Late Pleistocene Sedimentary History of Multiple Glacially Dammed Lake Episodes Along the Yarlung-Tsangpo River, Southeast Tibet
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Quaternary Research 82 (2014) 430–440 Contents lists available at ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres Late Pleistocene sedimentary history of multiple glacially dammed lake episodes along the Yarlung-Tsangpo river, southeast Tibet Shao-Yi Huang a,⁎, Yue-Gau Chen a, George S. Burr b, Manoj K. Jaiswal a,c,YunungNinaLina,d, Gongming Yin e, Jingwei Liu f, Shujun Zhao g, Zhongquan Cao h a Department of Geosciences, National Taiwan University, Taipei 106, Taiwan b Physics Department, University of Arizona, AZ 85721-0081, USA c Department of Earth Sciences, Indian Institute of Science Education and Research—Kolkata, 741252, India d Division of Geological and Planetary Sciences, California Institute of Technology, CA 91125, USA e State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China f Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China g Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029, China h Seismological Bureau of Tibet Autonomous Region, Lhasa, China article info abstract Article history: We present a reconstructed lithologic column compiled from a series of lacustrine outcrops along a tributary of Received 29 April 2013 the Nyang River, a major tributary of the Yarlung-Tsangpo in southeast Tibet. The deposits were preserved be- Available online 16 July 2014 tween terraces at altitudes of 2950–3100 m asl. The stratigraphic record features at least two sets of coarsening-upward sequences depicting episodic aggradation and progradation of a glacially dammed lake relat- Keywords: ed delta. Recognized facies changes illustrate the evolution cycles of depositional environments from pro-delta, Glacier-dammed lake delta front, to delta plain. Radiocarbon and optically stimulated luminescence dates reveal an aging-downward Yarlung-Tsangpo Zelunglung Glacier trend in stratigraphic order and provide an approximate timeline for the formation of glacially dammed lakes Tibet in late Pleistocene. This result reflects that the Zelunglung Glacier had progressively advanced to block the Yarlung-Tsangpo river and the dam materials had stepwise stacked up to an altitude of 3095 m asl during Marine Oxygen Isotope Stages 4 to 2. © 2014 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction Naturally dammed lakes are well studied worldwide (Costa and Schuster, 1988). They can cause two types of potential hazards to The Tibetan plateau features spectacular surface expressions on the humans: in the upstream area the blockage of a river would cause inun- high plateau and its margin (Zeitler et al., 2001). The rough terrains of dation where the reservoir will be filled with the subsequent the frozen peaks and steep gorges bring up numerous potential hazards impounded water and sediments, while in the downstream area the such as land/snow avalanche, moraine/glacially dammed lakes, and out- outburst flooding resulting from the failure of the dam would be the burst floods resulted from varied causes in the Tibet/Himalaya area most severe threat to be faced. Examples include historical failures of (Richardson and Reynolds, 2000). The eastward-flowing Yarlung- Himalayan moraine-dammed lakes in the Himalayas and the well- Tsangpo river flanks the southern margin of the Tibetan plateau and known Lake Missoula floods (e.g. Baker and Bunker, 1985; Richardson drops more than 2500 m over a horizontal distance of 300 km, creating and Reynolds, 2000). a deeply cut valley, merely 35 to 200 m wide. In recent years, the devas- Previous researchers have reported the occurrence of lacustrine tating floods released from the upstream of the gorge due to the block- sediments preserved in the terraces along the middle reach of the age of the Yarlung-Tsangpo have raised public concerns. Various studies Yarlung-Tsangpo, upstream of its gorge through the Himalayas have suggested that although the erosion rates on the plateau are in (Zhang, 1998; Montgomery et al., 2004). The origin of lake deposits in general low, focused denudation on the margins of the plateau appears this area has been variably interpreted, as a result of Pliocene normal- to be intense; a catastrophic outburst of impeded water may induce sig- faulting-related graben deposition (Zhang, 1998), or as the evidence nificant impacts on the morphology of the plateau margins (Fielding, of glacier-dammed lake events (Montgomery et al., 2004; Liu et al., 1996; Lal et al., 2003; Larsen and Montgomery, 2012; Lang et al, 2013). 2006). Montgomery et al. (2004) surveyed fluvial and lacustrine ter- races along the Yarlung-Tsangpo and Nyang rivers and suggested that the breakout of glacial dams could have resulted in a series of ⁎ Corresponding author at: No. 1 Section 4, Roosevelt Road, Department of Geosciences, National Taiwan University, Taipei 106, Taiwan. megafloods. They reported terraces recording paleolakes at two eleva- E-mail address: [email protected] (S.-Y. Huang). tion ranges and based on the areal extent of the paleolakes estimated http://dx.doi.org/10.1016/j.yqres.2014.06.001 0033-5894/© 2014 University of Washington. Published by Elsevier Inc. All rights reserved. S.-Y. Huang et al. / Quaternary Research 82 (2014) 430–440 431 the magnitude of possible outburst floods at up to 106 Wm−2, as the that vary from pro-delta, delta front, to delta plain and therefore help impounded water was released through the narrow Yarlung-Tsangpo to estimate the lake level (the facies model is explained in Supplemen- gorge. Korup and Montgomery (2008) later proposed that glacial dam- tary Fig. 2). ming could significantly induce river incision upstream and stabilize the position of the Tsangpo gorge and contribute to the rapid exhumation Results along the southeast margin of the Tibetan plateau. The reported timing and numbers of paleolakes are diverse: e.g., Observed profiles Montgomery et al. (2004) and Liu et al. (2006) both proposed Holocene ages for paleolakes (1.1–1.74 ka for the lower paleolake around 3100 m The column starts from the bottom of the lowest observed outcrop asl and 9.8–11.4 ka for the higher one around 3500 m asl); Zhu et al. (profile A1), ca. 2950 m asl. The A1 column (Fig. 2) begins with fine (2013) proposed three stages of dammed lakes: the early stage of the to medium sand beds and some peaty layers, occasionally alternating last glaciation (75–40 ka), the local last glacial maximum (12–13 ka), with few coarse grained cross-beds and lenticular beds (Fig. 3a). The and the Neoglaciation (1.8–1.2 ka) from the lake sediments ranging A2 column is adjacent to A1 and records overlapped sections (Figs. 3a, from 2900 to 3800 m asl. These studies lead to a series of questions: b1), the stratigraphy of the two columns can be traced with distinctive What would be the most likely origin of the paleolakes: monsoon- sandy layers (e.g., Fig. 3b2) and the relevant elevation of the two profiles driven valley glaciers, tectonic structures, regional slope failures, or can be confirmed with the laser range finder and portable GPS in other alternatives? How many lakes have there been during Quaternary the field. time? How long were they present and were there many breakout Moving upward to profiles B to D, the texture gradually changes floods over the life span of a lake? To answer these questions, we from laminated fine material (Fig. 3c, top of profile A2) to distinctive need a comprehensive picture of the stratigraphic framework for the in- rhythmic silty sections (Fig. 3d, bottom of profile B and Fig. 3e, rhythmic ferred paleolakes. occurrence in profile D). Column B is to the south of profiles A1 and A2; In this study, we present a reconstructed sediment column based on the relative altitude is determined by the laser range finder. Columns C a detailed field survey that records the evolution of a delta related to an and D are continuous outcrops above the column B. The relative altitude impounded lake from the Nyang River tributary. The archived stratigra- is determined by the GPS reading and the laser range finder. This phy provides good constraints on the timing, and fluctuations of the rhythmic section is about 25 m in thickness with abundant fine lamina- inferred paleolakes and further allows us to establish a depositional tions, convoluted structures, and some sub-angular to sub-rounded framework of the regional dammed lakes. dropstones. This rhythmic section preserves well developed varve layers and the cyclic of dark/light couplets changes through time (small- Strategies and observations er in the lower section and larger in the upper section), varying from 0.1 to 2 cm in width. Our main study area, Linzi, is located in the tributary drainage of Above the rhythmic section, the percentage of sandy components Nyang River, upstream from the confluence of the Nyang River and increase gradually and the texture transitions to medium-coarse sand the main course of the Yarlung-Tsangpo (Figs. 1a, b). The tributary interbedded with thick mud layers within the D profile (Fig. 3f). Coarse empties into the Nyang River 17 km upstream of the confluence and materials follow immediately with abundant cross-beddings, ripples, forms a well defined, fan-shaped delta with outcrops scattered among scour and fill marks, and lenticular gravel beds (Fig. 3g). Imbricated the terraces that are easily identified from digital elevation model gravels, trough cross-bedding and distinctive multiple erosional sur- (DEM) maps (Fig. 1c). Lacustrine strata were exposed at several levels faces mingled with mud blocks strongly indicate a high energy deposi- within the terrace outcrops, interbedded with alluvial deposits. Nine tional environment in this section. columns of the studied Linzi outcrops were thoroughly documented in From the upper section of profile D to profile F, the column contains terms of lithology, texture, and sedimentary structures (Fig.