Late Neogene Loess Deposition in Southern Tarim Basin: Tectonic and Palaeoenvironmental Implications

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Tectonophysics 375 (2003) 49–59 www.elsevier.com/locate/tecto

Late Neogene loess deposition in southern Tarim Basin: tectonic and palaeoenvironmental implications

Hongbo Zhenga,b,*, Chris McA Powellc, Katherine Butcherc, Junji Caob

a Laboratory of Marine Geology, School of Ocean and Earth Science, Tongji University, Shanghai 200092, China b Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710054, China c Tectonics Special Research Centre, School of Earth and Geographical Sciences, The University of Western Australia, Nedlands WA6009, Australia Received 11 February 2002; received in revised form 28 October 2002; accepted 5 June 2003

Abstract

Uplift of the Tibetan Plateau during the late Cainozoic resulted in a thick apron of molassic sediments along the northern piedmonts of the Kunlun and Altyn Mountains in the southern Tarim Basin. Early Neogene sediments are characterised by sandstone, siltstone and red mudstone, representing floodplain to distal alluvial fan environments. The Early Pliocene Artux Formation consists of medium-grained sandstone and sandy mudstone with thin layers of fine pebbly gritstone. The Late Pliocene to Early Pleistocene Xiyu Formation is dominated by pebble to boulder conglomerate typical of alluvial fan debris flow deposits. Sedimentological investigation, together with grain size and chemical analyses of siltstone bands intercalated with sandstone and conglomerate in the Xiyu and Artux Formations, point to an aeolian origin, suggesting desertic conditions in the Tarim Basin by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau inferred from the lithofacies change from fine-grained mudstone and sandstone to coarse clasts. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole. D 2003 Elsevier B.V. All rights reserved.

Keywords: Loess; Pliocene; Pleistocene; Tarim Basin; Taklimakan Desert; Tibetan Plateau

1. Introduction most continuous late Neogene terrestrial palaeoclimatic records on Earth can be observed. Loess was Aeolian dust deposition is widespread over central formed during glacial periods when climates were cold and eastern Asia, and maximum accumulation is cen- and dry. Strong northwesterly winds (winter monsoon) tred in the Chinese Loess Plateau where one of the deflated dust from the deserts of the Asian interior to construct the Loess Plateau in central China. Whereas during interglacial periods when the summer monsoon was stronger, moist southeasterly winds penetrated * Corresponding author. Laboratory of Marine Geology, School more deeply inland, a situation that favoured soil of Ocean and Earth Science, Tongji University, 1239 Siping Road, Shanghai 200092, China. Tel.: +86-21-3501-4114; fax: +86-21- development. The loess–palaeosol sequences thus 6598-8808. preserve an evolutionary history for the East Asian E-mail address: [email protected] (H. Zheng). monsoon regime (An et al., 1990).

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Provenance study of aeolian dust has suggested Presumably, the red dust has also originated from the that the Taklimakan Desert (Fig. 1) is one of the major same source area as the overlying yellow loess but the source areas for the loess deposits in central China provenance and its palaeoclimatic implications are yet (Liu, 1985). The evolution of the Taklimakan Desert to be fully explored. therefore exerts a profound influence on aeolian Uplift of the Tibetan Plateau during the late Cai- deposition in central and East Asia. However, our nozoic resulted in a thick apron of molassic sediments knowledge of the evolutionary history of the Takli- along the northern piedmonts of the Kunlun and Altyn makan Desert is generally sparse. Existing evidence Mountains in the southern Tarim Basin (Zheng et al., suggests that it may have come into existence around 2000). These Neogene clastic sequences not only 2.5 Myr as inferred from the onset of aeolian loess preserve the palaeoenvironmental history of the fore- deposition in the Chinese Loess Plateau, or latest, land basins but also provide valuable information on Miocene as suggested by scattered sedimentological the uplift of the northern Tibetan Plateau. Our previ- investigations of buried aeolian sand sequences in and ous field observations (Zheng et al., 2000) have around the desert (Dong et al., 1991). revealed that the Neogene molassic sequences contain Recent studies of the red clay formation that a substantial amount of siltstone that is aeolian in underlie the loess successions in the Chinese Loess origin (Zheng et al., 2000). In this paper, we focus on Plateau suggest that these red deposits are also aeolian the sedimentological study of these aeolian siltstones in origin. A magnetostratigraphic study of a red clay to further characterise and to evaluate their tectonic profile in northern Gansu province indicates that the and palaeoenvironmental significance in relation to onset of red clay deposition in the Loess Plateau could the evolution of the Taklimakan Desert and the uplift date back to as early as 8 Myr (Qiang et al., 2001). of the northern Tibetan Plateau.

Fig. 1. Location map showing the distribution of aeolian facies in the Tarim Basin (after Li et al., 1995). 中国科技论文在线 http://www.paper.edu.cn

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2. Aeolian facies in the southern Tarim Basin predominant component of loess. These facies are distributed in the hilly piedmont regions of the Kun- Aeolian facies in the southern Tarim Basin are well lun and Altyn, overlying Cainozoic or older strata. developed and have an arc-shaped distribution along They are commonly a few metres thick but can reach the northern piedmonts of the Kunlun and Altyn up to 100 m in places. The sandy loess and loess Mountains (Fig. 1). Three distinct aeolian facies can facies are distributed mostly at elevations between be recognized from the basin centre to the mountain 2000 to 3000 m a.s.l. but can reach as high as 4400– foot (Li et al., 1995). 4600 m where perennial snow and tundra begin. The aeolian loess sequences in the northern fronts 2.1. The desert facies of the Kunlun and Altyn Mountains have attracted attention for years, yet the age of the deposits has The desert facies dominates the Taklamakan Desert remained uncertain. Liu (1965) was the first to study (Tarim Basin). Light brown, red to yellow fine (125– these deposits and he assigned a late Quaternary age 250 Am) sand with minor fine (63–125 Am) sand is (last glacial period) to the aeolian facies in the the predominant component of this facies. The Takla- southern Tarim using stratigraphic correlations. Wen makan Desert is the second largest mobile sand sea in and Qiao (1995) dated the deposits developed on the the world, and its present landforms are characterised terraces of the Keliya River near Yutian by using by various types of sand dunes ranging in height from thermoluminescence methods and suggested that the a few metres up to 30 m. loess along the Kunlun Mountain front was formed during the last 200 kyr. More recently, Li et al. (1998) 2.2. The gobi facies studied the magnetostratigraphy of a 70-m-thick aeolian silt section at Yutian and concluded that aeolian The Gobi belt is the ground surface of the giant sedimentation in the southern Tarim Basin started as modern alluvial fan systems developed in the Kunlun early as 700 kyr. and Altyn foreland basins. This facies also includes The present study has revealed that aeolian loess the lower terraces of the braided fluvial systems deposition in the southern Tarim Basin commenced draining to the basin. The subsurface lithology of much earlier than the Pliocene. These loess deposits the Gobi belt consists of sand and gravel with bands have been preserved as buried intercalated bands in and/or lenses of silt. These silty lenses, ranging from the Neogene molassic sediments of the foreland tens of centimetres up to a metre in thickness, are of basins of the Kunlun and Altyn Mountains at the aeolian origin and are preserved because of burial by northern edge of the Tibetan Plateau. overlying material. Fine-grained material has been deflated, resulting in the formation of lag deposit forming the Gobi 3. Molassic sedimentation in the southern Tarim surface concentrated with gravel and coarse sand from Basin whence the Gobi (Mongolian gobi=stony desert) gets its name. The deflated fine material is transported, Uplift of the Tibetan Plateau in the late Cainozoic sorted and then redeposited in the loess belts further resulted in the formation of an apron of molassic south. Severe deflation is a major erosional process sediment in the foreland basins of the Kunlun Moun- occurring in the Gobi belt. Strong NE and NW winds, tains. These sediments, shed off the northern edge of associated with atmospheric depressions, are a very the Tibetan Plateau, preserve a nearly continuous common weather phenomenon in winter and spring in Neogene and Quaternary sedimentological succes- the southern Tarim area. sion, recording the uplift history of northern Tibet (Zheng et al., 2000) and the palaeoenvironmental 2.3. The sandy loess and loess facies changes in the Tarim Basin. Exploration for oil in the Tarim Basin in recent years provides new infor- The sandy loess facies consists mainly of fine sand mation about its subsurface geology (Zhou and Chen, and coarse silt (16–63 Am), the coarse silt being the 1990). 中国科技论文在线 http://www.paper.edu.cn

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The foreland basins in the southern Tarim contain a subsiding basin in the Miocene after the final regres- minimum of 5 km of Late Cainozoic clastic sediments sion of Peri-Tethys to the west in the Oligocene (Zhou (Fig. 2). Marine limestone is widespread in the Eo- and Chen, 1990). Early Neogene sediments are char- cene. By early Oligocene, marine limestone is restrict- acterised by red-coloured mudstone, siltstone and thin ed to the southwestern corner of the basin (Zhou and sandstone lenses, suggesting a floodplain to distal Chen, 1990). The entire Tarim region became a unified alluvial fan environment. Late Neogene to Pleistocene

Fig. 2. Simplified late Cainozoic stratigraphy of the molasse and overlying aeolian loess deposits in southern Tarim Basin. Thickness of the Xijiang and Wusu Groups is not available. Thickness (in kilometers) of the Xiyu and Artux Formations and Wuqia Group is taken as that of the Yecheng section (Cande and Kent, 1995). 中国科技论文在线 http://www.paper.edu.cn

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sediments are more widely distributed and are charac- poorly sorted, commonly have reversed grading, and terised by coarse sediments with pebbles to boulder- show little evidence of fluvial reworking. The middle sized clasts. part of the Yecheng section is known as the Artux The Yecheng section located at the foot of the Formation in the southern Tarim Basin. western Kunlun (Fig. 1) contains over 4.5 km of late The uppermost part of the section is dominated by Neogene to Pleistocene sediments (Fig. 3A).The pebble to boulder conglomerate typical of alluvial fan lower part of the section contains alternating red debris-flow deposits. The conglomerate is very poorly mudstone and fine-grained pale yellow sandstone. sorted, with clasts ranging from granules to boulders The sandstone was deposited in shallow (generally >2 m in diameter. There is no evidence of fluvial <1 m deep) streams, and as sheet flow deposits, with sorting or reworking of the debris flow conglomerates. intercalated red mudstone representing overbank Some channelling occurs at the base of individual deposits. The inferred depositional environment is layers, and reverse grading is common. This conglom- distal alluvial fan and/or floodplain. Palaeocurrent erate, known as the Xiyu Conglomerate, is widely information suggests distal alluvial fans sloping north- distributed along the margins of Tarim Basin, and has ward with shallow trunk streams draining westward. been assigned ages of either Pliocene (Zhou and Based on regional stratigraphic correlation, the lower Chen, 1990) or early Pleistocene (Li, 1984). The Xiyu part of the section is equivalent to the Miocene Wuqia Conglomerate overlies the Artux Formation conform- Group. ably in the studied section but unconformably in other The middle part of the section consists of medium- places (Zhou and Chen, 1990). grained sandstone and sandy mudstone, with silt beds Recent study of the magnetostratigraphy of the and thin layers of fine gravel. The gravel beds are middle and upper part of the section (the Artux and

Fig. 3. (A) Cross section of the Yecheng profile showing the Miocene red beds, Pliocene–Pleistocene conglomerate and Quaternary aeolian loess cover. (B) Sketch of a section at the lower part of the Xiyu Conglomerate showing aeolian silt bands intercalated with coarse pebble– boulder clasts. 中国科技论文在线 http://www.paper.edu.cn

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Xiyu formations) suggests that the Artux Formation Xiyu Conglomerate has revealed strong similarities was formed during 4.6–3.5 Myr, and the Xiyu Con- with the present-day Gobi deposits that contain gravel glomerate during 3.5 to <1.8 Myr (Zheng et al., 2000). intercalated with aeolian sand and silt. The upper limit of the Xiyu Formation is not clear in The Pliocene Artux Formation also contains a the studied section as it is covered by Quaternary substantial number of siltstone beds ranging from 2 loess. However, K–Ar studies of a basalt unit at the to 10 m thick. These siltstone beds are also internally uppermost part of the Xiyu Conglomerate in Yutian structureless and cemented by carbonate, and are suggested that the upper limit of the formation is intercalated with sandstone, sandy mudstone and grit- about 1.4–1.2 Myr (Liu and Yi, 1990). stone or conglomerate layers. Where the siltstone Tectonic movements during and after the deposi- bands are intercalated with gritstone or conglomerate tion of the Xiyu Conglomerate have deformed the layers, they become more recognizable in the field entire Miocene to early Pleistocene section, resulting and are similar in many ways to those of the Xiyu in all the strata in the Yecheng section dipping Formation. Sedimentological analysis has indicated northward toward the Tarim Basin (Fig. 3A).Dips that the Artux Formation consists of middle to distal of the Miocene and early Pliocene strata in the section alluvial fan or braided stream deposits. The inferred are uniformly around 70–75j, and become progres- slope was gentler than when the Xiyu Conglomerate sively shallower from the upper part of the Artux was deposited. Siltstone beds in the Artux Formation Formation into the Xiyu Conglomerate, where the are interpreted as aeolian deposits. dips at the top of the exposed section are around In the underlying Miocene Wuqia Group, thin beds 20j toward the north. All Miocene, Pliocene and of siltstone have also been observed. These are Pleistocene strata are overlain unconformably by the intercalated with red mudstone and sandstone, and horizontally bedded Wusu Conglomerate (middle are thinner and much less prominent than that in the Pleistocene), which is capped in turn by Quaternary Artux and Xiyu Formations. loess (Xinjiang Group). A feature of the Xiyu Conglomerate of special interest involves conglomeratic debris flows interca- 4. Grain-size analysis of the siltstone in the Xiyu lated with bands or lenses of pale grey to yellowish and Artux Formations siltstone (Fig. 3B). These siltstone bands are internally structureless, well to moderately cemented by carbon- Grain-size analysis is an effective method for ate, and distributed in rather uniform frequency identifying the origin of sediments and the way in throughout the entire 2-km formation (Fig. 3B). The which they are deposited because fluvial and aeolian thicknesses of the bands range from 1 to 10 m, with sediments have their own characteristic grain-size the majority being about 2 m thick. distribution patterns. Samples have been collected We interpret these siltstone bands as aeolian depos- systematically for grain-size analysis from the silt- its coeval with the debris flows for the following stone bands in the Xiyu and Artux Formations. For reasons: (a) Sedimentological analysis indicates that comparison, we also collected Quaternary loess (Xin- the Xiyu Conglomerate is an alluvial fan debris-flow jiang Group) samples from Yecheng and Yutian, deposit typical of those formed under arid to extreme- aeolian dune sand samples from the Taklimakan ly arid climatic conditions. Aeolian deflation and Desert, and fluvial mudstone and sandstone samples deposition are common geological processes in such from the Artux Formation and Wuqia Group. regions. (b) Alluvial fans or debris flows in the Powder samples of 1.5 g were dissolved in 10 ml of Kunlun foreland basins result from seasonal storms 10% hydrogen peroxide (H2O2) solution in a 300-ml or rapid melting of snow from the highlands, process- beaker in order to remove the organic materials. Ten es that are typically intermittent; alluvial and aeolian millilitres of 10% hydrochloric acid (HCl) solution processes can alternate. (c) The intercalated siltstone were added to the beaker and heated to the boil in order bands are internally structureless. There is little evi- to remove all the authigenic carbonate and pedogenic dence of fluvial sorting or reworking. (d) Field obser- clay minerals so that we could analyse the grain size of vation of the sedimentological characteristics of the the original particles transported to the site. While the 中国科技论文在线 http://www.paper.edu.cn

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solution was heating and reacting, it was necessary to single transportation agent. In contrast, grain-size add distilled water to avoid it drying out. The solution distribution curves of samples from the fluvial sand was then diluted with distilled water to a measured beds of the Artux Formation and Wuqia Group (Fig. consistency, and 10 ml of 0.05 N (equivalent concen- 4) all have two peaks, with the smaller one being tration) sodium hexametaphosphate [(NaPO3)6] was centred in the finer fraction. Note the upward-coars- added to disperse the grains. Finally, the beaker con- ening of aeolian samples from Artux and Xiyu taining the solution was vibrated ultrasonically for Formations and the overlying Quaternary loess, indi- about 10 min to keep the solution in a suspended state. cating a general trend of increasing intensity of the After these procedures, the suspension was measured. winter monsoon winds since the Pliocene (cf. Fig. Grain-size analysis has been carried out in the Institute 4A and B). of Earth Environment, Chinese Academy of Sciences at Xi’an by using Mastersizer MS-S laser grain-size analysis system. The grain-size range of the machine is 5. Geochemic characteristics of the siltstone in the 1000–0.2 Am. Xiyu and Artux Formations As shown in Fig. 4, the siltstone samples from the Xiyu and Artux Formation are composed mainly of To document the geochemistry of the siltstone coarse silt, with average grain size ranging from 20 bands in the Xiyu and Artux Formations, we have to 35 Am. When compared with typical loess samples analysed the main elements, trace elements and rare (ZHB 21, ZHB 44), they show strong similarity in earth elements (REE) of these siltstone specimens, terms of their grain-size distribution patterns. All and compared them with samples from the loess samples exhibit distinct narrowly shaped, single-peak profiles (Xinjiang Group) in the southern Tarim Basin distribution curves, indicating good sorting and a and the Loess Plateau of central China (Liu, 1985;

Fig. 4. Grain-size distribution of samples from the siltstone bands of Artux and Xiyu Formations in the southern Tarim Basin (A) and its comparison with that of typical aeolian loess (B) and fluvial sediments (C). 中国科技论文在线 http://www.paper.edu.cn

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Table 1 Main chemical components (%) of the siltstone from Xiyu and Artux Formations and their comparison with those of loess from the Xinjiang Group and the Chinese Loess Plateau

Sample Al2O3 Fe2O3 CaO MgO K2ONa2O Artux Formation (n=7) average 9.64 3.76 7.87 2.11 2.17 2.21 range 8.90–10.70 3.31–4.24 5.82–10.03 1.70–2.80 1.96–2.35 1.51–3.30 Xiyu Formation (n=8) average 8.81 3.47 7.68 2.14 2.22 2.87 range 7.75–9.66 2.39–4.26 4.97–9.47 1.72–2.54 1.51–2.99 1.66–3.81 Xinjiang Group (n=2) average 8.88 3.65 11.14 2.51 2.11 1.71 range 8.84–8.91 3.63–3.68 10.64–11.63 2.37–2.66 2.02–2.20 1.66–1.76 Luochuan (red clay) (n=3) average 11.9 4.80 7.73 2.06 2.08 1.49 range 11.49–12.4 4.46–5.07 6.50–8.86 1.98–2.13 1.88–2.26 1.42–1.60 Xifeng (red clay) (n=62) average 12.66 5.39 10.42 3.10 2.22 0.99 range 9.68–15.91 3.60–6.98 2.23–21.82 1.81–7.20 1.35–3.01 1.66–1.76 Sand (Artux Formation average 8.12 2.37 5.45 0.99 1.67 1.59 and Wuqia Group [n=5]) range 7.31–8.71 1.58–2.95 2.91–9.28 0.69–1.19 1.40–2.09 1.29–1.74

Wen, 1991). For comparison, chemical analysis has Sc, Cs, Hf, Th, U) of the siltstone also show a similar also been carried out on samples taken from the pattern with that of the loess and red clay (Table 2). sandstone in Wuqia Group. Element concentrations The average value of total REE (including Y) contents were determined by using SOLA ICP-MS, with indi- is 135.8Â10À6 for Artux siltstone (sample number, um and rhodium as internal standards. The samples n=7), 166.5Â10À6 for Xiyu siltstone (n=8), 166.4Â À6 À6 were first digested with a HF+HClO4+HNO3 mixed 10 for Tarim loess (n=2), 171Â10 for Luochuan solution. Replicate analysis of standard samples loess (Wen, 1991) and 154Â10À6 for Xifeng red clay. (GSS2) showed that the precision for most of the The chondrite-normalized REE patterns of all the elements is around 10% (RSD%). samples exhibit a generally identical pattern, charac- The contents of main elements (Al2O3,Fe2O3, terised by LREE enrichments with a relatively flat CaO, MgO, K2O, Na2O) of the siltstone in the Xiyu HREE (Fig. 5). A slightly negative Eu anomaly (Eu/ and Artux Formations are comparable with those of Eu*=0.59 for Artux siltstone, 0.61 for Xiyu siltstone, Quaternary loess (Xinjiang Group) and Pliocene red 0.59 for Xinjiang Group loess, 0.714 for Luochuan clay from Luochuan and Xifeng but different from loess and 0.625 for Xifeng red clay) can also be sand samples from the Artux Formation and Wuqia observed in all samples. The geochemical analyses Group (Table 1). The trace element contents (Ti, Mn, demonstrate the similarities of the siltstone bands in

Table 2 Trace element concentrations (Â10À6) of siltstone from Xiyu and Artux Formations and their comparison with those of loess from Xinjiang Group and the Chinese Loess Plateau Sample Ti Mn Sc Cs Hf Th U Artux Formation (n=7) average 2719 542 9.21 6.18 4.64 13.14 2.37 range 2106–3273 439–627 7.28–13.24 4.63–8.02 1.45–9.33 10.20–18.39 1.98–2.82 Xiyu Formation (n=8) average 3156 511 8.58 5.76 4.03 15.11 2.03 range 2143–4378 352–660 5.94–11.19 3.18–9.86 1.80–6.46 7.37–27.60 1.03–3.06 Xinjiang Group (n=2) average 3404 595 8.44 5.74 4.96 13.88 2.77 range 3375–3432 514–675 7.04–9.85 4.57–6.90 2.88–7.04 12.48–15.28 2.31–3.23 Luochuan (red clay) (n=4) average 3660 634 10 7.6 6.05 12.2 3.15 range 1679–4676 310–878 1–30 5.95–9.99 4.38–10.1 10.3–14.3 1.54–4.74 Xifeng (red clay) (n=2) average 3152 654 10.37 7.25 3.65 14.56 1.92 range 2257–3824 291–999 7.60–14.77 4.38–9.42 1.96–5.36 8.91–20.65 1.16–2.48 Sand (Artux Formation average 1674 323 5.92 3.98 3.18 9.05 1.36 and Wuqia Group (n=5) range 1231–2432 209–466 3.35–7.87 2.34–6.29 1.25–5.20 6.67–13.40 0.65–1.97 中国科技论文在线 http://www.paper.edu.cn

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subrounded. The surface of the grains is usually scabrous, with evident mechanical abrasion and im- pact (Fig. 6). Typical V-shaped hollows and parallel scoring are observed. Most of the grains show surface characters that are typical of aeolian processing.

7. Discussion and conclusions

The formation and evolution of the Taklimakan Desert is a regional response to widespread aridifica- tion caused by global changes during the late Caino- zoic. Among other factors, the uplift of the Tibetan Fig. 5. Chondrite-normalized REE patterns of siltstone from the Plateau and induced changes in atmospheric circula- Artux and Xiyu Formations and their comparison with that of loess tion may have provided the major driving forces. The and red clay from the Chinese Loess Plateau. evolutionary history of the Taklimakan Desert and the loess deposits in the surrounding regions have great the Artux and Xiyu Formation, the Quaternary loess potential for improved understanding of palaeocli- from both the Tarim Basin and the Loess Plateau, and matic changes both on regional and global scales, as the Pliocene red clay. well as the uplift history of the Tibetan Plateau. It has been claimed that the aeolian landforms of the Taklimakan Desert were formed during the mid- to 6. Surface characteristics of quartz grains late Quaternary (Zhu, 1981). Dong et al. (1991) studied aeolian sand sections buried in the Neogene Twenty-three samples were selected for surface sediments in the Taklimakan Desert and surrounding examination in an environmental scanning electronic regions and suggested that the desert may have been microscope (ESEM). The samples were pretreated in existence since the Pliocene or Latest Miocene, i.e., before coating. The <2 Am size fraction was sieved much earlier than previously thought. There is little away, and the samples were then separated into two doubt that buried aeolian sand provides strong evi- size fractions: coarser and finer than 30 Am. All dence of the existence of former arid landscapes. samples were ultrasonically cleaned and oven-dried However, as buried aeolian sand has been found only at temperatures below 40 jC. in a few scattered sites, it is far from conclusive as to The quartz grains are generally angular to suban- whether it represents relics of a relatively small-scale gular (over 90%), with a minor proportion being aeolian landscape, or those of a desert as vast in the

Fig. 6. SEM images of quartz grains from the Xiyu (a) and Artux (b) Formations. 中国科技论文在线 http://www.paper.edu.cn

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Late Miocene–Pliocene as the Taklimakan is today. climatic deterioration’’ in the Northern Hemisphere Because of strong deflation and reworking, geological that culminated in the Plio–Pleistocene ice age. evidence of an ancient desert is less likely to be found Our reconnaissance of late Cainozoic palaeoenvir- within the desert itself than in the surrounding regions onmental data from the Tibetan Plateau and the sur- with depositional sequences. rounding regions indicates that Tibetan uplift affected Sedimentological analysis of the Yecheng section palaeoclimates, especially that of the Tarim region, in provides valuable information about the provenance two ways. First, the high Tibetan Plateau stands as a of the strata, and consequently about the uplift history physical barrier impeding the incursion of the Indian of the northern edge of the Tibetan Plateau. Pliocene Ocean monsoonal moisture, and so imposes arid con- lithofacies of the Artux Formation are dominated by ditions across much of the plateau and the region north pale yellow medium- to fine-grained sand units. The of it. Second, a vast elevated region like the Tibetan pebbles are small, round to subrounded and appear in Plateau acts as a thermal amplifier. It enhances the thin (decimetres up to 1 m) clast-supported beds. Mongolian high-pressure system and thus strengthens Common pebble size types are chert, dark indurated the Asian winter monsoon. A stronger winter monsoon siltstone and greywacke, with some sheared mafic not only intensifies the aridity of the region but volcanic fragments and purple to red siltstone, inter- provides the northerly winds for entrainment, trans- preted as reworked Mesozoic to early Cainozoic portation and deposition of aeolian dust. sediments. This contrasts with the overlying Xiyu Formation, which contains massive clast-supported cobble–boulder units dominated (>70%) by rock Acknowledgements fragments from the sedimentary cover of the Kunlun such as chert, silicious limestone, marble and quartz- Prof. Christopher Powell died tragically on a field mica schist, with subordinate amounts (<30%) of trip while the manuscript was in the final stages of quartzo-feldspathic gneiss, amphibolite, granite and preparation. HZ thanks him for his support as a plutonic rocks from the deeper levels of the Kunlun. supervisor, a collaborator and a dear friend. This The proportion of metamorphic and plutonic rock paper is dedicated by all the authors to Prof. Powell, fragments increases upwards with sorting decreasing for the best way of remembering him is to continue in the same sense. the work he started. The work was financially The Yecheng section provides evidence of an supported by grants NSFC (40025107), NKBRSF abrupt change in the depositional gradient between Project (G2000078501), the Chinese Academy of 4.5 and 3.5 Myr (Zheng et al., 2000). Before then, Sciences through its ‘‘100-talents Program’’ and the deposition of red beds occurred in distal alluvial fan Ministry of Education of China through the ‘‘Chang- and/or flood plain environments. This was replaced by jiang Program’’. Prof. John Dodson kindly read an debris flow accumulation after 3.5 Myr. This abrupt early version of the manuscript. The authors also change is a direct result of the uplift of the northern thank Prof. Jim Bowler and Prof. Edward Derbyshire edge of the Tibetan Plateau. for reviewing the paper. The onset of aeolian sedimentation in the southern Tarim Basin coincided with the lithofacies change from fine-grained mudstone and sandstone to coarse References clasts. We interpreted the onset of full arid conditions in the Tarim region as a result of the reorganization of An, Z., Liu, T., Lu, Y., Porter, S.C., Kukla, G., Wu, X., Hua, Y., the general atmospheric circulation induced by uplift 1990. The long-term paleomonsoon variation recorded by the of the Tibetan Plateau. The forcing of the late Caino- loess–paleosol sequence in central China. Quaternary Interna- zoic climate by plateau uplift is yet to be fully tional 7/8, 91–95. explored. General Circulation Model sensitivity tests Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomag- netic polarity timescale for the Late Cretaceous and Cenozoic. run by Ruddiman and Kutzbach (1989) have, howev- Journal of Geophysical Research 100, 6093–6095. er, suggested that the Tibetan Plateau indeed played Dong, G., Chen, H., Jing, J., Wang, Y., 1991. Cenozoic paleo-eolian an important role in inducing the ‘‘late Cainozoic sands in the south marginal area of the Taklimakan Desert. 中国科技论文在线 http://www.paper.edu.cn

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Glaciers and Environments of West China. Science Press, Bei- monsoon, and Pliocene uplift of northern Tibet. Earth and Plan- jing, pp. 42–48. etary Science Letters 187, 83–93. Li, Y.T. (Ed.), 1984. Stratigraphy of China: 13. Tertiary. Geology Ruddiman, W.F., Kutzbach, J.E., 1989. Forcing of late Cenozoic Press, Beijing. 325 pp. Northern Hemisphere climate by plateau uplift in southern Asia Li, B., Dong, G., Zhang, J., Li, S., Jin, H., Chen, H., Wen, X., and the American west. Journal of Geophysical Research 94 Wang, Y., Zhu, Y., 1995. Aeolian facies distribution in and to (D15), 18409–18472. the south of the Taklimakan Desert. Acta Geologica Sinica 69 Wen, Q., 1991. Study of the Geochemistry of Loess in China. (1), 55–63. Science Press, Beijing, pp. 69–109. Li, B., Li, S., Wang, Y., Dong, G., Jin, H., Yan, M., Gao, Q., Wang, Wen, Q., Qiao, Y., 1995. Palaeoclimatic records of late Pleistocene G., 1998. Geological age of the sand and dust deposits of the sediments in the northern Kunlun. In: Tibetan Plateau Research Aqiang section in the extremely arid region of China. Acta Geo- Association (Ed.), Proceedings of the Tibetan Plateau and Glob- logica Sinica 72 (1), 83–92. al Change. Meterology Press, Beijing, pp. 96–104. Liu, T., 1965. Loess Deposits in China. Science Press, Beijing. Zheng, H., Powell, C.McA., An, Z., Zhou, J., Dong, G., 2000. 115 pp. Pliocene uplift of the northern Tibetan Plateau. Geology 28, Liu, T., 1985. Loess and the Environment. Science Press, Beijing. 715–718. 481 pp. Zhou, Z.Y., Chen, P.J. (Eds.), 1990. Biostratigraphy and Geological Liu, J., Yi, M., 1990. Quaternary volcanics in the western Kunlun Evolution of Tarim Basin. Science Press, Beijing. 439 pp. and their K–Ar ages. Science in China, B 20 (2), 180–187. Zhu, Z., 1981. The Aeolian Landforms of the Taklimakan Desert. Qiang, X.K., Li, Z.X., Powell, C., Zheng, H., 2001. Magnetostrati- Science Press, Beijing. 350 pp. graphic record of the Late Miocene onset of the East Asian