Quaternary Glaciation of the Bailang River Valley, Qilian Shan

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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: [email protected] (S. Zhou). 3800 m a.s.l. at the edge of the mountains, and moraines

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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

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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

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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. 9) 2.5 km below Zhonglianggan on the west bank of the and it is overlain by an 18 m thick succession of loess. Bailang River. The surface of this terrace is 250 m above Their stratigraphic and sedimentologic relationship the present river level at an altitude of 2742 m a.s.l. The shows that the gravel of terrace 5 should be synchronous glacioﬂuvial gravel is 30 m thick and overlies Tertiary 中国科技论文在线 http://www.paper.edu.cn

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Fig. 8. The outwash terraces and their dates near Changgousi.

buried soils in some younger terraces. The ESR, TL and radiocarbon dating were undertaken in Qingdoa In- stitute of Marine Geology, Guangzhou Institute of Geochemistry (Chinese Academy of Sciences) and Lanzhou University, respectively. The samples for ESR dating were taken from Changgousi and Zhonglianggan tills, and outwash deposits of terrace 5. No natural sections are present for the two tills, and pits to 1–1.2 m depth were dug to collect samples. The sample from terrace 5 was collected from the section within a 1 m deep pit. The samples were divided into two parts in the laboratory. One part was used to extract quartz. The sample was washed in distilled water and was sieved Fig. 9. The outwash gravel on the terrace 5 and the highest terrace. into particle size of 0.06–0.25 mm. The organic matter was removed using H2O2. The carbonate was dissolved by soaking it in HCl with a concentration Red Clays (Fig. 9). The contact between these suggests of 6 ml/l for one week. The feldspar and surface of that this outwash terrace is synchronous with the the quartz were removed by soaking them in HF for erratics of the Zhongliangan Glacial Stage. This terrace 6 h. This was washed clean with distilled water and gravel is also present on the west bank of the Xicha dried in an oven at B401C. The magnetic minerals River, but here it overlies the Yumen Conglomerate, an were removed using a magnetic sorter, to leave the extensive Early Pleistocene gravel in the Hexi Corridor purer quartz particles (>90% by X-ray diffraction region. The top of the Yumen Conglomerate is dated at analysis). Finally, the quartz particles were separated 930 ka BP using palaeomagnetic and electron spin into nine portions (300 mg per portion) and each resonance (ESR) methods (Zhao, 2000). The overlying portion was artiﬁcially irradiated in different doses outwash and the Zhonglianggan Glacial Stage is there- (0, 225, 400, 615, 824, 1149, 1336, 2246 and 2593 Gy) fore younger than 930 ka BP. using the 60Co radioactive source. The irradiation rate was 45 Gy/min. The irradiated quartz particles were tested after 10 days using ECS105ESR spectral analyzer 5. Dating at a temperature of 201C, X-wave band, 2 mW in microwave power, 0.1 mT in amplitude modulation, We have used ESR, thermoluminescence (TL), and 348 mT in Central magnetic ﬁeld and 5 mT in scanning radiocarbon methods to date the glacial deposits and its amplitude. The Ge center was regarded as the dating outwash terrace deposits, loess on the terraces and signal. 中国科技论文在线 http://www.paper.edu.cn

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Table 2

Contents of U, Th, K2O in the samples and the ESR dates

Samples U (ppm) Th (ppm) K2O(%) Water (%) Annual dose (Gy/ka) Accumulated dose (Gy) ESR date (ka) Changgousi till 3.04 18.9 1.73 9.7 3.48 471.10 135.3 Zhonglianggan till 2.92 11.4 2.12 7.5 3.31 1532.20 462.9 Outwash of T5 1.63 8.1 1.68 3.75 2.48 322.78 130.1

The other part of the sample was used to measure the concentrations of U; Th and K: These were measured using a laser ﬂuorescence method, colorimetric spectro- photometry and atomic absorption technique, respectively. Based on the concentrations, their radiation annual doses were calculated using the Bell formula. The results are shown in Table 2. The Zhonglianggan till was dated to 462.9 ka BP, which probably corresponds to MIS12. The Changgousi till and its outwash were dated at 135.3 and 130.1 ka BP, respectively. Furthermore, the basal and the central part of the 18 m thick loess on terrace 5 were dated to 141.7711.4 and 43.773.5 ka BP, respectively, using TL methods (Qiao et al., 1994). This basal date agrees well with the ESR dating results from the Changgousi till Fig. 10. The moraines of Last Glacial in Laolongwan Valley near Menyuan. and its outwash. We therefore believe that the Chang- gousi Glacial Stage occurred during the Penultimate Glacial (MIS6). The bottom of the loess on the outwash terrace 3 is period of stability before being uplifted, but began to dated at 15.671.2 ka BP by TL, showing that the dissect when the mountains began to rise at about outwash gravel of terrace 3 is the product of the last 3.6 Ma BP (Li, 1999). During more than 2 million years glacial maximum (LGM). Its correlative glacial deposit of uplift, Yumen conglomerate rock was deposited. The should be the upper till. Based on this, the reasonable Kunlun–Huanghe movement of Qinghai–Tibetan Pla- conclusion is that the two glacial tills overlying the teau occurred at 1.1–0.6 Ma BP (Li and Fang, 1998), Changgousi were formed during MIS4 and MIS2, greatly changing the geomorphologic processes and respectively. uplifting the Qilian Shan into the cryosphere of that In addition, we collected organic matertial from a time. The highest parts of the mountains probably buried soil sample from the top of terrace 1 (20 m above became glaciated. The Bailang River area attained this the present river level and 1 km upstream of Changgou- height at least 460 ka BP. The fact that the highest si). This was dated to 6920778 years BP by 14C, outwash terrace has been cut down to 250 m above the indicating that the terrace was formed in early present river level indicates that after the Zhonlianggan Holocene. glaciation, the mountains experienced substantial uplift. Strong tectonic movement is also indicated by the active faults that stretch along the northern and southern sides 6. Discussion and conclusion of the mountain range. Along the southern slope of the eastern Qilian Shan, The Zhonglianggan till is the oldest glacial deposit research work was undertaken since the 1990s. The we have found in the Bailang River Basin. Its ESR age older moraine of the Laolongwan valley near Menyuan is signiﬁcant for understanding the geomorphologic County was ascertained as the product of the penulti- evolution and environmental change in the northeastern mate glaciation based on the palaeosol S1 buried in the part of the Qinghai–Tibetan Plateau. It is believed overlying loess on the outwash terrace which was that the Qilian Shan experienced rapid uplift synchro- thought to be contemporaneous with the moraine nous with the Tibetan Plateau, and helped force (Guo et al., 1995). The moraines of the Last Glacial in climatic deterioration, leading to glaciation at least by Laolongwan are very distinguishable (Fig. 10). Particu- 463 ka BP. larly, great attention has been paid to the glaciations An extensive planation surface at 3000–3200 m a.s.l. is in Gangshiga valley (Li and Li, 1991; Kang, 1992; present in the Qilian Shan. This has witnessed a long Rose et al., 1998). The tills in Gangshiga valley were 中国科技论文在线 http://www.paper.edu.cn

S. Zhou et al. / Quaternary International 97–98 (2002) 103–110 109

widespread in the Northern hemisphere during MIS18 and/or 16.

Acknowledgements

The ESR dating, TL dating and 14C dating were undertaken, respectively, in ESR Laboratory of Qing- dao Institute of Marine Geology, China, TL Laboratory of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences; and 14C Laboratory of Lanzhou University. We thank Professors Ye Yujiang and Lu Liangcai for ESR and TL measurements. We thank Lewis A. Owen and Martin P. Kirkbride for their conscientious suggestions. This research was undertaken supported by NSFC projects ‘‘The Environmental changes and its relationship with humans in the Hexi Region’’ (49731010) and ‘‘Study on Quaternary glaciation in Shaluli Shan’’ (49971080).

Fig. 11. The moraine section and its dating in Gangshiga Valley (based on Li and Li, 1991; Kang, 1992). References

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