Glacial Geomorphology and Ice Ages in the Bailang River Basin, Qilian Mountains
ZHOU Shang-zhe, LI Ji-jun,ZHANG Shi-qiang,ZHAO Jin-dong (Department of Geography, Lanzhou University, Lanzhou Gansu 730000, China)
The Bailang River, a tributary of the Heihe River and 40 km in length, rises in the Zoulangnanshan Mountain (5 100 m a.s.l.) and flows northward with a discharge of 39×10 6m 3 / year.It disappears in the diluvial area of Hexi corridor. The coordinates are 38°52′～39°10′ N, 99°15′～99°28′ E. There are 23 glaciers over there, the ELA of which is about 4 500 m a.s.l. Among them, the Glaciers No.14 and 16 are most typical ones in glacial sedimentation. The glacial sediments are recognizable both in the U shaped valley and in front of the mountain. In the U shaped valley, the moraine of the Little Ice Age is just distributed around the modern moraine, and the lateral moraines of the Neoglaciation stretch along the two sides of the river. In front of the mountain, there are three sets of imbricated fanlike moraines, of which the lowest one is near Changgousi and named Changgousi Glaciation. This moraine is dated 135 ka BP with ESR method. The bottom of an overlying loess (18 m in depth) of an outwash terrace, that is corresponding to the Changgousi moraine, is dated in (141.7±11.4) ka BP with TL method.This means that the Changgousi moraine was formed during penultimate glacial age, i.e., MIS6. It is reasonable that the two overlying sets of moraine represent the early stage of LG and the LGM, respectively. The oldest glacial deposit is on the top of a high hill, named Zhonglianggan (2 997 m a.s.l. and 500 m above the river level), from which a ESR dating of (460±45) ka BP is obtained, showing that this till deposit belongs to the Stage 12 of oxygen isotope. This dating just coincides well with the Shangwangfeng Glaciation in the *Brümqi River Basin of eastern Tianshan Mountains, Xinjiang, which is ESR dated 470 ka BP. Based on above mentioned, it is sure that there existed many glaciations, including MIS 12, 6, 4, 2 of the Pleistocene, and Neoglaciation and Little Ice Age in the Holocene. In the Qilian Mountains, planations, 3 000～ 3 200 m in height, are widespread which are witness showing that there was a long stable period of the crust before its strong uplifting. These planations disintegrated when the mountains began to rise at about 3.7 Ma BP.Afterwards, it was a long time(about 2.8 Ma) for the sedimentation of Yumen conglomerate rock. The famous Kunlun-Yellow River tectonic movement of the Tibetan Plateau occurred at 1.1 Ma BP, which greatly changed the geomorphologic process and made the Qilian Mountains up to the cryosphere, and some peaks of the Qilian Mountains came into a glacial-periglacial environment. The Bailang River basin was up to such a height at least at 460 ka BP.The fact that the highest outwash terrace has been cut down to 250 m indicates that after Zhonlianggan Ice Age, the mountains had a great uplift. According to an ESR dating on a moraine from Kunlun Shan Pass and a 36Cl dating on the bottom of a 309 m deep ice core from Guliya Ice Cap of the West Kunlun Mountains, Shi Yafeng suggested that the Maximum Glaciation on the Tibetan Plateau occurred in 0.6～0.8 Ma BP, or MIS 16. During the Maximum Glaciation, glaciers had an area of 500 000 km 2 in total. This maximum glaciation was also called the Third Glaciation from the last. Was there no other glaciation occurred in such a long time between 600 ka BP and 125～180 ka BP, the penultimate ice age?The discovery of glaciations for MIS 12 in the Qilian Mountains and Tianshan Mountains is quite significant.