Quaternary Glaciation of the Bailang River Valley, Qilian Shan
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http://www.paper.edu.cn Quaternary International 97–98 (2002) 103–110 Quaternary glaciation of the Bailang River Valley, Qilian Shan Shangzhe Zhou*, Jijun Li, Shiqiang Zhang Department of Geography, Lanzhou University, Lanzhou 730000, People’s Republic of China Abstract The Qilian Shan, on the Northeast margin of the Qinghai–Tibetan Plateau, is weakly influenced by the Asian monsoon. Until recently, the Quaternary glacial geology of this region has been poorly understood. This paper describes a sequence of Quaternary glacial deposits in the upper reaches of the Bailang River. Using electron spin resonance (ESR), thermoluminescence (TL) and radiocarbon dating, tills, loess, buried soils and landforms were dated. The oldest till was dated B463 ka BP by ESR. A younger till and its outwash terrace were dated to B135 and 130 ka BP. The loess on this outwash terrace was dated to 141.7711.4 ka BP at its base and 43.773.5 ka BP in its central part by TL dating. A buried soil on a younger till was dated at 6920778 ka BP using 14C. The glacial landforms and these dating results show that glacial advances occurred during the Little Ice Age, the Neoglacial, MIS2–4, MIS6, and MIS12. Glaciation during MIS12 implies that the Qilian Mountains were rising coevally with the Qinghai–Tibetan Plateau, and were probably at a sufficient elevation for glaciation since at least 463 ka BP. r 2002 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction were unable to date each advance. Recently, we have applied modern dating techniques to define the timing of The Qilian Shan (Shan=Mountains), lying between glaciations in a succession of well-preserved sediments Gansu and Qinghai Provinces of China, is an impressive and landforms along the Bailang River. This paper mountain system at the northeastern margin of the describes this research and presents a new framework Qinghai–Tibetan Plateau (Fig. 1). It comprises many for the glaciation in the Qilian Shan. parallel mountains and valleys that trend NWW. The highest peak rises to 5826.8 m above an average topography of between 4000 and 5200 m a.s.l. The Qilian Mountains are close to the arid regions of central 2. The Bailang River drainage basin Asia. The annual precipitation along the northern front of the mountains, as in Dunhuang and Jiuquan, is The Bailang River (381520–391100 N, 991150–280 E), a o100 mm. Nevertheless, because of its high altitude, seasonal tributary of the Heihe River, rises in Zoulang- precipitation of >400 mm is present above the snowline nanshan of the middle section of Qilian Shan and (4200–4400 m a.s.l.). Together with the Tian Shan, the flows northwards with an average annual discharge of Qilian Shan plays an important role in transforming B39 Mm3. It infiltrates into the 40 km long diluvial area atmospheric moisture into surface runoff in the tempe- of the Heixi Corridor. The source of Bailang River is at rate arid regions of Central and East Asia. In the Qilian 5121 m a.s.l., and there are 23 modern glaciers with a Mountains there are 3306 modern glaciers with a total snowline altitude of about 4500 m that feed the river area of 2062.72 km2 and an ice volume of 146.63 km3 (Fig. 1: Wang, 1981). All the glaciers are smaller than (Wang, 1981). The extent of these glaciers has fluctuated 2km2 (Table 1). repeatedly throughout the Quaternary. Quaternary glaciation in the Qilian Shan has been studied by Liu (1946, 1963), Weng (1946), Li (1963), Qin (1981), Wu (1984), Guo (1984) and Rose et al. (1998). 3. Distribution of Quaternary glacial deposits These workers recognized several glacial advances, but Large U-shaped valleys extend 3–4 km from the *Corresponding author. termini of the largest modern glaciers to an altitude of E-mail address: zhsz@lzu.edu.cn (S. Zhou). 3800 m a.s.l. at the edge of the mountains, and moraines 1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. 转载 PII: S 1040-6182(02)00055-1 中国科技论文在线 http://www.paper.edu.cn 104 S. Zhou et al. / Quaternary International 97–98 (2002) 103–110 Fig. 1. Modern glaciers in the headwater of Bailang River. Table 1 Dimensions of the largest modern glaciers in the Bailang River Drainage Basin. Locations are shown on Fig. 1 Glacier Area (km) Length Snowline Terminus number (km) (m a.s.l) (m a.s.l) 11 0.9 2.2 4500 4280 13 0.92 2.0 4480 4250 14 1.08 2.2 4470 4260 16 1.53 2.8 4500 4230 18 1.42 2.8 4500 4260 19 1.26 2.3 4500 4300 21 1.51 2.5 4520 4340 are present in the foreland regions. The glacial deposits Fig. 2. The two sets of moraines surrounding the glacial lobe of glacier can be subdivided into two parts. No. 14. 3.1. Glacial deposits in U-shaped valleys however, the LIA moraines are distributed far beyond The contemporary glaciers are surrounded by mor- modern moraines. aines. For example, at the end of glacier No. 14, an 80 m A 60–80 m high lateral moraine stretches from near high moraine dams a small glacial lake. This moraine the LIA moraine of glacier No.14 along both sides of the is comprised of two parts: the upper part is fresh, while valley to an altitude of 3800 m a.s.l. (Fig. 3); but no end the underlying part is weakly weathered and glacial moraine is present. A 100 m high compound moraine is boulders are covered with lichens. Glacial meltwater present at an altitude of 3980 m a.s.l., indicating that flows between the two parts (Fig. 2). As in other regions glacier Nos. 14 and 16 were once joined at this location. in Western China (Chen, 1988), such moraines can These moraines are covered by abundant alpine vegeta- be thought to be the products of the Little Ice Age tion. According to the common pattern in Western (LIA), that is, the two parts represent the termini of China, these moraines are assumed to have formed recent and of LIA glaciation. In the other valleys, during the Neoglacial. 中国科技论文在线 http://www.paper.edu.cn S. Zhou et al. / Quaternary International 97–98 (2002) 103–110 105 3.2. Glacial deposits in front of the mountain This is called ‘‘Zhonglianggan’’ and is 500 m above the present river level. Over this ridge, till and conglomerate The U-shaped valley terminates at an altitude of rock erratics (1–2 m in diameter) are present (Fig. 5). B3800 m a.s.l. Below this, the topography is very steep Their high position indicates that these deposits are and the river flows through steep falls as a consequence quite old representing another glaciation. We have of tectonic faulting uplift. The glacial record is mainly called this glaciation the ‘‘Zhonglianggan Glacial distributed between the Bailang River and its tributary Stage’’. Postdating this glaciation, the landform suffered Xicha River in front of glaciers Nos. 14 and 16. Four much erosion. The past landscape during the glaciation sets of tills are clearly recognized. The biggest one is near would have been quite different from the present the Changgousi temple with an area of about 8 km2. The morphology. It is possible that the Zhonglianggan margin of this deposit is fluvially eroded. The highest glacial sediment was formed not only by glaciers Nos. point of this deposit is 2968 m a.s.l. and it is 268 m above 14 and 16, but also by other glaciers from adjacent the present river level. This till deposit represents a tributary valleys. There are six sets of glacial successions major glaciation, which we call the ‘‘Changgousi glacial (including two in the U-shaped valley and four in front stage’’. of the mountain) in the Bailang River basin (Figs. 6 Another two tills overlie the Changgousi till. The and 7). boundary between the two tills is at an altitude of 3450 m a.s.l. The three tills form an imbricate structure associated with the steep landform (Fig. 4), showing that the glaciers developed from hanging valley glaciers at 4. Terrace stratigraphy that time. A high ridge is present at about 2000 m from the There are at least five main outwash terraces along the center of Changgousi till fan, downwards the lower Bailang River (Fig. 8). The largest terrace (terrace 5) is reaches of Bailang River at an altitude of 2996 m a.s.l. widely distributed in the Bailang Valley and its adjacent Fig. 5. The erratics on the top of Zhonglianggan (500 m above river Fig. 3. The Neoglacial moraine in the U shaped valley. level). Fig. 4. Landforms and glacial deposit in the Bailang River Basin. 中国科技论文在线 http://www.paper.edu.cn 106 S. Zhou et al. / Quaternary International 97–98 (2002) 103–110 Fig. 6. Distribution map of glacial sediments in the Bailang River Basin. Fig. 7. The longitudinal section of the glacial sediments in the Bailang River Basin. tributary, the Xicha Valley. This terrace extends from with the moraine of the Changgousi Glacial Stage. the moraine of the Changgousi Glacial Stage to the Other younger terraces correspond to the overlying piedmont alluvial plain. The terrace is comprised of moraines of Changgousi age, but there is no apparent poorly sorted grey coarse gravel containing boulders direct relationship between them. between 1 and 2 m in diameter. This grey coarse gravel is The highest outwash terrace in the basin is present 50 m thick and underlain by Tertiary Red Clays (Fig.