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Pseudo-feathery dunes in the Kumtagh Desert
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Geomorphology 100 (2008) 328 – 334 www.elsevier.com/locate/geomorph
Pseudo-feathery dunes in the Kumtagh Desert Zhibao Dong ⁎, Jianjun Qu, Xunming Wang, Guangqiang Qian, Wanyin Luo, Zhenhai Wei Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, China Received 9 August 2007; received in revised form 30 December 2007; accepted 2 January 2008 Available online 16 January 2008
Abstract The Kumtagh Desert is the last explored desert in China. The unique patterns on aerial photographs and satellite images have led previous researchers to the conclusion that the Kumtagh Desert is the only place over the world where typical feathery dunes are developed, and that some unique wind regimes are at work. Recent field investigation reveals that the feathery patterns in fact reflect the albedo contrasts between the seif dunes, bright dune-like drifts and the dark inter-dune lag sediments. The wind regime in the area containing the so-called feathery dunes is typical of the regime that produces seif dunes that were considered to be shafts of the feathery dunes, and the vanes of the feathery dunes are in fact bright dune-like drifts with indistinct height from the surroundings rather than true transverse dunes. Further analysis indicates that the sediments of seif dunes, dune-like drifts and dark inter-dune flats differ in grain size distribution, mineralogy, reflectance spectrum and particle micro-morphology. Sediments that constitute seif dunes and dune-like drifts are finer and poorly rounded, contain relatively more quartz, and hence have higher albedo, while the sediments that constitute the dark inter-dune flats are coarser and better rounded, contain far less quartz but relatively more andesite and dacite, and hence have lower albedo. As a result, what was originally perceived as unique feathery dunes are in fact only pseudofeathery dunes composed primarily of seif dunes. This discovery also demonstrates that blowing sand drifts can form dune-like patterns that remain visible for long periods of time. It is important to distinguish between the patterns of true dunes and those of dune-like drifts (pseudodunes) that cause albedo contrasts in interpreting remote sensing images. © 2008 Elsevier B.V. All rights reserved. Keywords: Aeolian processes; Dune geomorphology; Feathery dunes
1. Introduction Wind–bedform interactions that result in various dune forms are an important branch of aeolian geomorphology. Exploration of the diverse dune fields that bear rich information on wind– bedform interactions on Earth and other planets has been of continuing significance because of the insights it provides into the universality of geomorphological processes (Bagnold, 1941; Greeley and Iversen, 1985; Anderson, 1996; Lorenz et al., 2006). The Kumtagh Desert is the last desert to be explored in China, and the discovery of unique feather-like veined patterns in aerial photographs and satellite images of the northern part of ⁎ Corresponding author. Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 260, West Donggang Road, Lanzhou, Gansu 730000, China. Tel.: +86 931 8271167; fax: +86 931 8277169. E-mail address: [email protected] (Z. Dong). 0169-555X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.01.004
this desert have led previous researchers to conclude that some unique process is at work to produce the feathery dunes responsible for these patterns. The “shafts” of the feathers have been interpreted as longitudinal dunes, whereas the vanes have been interpreted as transverse dunes (Zhu et al., 1980). For several decades, the Kumtagh Desert has been believed to be the only place in the world where such typical feathery dunes developed (Wang et al., 2005). However, it is unusual for longitudinal and transverse dunes to appear under the same wind regime because they are formed by very different dynamical mechanisms (Cook et al., 1993; Livingstone and Warren, 1996; Lancaster, 2006). The harsh environments of the Kumtagh have prohibited researchers from in-depth work except some geological and morphological interpretations on aerial photographs and satellite images, although an old Silk Route passed close to the northern edge and several scientific explorers set their feet there as early as 1901 (Zhu et al., 1980; Qu et al., 2005). Some
Z. Dong et al. / Geomorphology 100 (2008) 328–334
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Fig. 1. Satellite image of the main body of the Kumtagh Desert. Pseudo-feathery dunes have developed in the north, but dune types become more complex in the south as a result of the complex wind regimes and the alluvial processes that are sculpting the underlying relief, which is affected by the presence of the huge Altyn Tagh Mountains which rise more than 1500 m above the desert floor.
speculations have been proposed on the formation of the socalled feathery dunes. Xia (1987) proposed that the conditions favoring feathery dunes are broad field, abundant sediment supply, and unidirectional wind regime. He speculated that the roll-vortex flows were responsible for the formation of longitudinal dunes (shafts of feathery dunes), but they could only transport a fraction of the rich inter-dune sediments to the longitudinal dunes, as a result some of the inter-dune sediments had to be left behind to form long narrow mounds (vanes of feathery dunes) roughly perpendicular to the longitudinal dunes. Qu et al. (2005) speculated that the feathery dunes were controlled by the NE–SW aligned fault structures to the north of the Altyn Tagh Mountains. Other researchers (G. Yang's personal communication, 1973) thought the feathery dunes were controlled by the underlying relief. The shafts of feathery dunes were then formed by aeolian sediments covering old yardangs that were aligned in the NE–SW direction. None of the previously proposed speculations can provide convincing evidence. Researchers from the Gansu Institute of Desert Research made the first scientific investigation into the Kumtagh Desert in 2004, primarily on the desert vegetation. We have had the opportunities to get into the Kumtagh Desert four times since 2005. We have made field investigation on the sediments, morphology of the so-called feathery dunes and set up a wind tower to get more understanding of the geomorphology of the desert. The present paper summarizes some preliminary results obtained from field survey, interpretation of remote sensing images and analysis of sediments. It focuses on a basic issue: are there unique processes at work to produce the so-called feathery dunes in the Kumtagh Desert?
2. Physiographical setting The Kumtagh Desert is located in the eastern Tarim Basin and to the north of the high Altyn Tagh Mountains and south of Kezile Tagh Mountain, between 89°57.8′E and 94°54.1′E longitude, and between 39°7.8′N and 41°0.1′N latitude, covering an area of 22 000 km2. The desert is separated from the Taklimakan Desert by the Lop Depression. Various types of aeolian dunes combine to give the Kumtagh Desert the general appearance of a huge whiskbroom overlying the SW–NEinclined alluvial–diluvial plain north of the Altyn Tagh Mountains (Fig. 1). Hence, sediments of the Kumtagh Desert mainly come from the Altyn Tagh Mountains. The annual precipitation is less than 30 mm. The so-called feathery dunes have developed in the north, near the handle of the whiskbroom, between 91°24.8′E and 92°48.4′E longitude, and 40°1.2′N and
Fig. 2. Sand-driving wind rose in the northern Kumtagh Desert (Qu et al., 2007).
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Fig. 3. Aerial photograph of an area of the feather-like veined patterns of pseudo-feathery dunes in the northern Kumtagh Desert in 1971 (central point: 40°14′24.12″N, 92°26′24.53″E).
40°27.8′N latitude, covering an area of 4000 km2 (about 18% of the total area of the desert). Wind records from the wind tower set up in 2004 in the northern edge of Kumtagh Desert indicate that the sand-driving wind regime (≥ 5 m s− 1, the threshold wind velocity) measured at 2 m above the surface is acutely bimodal according to the wind environment classification of Fryberger (1979). The resultant sand-driving direction is towards SW205° (Fig. 2) (Qu et al., 2007). There are two dominant groups of winds: one group includes the NNE, NE and ENE winds, accounting for about 45% of winds, and the other group includes the ESE and E winds, accounting for 23%. Such a wind regime is commonly observed where seif dunes form. It is speculated that the wind regime becomes more complicated in the south due to the effects of the Altyn Tagh Mountains on local air circulation; this structure rises more than 1500 m above the Kumtagh Desert (Li, 2003). Therefore, the dune types are more complex in the south. 3. Pseudo-feathery dunes and their formation Our recent field investigations reveal that the unique featherlike patterns seen in aerial photographs and satellite images (Figs. 3–5) are composed of bright seif dunes with sinuous ridges and occasional slip-faces that comprise the shafts of the
Fig. 4. Quickbird satellite image taken in 2005 with a resolution of 0.6 m at the same location as the aerial photograph in Fig. 3.
Fig. 5. A low-angle aerial photograph taken in 2006 at about 150 m above the ground (location: 40°12′40.62″N, 92°06′25.98″E, looking east), showing clearly that the bright dune-like drifts have an albedo that contrasts strongly with the surrounding dark lag deposits, but their height above the surrounding area is indistinct.
feathers and bright dune-like sand drifts that comprise the vanes of the feathers and that lie between the seif dunes. The so-called feathery dunes are in fact pseudo-feathery dunes. Morphometry of the seif dunes and dune-like drifts was analyzed on aerial photographs taken in 1971 and Quickbird satellite images of 2005. We selected five sample plots (A, B, C, D and E in Fig. 1) to analyze the height, width, spacing and alignment of seif dunes. There are about 30 lines of seif dunes in the pseudofeathery dune area in the northern Kumtagh Desert. They are aligned from 55° to 235° (with 0° as north), generally extending over several or several tens of kilometers. They are 8 to 21 m tall (mostly 10 to 17 m) (Fig. 6), 40 to 120 m wide, and are spaced 100 to 1000 m (mostly 100 to 400 m apart) (Fig. 7). The height and spacing of the seif dunes are poorly correlated (Fig. 8), especially in plots C, D and E. The height of the seif dunes tends to decrease from the northeast (Plot A) to the southwest (Plot E). Ridges of the seif dunes are sinuous with relatively regularly spaced “teardrops” (Fig. 5). The teardrops are actually the remains of barchan dunes with slip-faces on the northwestern side and are oriented roughly in the same direction as the seif
Fig. 6. Probability distribution of height of the seif dunes.
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dunes. This suggests that the seif dunes have evolved from a line of barchan dunes. The formation of seif dunes in the northern Kumtagh Desert can be explained by the evolution model proposed by Bagnold (1941). The most frequent (NE) wind (the prevailing gentle wind, g) forms barchan dunes with southeastern and northwestern horns, the strongest but less frequent (E, SSE) winds (sand-bearing storm winds, s) elongate the southeastern horns (the windward wings) to form seif dunes, resulting in the alignment of the seif dunes from 55° to 235° (with 0° as north), oblique to both the NE and the E winds. The NNE wind tends to elongate the northwestern horns of the barchan dunes, but its action is limited compared with that of the E winds, resulting in the clear appearance of barchan horns on the northwestern side of the seif dunes (Fig. 5). The bright dune-like drifts between the seif dunes are transverse to the prevailing wind, and perpendicular to the bright seif dunes. They are analogous to the bright dune-like drifts that have been observed on Mars (Thomas et al., 1999), which are relatively regularly spaced at 80 to 350 m apart (mostly 80 to 220 m) (Fig. 9), and the dune-like drifts can be very easily interpreted as transverse dunes based on the veined patterns observed on aerial photographs and satellite images (Figs. 3 and 4). However, almost all of the bright dune-like drifts in the pseudo-feathery dune area are convex facing downwind (the prevailing wind is towards SW205°), contrary to the barchan dunes which are concave facing downwind. In addition, the wind regime typical of areas in which seif dunes form cannot foster the formation of transverse dunes (Fryberger, 1979). Field investigations have shown that the height of the bright dune-like drifts above their surroundings is indistinct, and that these structures arise at the leeward slope of undulating surfaces in some places (Fig. 5). Their transverse dune-like patterns on aerial photos and satellite images thus result from contrasts in albedos rather than the strong differences in elevation that characterize true dunes. The dune-like drifts are often composed of small patches of drifted sand with an extent on the order of meters and spaced at 15 to 50 m (Figs. 4 and 5). The albedo contrasts must be formed by differences in the constituting materials. For example, the bright dunes and dune-
Fig. 7. Probability distribution of spacing of the seif dunes.
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Fig. 8. The relationship between the spacing and height of the seif dunes (The sample plots are marked in Fig. 1).
like drifts observed on Mars have been interpreted as being composed of soft materials, possibly high in sulphates (Thomas et al., 1999). Sediments on 15 sets of seif dunes, bright dunelike drifts and dark inter-dune flats in the northern Kumtagh Desert were sampled to analyze their grain size distribution, mineral composition, reflectance spectrum and micro-morphology of sediment particles to obtain more information on the formation of feather-like patterns. The samples collected at similar geomorphological locations (seif dunes, bright dune-like drifts and dark inter-dune flats) were mixed before analysis to obtain their mean values. The grain size distribution was analyzed by dry sieve method in the Key Laboratory of Desert and Desertification, Chinese Academy of Sciences. Mineralogical analysis was conducted by the Mineralogy Analysis Center of Gansu Central Laboratory. The reflectance spectrum of the sediment samples was measured by a 7ISW15 spectrometer provided by the Beijing 7-Star Optical Instrument Co. Ltd. Micro-morphology of sediment particles was analyzed by an EPM-810Q electronic probe made by Shimadzu Corporation. It is revealed that the seif dunes and bright dune-like drifts between seif dunes that comprise the pseudo-feathery dunes in the Kumtagh Desert are composed of finer materials blown out
Fig. 9. Probability distribution of spacing of the bright dune-like drifts.
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of the inter-dune sediments, which have a greater percentage of minerals with high albedos such as quartz (Fig. 11). Consequently, the inter-dune flats are composed of coarse lag deposits with a higher percentage of minerals with low albedos such as dacite. The mean diameter of the dark surface sediments in the inter-dune flats is greater than 1.0 mm, whereas that of the bright seif dunes, bright dune-like drifts, and subsurface sediments in the inter-dune flats is less than 0.4 mm (Fig. 10). The coarser the sediments, the lower the content of minerals with high albedos such as quartz, plagioclase, and potassium feldspar, and the greater the dacite content with low albedos (Fig. 11). Consequently, the coarser sediments of the inter-dune surface have lower albedos (0.16) that are close to those of the dark aeolian dunes on Mars (Edgett and Parker, 1998), whereas those of the bright seif dunes and sand-like drifts in the interdune flats have higher albedos (0.31) (Fig. 12) that are close to those of the bright dunes on Mars (Thomas et al., 1999). Fig. 13 shows the micro-morphology of sediment particles photographed by an electronic probe. The pockmark-like structures and dishing pits on sediment particles in the pseudo-feathery dune area are typical properties of aeolian sediments. The particles are rounded to different extent, but the dark coarser sediment particles are better rounded, suggesting that they are more resistant to weathering and wind abrasion than the bright finer sediments. This explains why the bright sediments are sorted out of the dark inter-dune sediments to form seif dunes and bright dune-like drifts. While the shafts of the feather-like patterns can be successfully explained by the Bagnold's development model for seif dunes, it is also essential to explain the formation of the bright dune-like drifts. Retention of the vane patterns between seif dunes revealed by albedo contrasts over time requires transport and deposition of bright fine sediments over the dark surface of coarse sediments. This can be attributed to saltationinitiated activity. The sediments that form the bright seif dunes and the dune-like drifts belong to the saltation category. They have a propensity for self-accumulation into sand dunes in the absence of obstacles because saltating grains bounce off a hard desert surface more effectively than they do over a bed of loose
Fig. 10. The grain size distribution of the sediments in the area with pseudofeathery dunes.
Fig. 11. The mineral composition of the sediments in the area with pseudofeathery dunes.
sand (Bagnold, 1941). The sand transport rate over the bright patches of fine sediments is, therefore, lower than that over the surrounding dark coarse sediments, leading to the development of bright drifts and cleaning of the dark surfaces. However, the bright dune-like drifts in the inter-dune flats have not developed into transverse dunes, such as barchan dunes. This is because the coarse surfaces of the inter-dune flats produce insufficient fine sediments to cause the bright patches to evolve into dunes in the inter-dune flats. Field investigations indicated that the dark areas in the inter-seif dune corridors were generally covered by lag sediments with mean diameters greater than 1 mm that are usually non-erodible (Chepil, 1952) although there were abundant erodible sediments under the dark lag cover. The erodible fractions of the surface sediments have been transported to form seif dunes. Coarse sand ripples (or fine gravel ridges) with a wavelength of 1 to 2 m and a height of 0.1 to 0.3 m occasionally appear in the dark areas. How the original dune-like patches were initiated is not yet well understood. The convex-downwind patterns suggest that these patches depend
Fig. 12. The reflectance spectrum of the sediments in the area with pseudofeathery dunes.
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Fig. 13. Micro-morphological features of sand particles in the Kumtagh Desert. (a) d N 1.00 mm, (b) d = 1.00 mm–0.50 mm, and (c) 0.50 mm–0.25 mm. The finer the sediments, the less rounded and brighter they are.
on wind action, which is strongest towards the middle of the inter-dune flats. Thus, the patches might have formed during a recent period of stronger-than-normal winds, when the bright fine sediments were sorted out and drifted over the inter-dune flats like waves under the influence of wind turbulence. Dunelike drifts were then left behind on the surface when the winds abated. Another explanation is that the bright fine sediments constituting the dune-like drifts in the inter-seif dune corridors are blown from the seif dunes by those winds that are oblique to the seif dunes. At the same time, some of the dune-like drifts were blown onto the seif dunes. This paper only presents some preliminary results. There are various types of dunes in the Kumtagh Desert besides the seif dunes. The spatial variations in dune geomorphology must be related to the variations in wind regime. However, as a part of the Qinghai-Tibetan Plateau, the Altyn Tagh Mountains have been experiencing rapid uplift since the early Quaternary (Li et al., 1979), leading to the complex geological structures in the Kumtagh Desert. Intermittent rivers originating from the Altyn Tagh Mountains have formed many wadis in the desert. To fully understand the seif dunes in the northern Kumtagh Desert requires understanding the variations in wind regime, geological structures and alluvial processes in the whole desert. This will be our future work. 4. Conclusions Recent field investigation reveals that what was originally perceived as unique feathery dunes in the Kumtagh Desert of China are in fact only pseudo-feathery dunes composed primarily of seif dunes. The feathery patterns in the northern Kumtagh Desert reflect the albedo contrasts between seif dunes, dune-like drifts and the coarse lag deposits in the inter-seif dune corridors. What we have learned from the pseudo-feathery dunes of the Kumtagh Desert is that occasional strong winds may form dune-like drifts that remain in place over long periods of time, with patterns that are easily interpreted as dunes on remote sensing images. Existence of dune-like drifts indicates an inadequate supply of sediments that can be transported by wind.
Acknowledgments We gratefully acknowledge the funding received from the Natural Science Foundation of China (40638038), and the Ministry of Science and Technology of China. References Anderson, R.S., 1996. Attraction of sand dunes. Nature 379, 24–25. Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. William Morrow & Company, New York. Chepil, W.S., 1952. Dynamics of wind erosion: initiation of soil movement by wind I. Soil structure. Soil Science 75, 473–483. Cook, R.U., Warren, A., Goudie, A., 1993. Desert Geomorphology. UCL Press Limited, London. Edgett, K.S., Parker, T.J., 1998. Bright aeolian dunes on mars: Viking observer observations. Lunar and Planetary Science XXIX, 1338. Fryberger, S.G., 1979. Dune forms and wind regime. In: McKee, E.D. (Ed.), A Study of Global Sand Seas. U.S. Government Printing Office, Washington, pp. 137–160. Greeley, R., Iversen, J.D., 1985. Wind as a Geological Process. Cambridge University Press, Cambridge. Lancaster, N., 2006. Linear dunes on Titan. Science 312, 702–703. Li, J., 2003. The Weather Systems and Climate in the Taklimakan Desert and its Surrounding Mountain Areas. Science Press, Beijing. (in Chinese). Li, J., Wen, S., Zhang, Q., Wang, F., Zheng, B., Li, B., 1979. The time, magnitude and modes of uplift of the Qinghai-Tibetan Plateau. Science in China (Series A) 6, 608–616 (in Chinese). Livingstone, I., Warren, A., 1996. Aeolian Geomorphology: An Introduction. Longman Singapore Publishers (Pte) Ltd., Singapore. Lorenz, R.D., Wall, S., Radebaugh, J., Boubin, G., Reffet, E., Janssen, M., Stofan, E., Lopes, R., Kirk, R., Elachi, C., Lunine, J., Mitchell, K., Paganelli, F., Soderblom, L., Wood, C., Wye, L., Zebker, H., Anderson, Y., Ostro, S., Allison, M., Boehmer, R., Callahan, P., Encrenaz, P., Ori, G.G., Francescetti, G., Gim, Y., Hamilton, G., Hensley, S., Johnson, W., Kelleher, K., Muhleman, D., Picardi, G., Posa, F., Roth, L., Seu, R., Shaffer, S., Stiles, B., Vetrella, S., Flamini, E., West, R., 2006. The sand seas of Titan: Cassini RADAR observations of longitudinal dunes. Science 312, 724–727. Qu, J., Zuo, G., Zhang, K., Zu, R., 2005. Relationship between the formation and evolution of the Kumtagh Desert and the regional neotectonic movement. Arid Land Geography 28, 424–428 (in Chinese with English abstract). Qu, J., Liao, K., Zu, R., Xia, X., Jing, Z., Dong, Z., Zhang, K., Yang, G., Wang, X., Dong, G., 2007. Study on the formation of feather-shaped sand ridge in Kumtagh Desert. Journal of Desert Research 27, 349–353 (in Chinese with English abstract).
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Thomas, P.C., Malin, M.C., Carr, M.H., Danielson, G.E., Daviesk, M.E., Hartmann, W.K., Ingersoll, A.P., James, P.B., McEwen, A.S., Soderblom, L.A., Veverka, J., 1999. Bright dunes on Mars. Nature 397, 592–594. Wang, J., Liao, K., Er, Y., Su, Z., Zhai, X., Liu, H., Kang, J., Ding, F., Zhang, J., Zheng, Q., 2005. Some results of the scientific expedition in the Kumtagh Desert. Gansu Science and Technology 21 (10), 6–8, 222 (in Chinese).
Xia, X., 1987. Some characteristics of the Kumtagh Desert. In: Xia, X. (Ed.), Scientific Expedition in Lop Nor. Science Press, Beijing. (in Chinese). Zhu, Z., Wu, Z., Liu, S., Di, X., 1980. Deserts in China. Science Press, Beijing. (in Chinese).
Geomorphology 100 (2008) 328 – 334 www.elsevier.com/locate/geomorph
Pseudo-feathery dunes in the Kumtagh Desert Zhibao Dong ⁎, Jianjun Qu, Xunming Wang, Guangqiang Qian, Wanyin Luo, Zhenhai Wei Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, China Received 9 August 2007; received in revised form 30 December 2007; accepted 2 January 2008 Available online 16 January 2008
Abstract The Kumtagh Desert is the last explored desert in China. The unique patterns on aerial photographs and satellite images have led previous researchers to the conclusion that the Kumtagh Desert is the only place over the world where typical feathery dunes are developed, and that some unique wind regimes are at work. Recent field investigation reveals that the feathery patterns in fact reflect the albedo contrasts between the seif dunes, bright dune-like drifts and the dark inter-dune lag sediments. The wind regime in the area containing the so-called feathery dunes is typical of the regime that produces seif dunes that were considered to be shafts of the feathery dunes, and the vanes of the feathery dunes are in fact bright dune-like drifts with indistinct height from the surroundings rather than true transverse dunes. Further analysis indicates that the sediments of seif dunes, dune-like drifts and dark inter-dune flats differ in grain size distribution, mineralogy, reflectance spectrum and particle micro-morphology. Sediments that constitute seif dunes and dune-like drifts are finer and poorly rounded, contain relatively more quartz, and hence have higher albedo, while the sediments that constitute the dark inter-dune flats are coarser and better rounded, contain far less quartz but relatively more andesite and dacite, and hence have lower albedo. As a result, what was originally perceived as unique feathery dunes are in fact only pseudofeathery dunes composed primarily of seif dunes. This discovery also demonstrates that blowing sand drifts can form dune-like patterns that remain visible for long periods of time. It is important to distinguish between the patterns of true dunes and those of dune-like drifts (pseudodunes) that cause albedo contrasts in interpreting remote sensing images. © 2008 Elsevier B.V. All rights reserved. Keywords: Aeolian processes; Dune geomorphology; Feathery dunes
1. Introduction Wind–bedform interactions that result in various dune forms are an important branch of aeolian geomorphology. Exploration of the diverse dune fields that bear rich information on wind– bedform interactions on Earth and other planets has been of continuing significance because of the insights it provides into the universality of geomorphological processes (Bagnold, 1941; Greeley and Iversen, 1985; Anderson, 1996; Lorenz et al., 2006). The Kumtagh Desert is the last desert to be explored in China, and the discovery of unique feather-like veined patterns in aerial photographs and satellite images of the northern part of ⁎ Corresponding author. Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 260, West Donggang Road, Lanzhou, Gansu 730000, China. Tel.: +86 931 8271167; fax: +86 931 8277169. E-mail address: [email protected] (Z. Dong). 0169-555X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.01.004
this desert have led previous researchers to conclude that some unique process is at work to produce the feathery dunes responsible for these patterns. The “shafts” of the feathers have been interpreted as longitudinal dunes, whereas the vanes have been interpreted as transverse dunes (Zhu et al., 1980). For several decades, the Kumtagh Desert has been believed to be the only place in the world where such typical feathery dunes developed (Wang et al., 2005). However, it is unusual for longitudinal and transverse dunes to appear under the same wind regime because they are formed by very different dynamical mechanisms (Cook et al., 1993; Livingstone and Warren, 1996; Lancaster, 2006). The harsh environments of the Kumtagh have prohibited researchers from in-depth work except some geological and morphological interpretations on aerial photographs and satellite images, although an old Silk Route passed close to the northern edge and several scientific explorers set their feet there as early as 1901 (Zhu et al., 1980; Qu et al., 2005). Some
Z. Dong et al. / Geomorphology 100 (2008) 328–334
329
Fig. 1. Satellite image of the main body of the Kumtagh Desert. Pseudo-feathery dunes have developed in the north, but dune types become more complex in the south as a result of the complex wind regimes and the alluvial processes that are sculpting the underlying relief, which is affected by the presence of the huge Altyn Tagh Mountains which rise more than 1500 m above the desert floor.
speculations have been proposed on the formation of the socalled feathery dunes. Xia (1987) proposed that the conditions favoring feathery dunes are broad field, abundant sediment supply, and unidirectional wind regime. He speculated that the roll-vortex flows were responsible for the formation of longitudinal dunes (shafts of feathery dunes), but they could only transport a fraction of the rich inter-dune sediments to the longitudinal dunes, as a result some of the inter-dune sediments had to be left behind to form long narrow mounds (vanes of feathery dunes) roughly perpendicular to the longitudinal dunes. Qu et al. (2005) speculated that the feathery dunes were controlled by the NE–SW aligned fault structures to the north of the Altyn Tagh Mountains. Other researchers (G. Yang's personal communication, 1973) thought the feathery dunes were controlled by the underlying relief. The shafts of feathery dunes were then formed by aeolian sediments covering old yardangs that were aligned in the NE–SW direction. None of the previously proposed speculations can provide convincing evidence. Researchers from the Gansu Institute of Desert Research made the first scientific investigation into the Kumtagh Desert in 2004, primarily on the desert vegetation. We have had the opportunities to get into the Kumtagh Desert four times since 2005. We have made field investigation on the sediments, morphology of the so-called feathery dunes and set up a wind tower to get more understanding of the geomorphology of the desert. The present paper summarizes some preliminary results obtained from field survey, interpretation of remote sensing images and analysis of sediments. It focuses on a basic issue: are there unique processes at work to produce the so-called feathery dunes in the Kumtagh Desert?
2. Physiographical setting The Kumtagh Desert is located in the eastern Tarim Basin and to the north of the high Altyn Tagh Mountains and south of Kezile Tagh Mountain, between 89°57.8′E and 94°54.1′E longitude, and between 39°7.8′N and 41°0.1′N latitude, covering an area of 22 000 km2. The desert is separated from the Taklimakan Desert by the Lop Depression. Various types of aeolian dunes combine to give the Kumtagh Desert the general appearance of a huge whiskbroom overlying the SW–NEinclined alluvial–diluvial plain north of the Altyn Tagh Mountains (Fig. 1). Hence, sediments of the Kumtagh Desert mainly come from the Altyn Tagh Mountains. The annual precipitation is less than 30 mm. The so-called feathery dunes have developed in the north, near the handle of the whiskbroom, between 91°24.8′E and 92°48.4′E longitude, and 40°1.2′N and
Fig. 2. Sand-driving wind rose in the northern Kumtagh Desert (Qu et al., 2007).
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Fig. 3. Aerial photograph of an area of the feather-like veined patterns of pseudo-feathery dunes in the northern Kumtagh Desert in 1971 (central point: 40°14′24.12″N, 92°26′24.53″E).
40°27.8′N latitude, covering an area of 4000 km2 (about 18% of the total area of the desert). Wind records from the wind tower set up in 2004 in the northern edge of Kumtagh Desert indicate that the sand-driving wind regime (≥ 5 m s− 1, the threshold wind velocity) measured at 2 m above the surface is acutely bimodal according to the wind environment classification of Fryberger (1979). The resultant sand-driving direction is towards SW205° (Fig. 2) (Qu et al., 2007). There are two dominant groups of winds: one group includes the NNE, NE and ENE winds, accounting for about 45% of winds, and the other group includes the ESE and E winds, accounting for 23%. Such a wind regime is commonly observed where seif dunes form. It is speculated that the wind regime becomes more complicated in the south due to the effects of the Altyn Tagh Mountains on local air circulation; this structure rises more than 1500 m above the Kumtagh Desert (Li, 2003). Therefore, the dune types are more complex in the south. 3. Pseudo-feathery dunes and their formation Our recent field investigations reveal that the unique featherlike patterns seen in aerial photographs and satellite images (Figs. 3–5) are composed of bright seif dunes with sinuous ridges and occasional slip-faces that comprise the shafts of the
Fig. 4. Quickbird satellite image taken in 2005 with a resolution of 0.6 m at the same location as the aerial photograph in Fig. 3.
Fig. 5. A low-angle aerial photograph taken in 2006 at about 150 m above the ground (location: 40°12′40.62″N, 92°06′25.98″E, looking east), showing clearly that the bright dune-like drifts have an albedo that contrasts strongly with the surrounding dark lag deposits, but their height above the surrounding area is indistinct.
feathers and bright dune-like sand drifts that comprise the vanes of the feathers and that lie between the seif dunes. The so-called feathery dunes are in fact pseudo-feathery dunes. Morphometry of the seif dunes and dune-like drifts was analyzed on aerial photographs taken in 1971 and Quickbird satellite images of 2005. We selected five sample plots (A, B, C, D and E in Fig. 1) to analyze the height, width, spacing and alignment of seif dunes. There are about 30 lines of seif dunes in the pseudofeathery dune area in the northern Kumtagh Desert. They are aligned from 55° to 235° (with 0° as north), generally extending over several or several tens of kilometers. They are 8 to 21 m tall (mostly 10 to 17 m) (Fig. 6), 40 to 120 m wide, and are spaced 100 to 1000 m (mostly 100 to 400 m apart) (Fig. 7). The height and spacing of the seif dunes are poorly correlated (Fig. 8), especially in plots C, D and E. The height of the seif dunes tends to decrease from the northeast (Plot A) to the southwest (Plot E). Ridges of the seif dunes are sinuous with relatively regularly spaced “teardrops” (Fig. 5). The teardrops are actually the remains of barchan dunes with slip-faces on the northwestern side and are oriented roughly in the same direction as the seif
Fig. 6. Probability distribution of height of the seif dunes.
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dunes. This suggests that the seif dunes have evolved from a line of barchan dunes. The formation of seif dunes in the northern Kumtagh Desert can be explained by the evolution model proposed by Bagnold (1941). The most frequent (NE) wind (the prevailing gentle wind, g) forms barchan dunes with southeastern and northwestern horns, the strongest but less frequent (E, SSE) winds (sand-bearing storm winds, s) elongate the southeastern horns (the windward wings) to form seif dunes, resulting in the alignment of the seif dunes from 55° to 235° (with 0° as north), oblique to both the NE and the E winds. The NNE wind tends to elongate the northwestern horns of the barchan dunes, but its action is limited compared with that of the E winds, resulting in the clear appearance of barchan horns on the northwestern side of the seif dunes (Fig. 5). The bright dune-like drifts between the seif dunes are transverse to the prevailing wind, and perpendicular to the bright seif dunes. They are analogous to the bright dune-like drifts that have been observed on Mars (Thomas et al., 1999), which are relatively regularly spaced at 80 to 350 m apart (mostly 80 to 220 m) (Fig. 9), and the dune-like drifts can be very easily interpreted as transverse dunes based on the veined patterns observed on aerial photographs and satellite images (Figs. 3 and 4). However, almost all of the bright dune-like drifts in the pseudo-feathery dune area are convex facing downwind (the prevailing wind is towards SW205°), contrary to the barchan dunes which are concave facing downwind. In addition, the wind regime typical of areas in which seif dunes form cannot foster the formation of transverse dunes (Fryberger, 1979). Field investigations have shown that the height of the bright dune-like drifts above their surroundings is indistinct, and that these structures arise at the leeward slope of undulating surfaces in some places (Fig. 5). Their transverse dune-like patterns on aerial photos and satellite images thus result from contrasts in albedos rather than the strong differences in elevation that characterize true dunes. The dune-like drifts are often composed of small patches of drifted sand with an extent on the order of meters and spaced at 15 to 50 m (Figs. 4 and 5). The albedo contrasts must be formed by differences in the constituting materials. For example, the bright dunes and dune-
Fig. 7. Probability distribution of spacing of the seif dunes.
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Fig. 8. The relationship between the spacing and height of the seif dunes (The sample plots are marked in Fig. 1).
like drifts observed on Mars have been interpreted as being composed of soft materials, possibly high in sulphates (Thomas et al., 1999). Sediments on 15 sets of seif dunes, bright dunelike drifts and dark inter-dune flats in the northern Kumtagh Desert were sampled to analyze their grain size distribution, mineral composition, reflectance spectrum and micro-morphology of sediment particles to obtain more information on the formation of feather-like patterns. The samples collected at similar geomorphological locations (seif dunes, bright dune-like drifts and dark inter-dune flats) were mixed before analysis to obtain their mean values. The grain size distribution was analyzed by dry sieve method in the Key Laboratory of Desert and Desertification, Chinese Academy of Sciences. Mineralogical analysis was conducted by the Mineralogy Analysis Center of Gansu Central Laboratory. The reflectance spectrum of the sediment samples was measured by a 7ISW15 spectrometer provided by the Beijing 7-Star Optical Instrument Co. Ltd. Micro-morphology of sediment particles was analyzed by an EPM-810Q electronic probe made by Shimadzu Corporation. It is revealed that the seif dunes and bright dune-like drifts between seif dunes that comprise the pseudo-feathery dunes in the Kumtagh Desert are composed of finer materials blown out
Fig. 9. Probability distribution of spacing of the bright dune-like drifts.
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of the inter-dune sediments, which have a greater percentage of minerals with high albedos such as quartz (Fig. 11). Consequently, the inter-dune flats are composed of coarse lag deposits with a higher percentage of minerals with low albedos such as dacite. The mean diameter of the dark surface sediments in the inter-dune flats is greater than 1.0 mm, whereas that of the bright seif dunes, bright dune-like drifts, and subsurface sediments in the inter-dune flats is less than 0.4 mm (Fig. 10). The coarser the sediments, the lower the content of minerals with high albedos such as quartz, plagioclase, and potassium feldspar, and the greater the dacite content with low albedos (Fig. 11). Consequently, the coarser sediments of the inter-dune surface have lower albedos (0.16) that are close to those of the dark aeolian dunes on Mars (Edgett and Parker, 1998), whereas those of the bright seif dunes and sand-like drifts in the interdune flats have higher albedos (0.31) (Fig. 12) that are close to those of the bright dunes on Mars (Thomas et al., 1999). Fig. 13 shows the micro-morphology of sediment particles photographed by an electronic probe. The pockmark-like structures and dishing pits on sediment particles in the pseudo-feathery dune area are typical properties of aeolian sediments. The particles are rounded to different extent, but the dark coarser sediment particles are better rounded, suggesting that they are more resistant to weathering and wind abrasion than the bright finer sediments. This explains why the bright sediments are sorted out of the dark inter-dune sediments to form seif dunes and bright dune-like drifts. While the shafts of the feather-like patterns can be successfully explained by the Bagnold's development model for seif dunes, it is also essential to explain the formation of the bright dune-like drifts. Retention of the vane patterns between seif dunes revealed by albedo contrasts over time requires transport and deposition of bright fine sediments over the dark surface of coarse sediments. This can be attributed to saltationinitiated activity. The sediments that form the bright seif dunes and the dune-like drifts belong to the saltation category. They have a propensity for self-accumulation into sand dunes in the absence of obstacles because saltating grains bounce off a hard desert surface more effectively than they do over a bed of loose
Fig. 10. The grain size distribution of the sediments in the area with pseudofeathery dunes.
Fig. 11. The mineral composition of the sediments in the area with pseudofeathery dunes.
sand (Bagnold, 1941). The sand transport rate over the bright patches of fine sediments is, therefore, lower than that over the surrounding dark coarse sediments, leading to the development of bright drifts and cleaning of the dark surfaces. However, the bright dune-like drifts in the inter-dune flats have not developed into transverse dunes, such as barchan dunes. This is because the coarse surfaces of the inter-dune flats produce insufficient fine sediments to cause the bright patches to evolve into dunes in the inter-dune flats. Field investigations indicated that the dark areas in the inter-seif dune corridors were generally covered by lag sediments with mean diameters greater than 1 mm that are usually non-erodible (Chepil, 1952) although there were abundant erodible sediments under the dark lag cover. The erodible fractions of the surface sediments have been transported to form seif dunes. Coarse sand ripples (or fine gravel ridges) with a wavelength of 1 to 2 m and a height of 0.1 to 0.3 m occasionally appear in the dark areas. How the original dune-like patches were initiated is not yet well understood. The convex-downwind patterns suggest that these patches depend
Fig. 12. The reflectance spectrum of the sediments in the area with pseudofeathery dunes.
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Fig. 13. Micro-morphological features of sand particles in the Kumtagh Desert. (a) d N 1.00 mm, (b) d = 1.00 mm–0.50 mm, and (c) 0.50 mm–0.25 mm. The finer the sediments, the less rounded and brighter they are.
on wind action, which is strongest towards the middle of the inter-dune flats. Thus, the patches might have formed during a recent period of stronger-than-normal winds, when the bright fine sediments were sorted out and drifted over the inter-dune flats like waves under the influence of wind turbulence. Dunelike drifts were then left behind on the surface when the winds abated. Another explanation is that the bright fine sediments constituting the dune-like drifts in the inter-seif dune corridors are blown from the seif dunes by those winds that are oblique to the seif dunes. At the same time, some of the dune-like drifts were blown onto the seif dunes. This paper only presents some preliminary results. There are various types of dunes in the Kumtagh Desert besides the seif dunes. The spatial variations in dune geomorphology must be related to the variations in wind regime. However, as a part of the Qinghai-Tibetan Plateau, the Altyn Tagh Mountains have been experiencing rapid uplift since the early Quaternary (Li et al., 1979), leading to the complex geological structures in the Kumtagh Desert. Intermittent rivers originating from the Altyn Tagh Mountains have formed many wadis in the desert. To fully understand the seif dunes in the northern Kumtagh Desert requires understanding the variations in wind regime, geological structures and alluvial processes in the whole desert. This will be our future work. 4. Conclusions Recent field investigation reveals that what was originally perceived as unique feathery dunes in the Kumtagh Desert of China are in fact only pseudo-feathery dunes composed primarily of seif dunes. The feathery patterns in the northern Kumtagh Desert reflect the albedo contrasts between seif dunes, dune-like drifts and the coarse lag deposits in the inter-seif dune corridors. What we have learned from the pseudo-feathery dunes of the Kumtagh Desert is that occasional strong winds may form dune-like drifts that remain in place over long periods of time, with patterns that are easily interpreted as dunes on remote sensing images. Existence of dune-like drifts indicates an inadequate supply of sediments that can be transported by wind.
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