Tuolin Fm. Pine wasthe main part main part of the AP(arborescent pollen) in the sporopollen assemblage, and wormwood is the of the frutice in the sporopollen. Sporopollen assemblage indicated the climate change from warm humid environment to cold and dry environment. Some Ostracoda and Gastropod fossils from the third member of Tuolin Fm. and Xiangzi Fm. indicated that the lake water changed from fresh water to mild brackish water. Hipparion Christol, the Placeotragus and microden Kolken from the second member of Tuolin Fm., discovered in the north Zhada County, are similar to the Hipparion in the North China and comparable with Hipparion-occurring Woma Fm. in Jilong basin of south Tibet. So the age of the stratum was Pliocene. According to the results of heavy mineral assemblage and...