The Significance Of The Cenozoic Molasses Formation In The Southwest Depression Of Tarim Basin

Since Cenozoic, the collide of India plate and Eurasian plate have caused the uplift of Qinghai-Tibet plateau and the uplift of The West Kunlun, which lies in Qinghai-Tibet's northwestern margin , and have deposited massive sediments in the southwest depression of Tarim basin. These sediments were the result of the uplift of the northwest of Qinghai-Tibet plateau in late Cenozoic, and to a large extent, well documented the uplifting of the plateau .This thesis is to recover some of the qualities and characteristics of the uplift of the northwest of Qinghai-Tibet plateau in late Cenozoic through analyzing the sediment record in the southwest depression of Tarim basin.The subsidence of the southwest depression of Tarim basin is relatively well coupled with the uplifting of the Qinghai-Tibet plateau. The relative of the couple lies is in time ,space, mechanism and the sediment supply between the sediment of the southwest depression of Tarim and the orogeny of the West Kunlun. After analysising the sequence and unconformities of the southwest depression of Tarim basin, in Cenozoic, we found there were three times of unconformities in the southwest depression of Tarim: Paleogene and pre-Paleogene, Neogene and Paleogene, late Pleistcene and early Pleistocene. These three times of unconformities showed three times vialent tectonic movement, caused highly uplift and denudation of the West Kunlun, rapidely the subsidence rate and the sediment of the southwest depression of Tarim.By contrasting the thickness, lithology , colour , ingredient of the sediments, and analyzing the size, heavy mineral ,chronology ,the molasses formation can be divided into 4 period: the lower Atushi formation is the first stage, the upper Atushi formation is the second period ,Xiyu formation is the third stage ,later Pleistocene and Holocene is the forth stage.By analyzing the sedimentology character of the molasses, and with combining the characteristics of fission track and magnetic, the uplift history of the West Kunlun can be divided into 5 stage:①Before Pliocene, there is no molasses formation, which shows slow uplift of the West Kunlun ;②Early Pliocene, there is conglomerate sediment in the foreland of the West Kunlun, which shows rapid uplift of the West Kunlun in the late Miocene;③Late Pliocene, the lithology changed obviously and Xiyu Conglomerate occured, which shows that more rapid uplift of the West Kunlun;④Eatly Pleistocene, there is unconformities at some places and shows violent tectonic movement in the southwest depression of Tarim. After that, the lithology more and more thicker ,the uplift rate of the West Kunlun continuously rapid;⑤One tectonic movement evoled the former formation into it in the early Pleistocene, shows that this movement is violent, the West Kunlun is still at the stage of rapidly uplifting.after this movement, the molasses sediment is still on the rapidly uplifting stage.