From Dust to Dust: Quaternary Wind Erosion of the Mu Us Desert and Loess Plateau, China

From dust to dust: Quaternary wind erosion of the Mu Us Desert and Loess Plateau, China

Paul Kapp1, Alex Pullen1,2, Jon D. Pelletier1, Joellen Russell1, Paul Goodman1, and Fulong Cai1* 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 2Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA

ABSTRACT weakly cemented Cretaceous strata with meters The Ordos Basin of China encompasses the Mu Us Desert in the northwest and the Chinese to several tens of meters of relief, and lengths Loess Plateau to the south and east. The boundary between the mostly internally drained Mu ranging from hundreds of meters to several ki- Us Desert and fluvially incised Loess Plateau is an erosional escarpment, up to 400 m in relief, lometers (Fig. 3A). The distribution and orienta- composed of Quaternary loess. Linear ridges, with lengths of ~102–103 m, are formed in Cre- tions of the ridges are indicated by the red shad- taceous–Quaternary strata throughout the basin. Ridge orientations are generally parallel to ing and red arrows in Figure 1; the red arrows near-surface wind vectors in the Ordos Basin during modern winter and spring dust storms. show mean orientations of multiple individual Our observations suggest that the Loess Plateau previously extended farther to the north and ridges, the number of which scale inversely west of its modern windward escarpment margin and has been partially reworked by eolian with their size. Many of the ridges exhibit steep processes. The linear topography, Mu Us Desert internal drainage, and escarpment retreat are windward faces and/or show evidence of being all attributed to wind erosion, the aerial extent of which expanded southeastward in China in streamlined in map view, and thus can be clas- response to Quaternary amplification of Northern Hemisphere glaciation. sified as yardangs, whereas other linear bedrock ridges are not obviously streamlined. Where INTRODUCTION during the Quaternary? How was the Loess present, elongate troughs between the ridges are The ~750 km (north-south) by ~450 km Plateau built? Central Asia became more arid variably floored by bedrock (Fig. 3A), vegeta- (east-west) Ordos Basin in China is bound by during the Quaternary, concomitant with the tion, or dunes (both active and vegetation stabi- late Cenozoic rift-flank mountain ranges (Zhang increase in Northern Hemisphere ice volume lized). Linear bedrock ridges and locally well- et al., 1998) and encompasses the Mu Us Desert (Tungsheng and Zhongli, 1993), and likely developed fields of yardangs are also present in the northwest and a large portion of the Chi- resulted in net desert expansion. Based on an along the windward and leeward margins of an nese Loess Plateau in the south and east (Fig. 1). upsection increase in the size and amount of approximately north-south–aligned, ~60-km- The Yellow River flows northward into the basin sand in loess along the northern margin of the long, ~50-m-high, and to 8-km-wide mesa of along the Yinchuan graben, eastward along the central Loess Plateau, Ding et al. (2005) pro- Cretaceous bedrock west of Otog city (Fig. 1; Hetao graben, southward through the eastern posed that the desert region was located ~200 Figs. DR1A and DR1B in the GSA Data Re- Loess Plateau to where it joins the Wei River, km farther windward (inland) at the onset of the pository1). East of Otog Mesa is an ~8-km-wide and then exits the basin to the east (Fig. 1); it Quaternary compared to its position during the mesa-parallel trough, presumably wind exca- may have followed this course since at least 2 Ma last glacial period. If correct, this implies a spa- vated, and then dunes of sand at higher eleva- (Craddock et al., 2010; Pan et al., 2011). Much tial migration in regions characterized by net eo- tion (Fig. 2B; Fig. DR1A). These observations of the deflationary Mu Us Desert exhibits inter- lian erosion versus accumulation and a retreat- indicate the importance of eolian processes in nal drainage and closed topographic depressions ing windward margin of the Loess Plateau. To sculpting the landscape of the Mu Us Desert. (Fig. 2). It exposes mostly Mesozoic bedrock test this hypothesis, we investigated the geology in its western part and variably active and sta- and geomorphology of the Ordos Basin (Fig. 1), LOESS PLATEAU WINDWARD bilized dune fields above Mesozoic bedrock in with emphasis on mapping landforms in the ESCARPMENT its eastern and southern parts (Fig. 1; Li, 2006). field and with satellite imagery. We also com- The spatial transition in the Ordos Basin In stark contrast, the Chinese Loess Plateau is pared wind patterns resolved from the geomor- from bedrock erosion to dunes to loess in the composed of Earth’s largest accumulation of phology with modern near-surface wind vectors windward (and increased-precipitation gradi- Quaternary loess and is strongly incised by the observed seasonally and during dust storms to ent) direction (Fig. 1) is observed globally and Yellow River and its tributaries (Figs. 1 and 2). evaluate our observations within a climatologic is an intuitive pattern. Remaining enigmatic, The loess strata are as much as several hundreds context. Our findings demonstrate the impor- but also widely documented, are abrupt transi- of meters thick and commonly interlayered with tance of wind erosion in sculpting local and re- tions between regions of wind erosion and thick paleosols. Loess accumulation occurred primar- gional topography, generating internal drainage, loess accumulation (e.g., Mason et al., 1999). ily during glacial periods when Central Asia was and simultaneously building and reworking a The boundary between the Mu Us Desert and colder and drier, whereas paleosols developed loess plateau. Loess Plateau provides an impressive example. during interglacial periods when the East Asian Locally along the western margin of the eastern Monsoon penetrated farther inland (Tungsheng BEDROCK WIND EROSION IN THE MU Loess Plateau, tributaries of the Yellow River and Zhongli, 1993; Porter, 2007). US DESERT mark boundaries between sand dunes of the There are several motivating questions for Wind streaks and dune geometries in the Mu Mu Us Desert and Loess Plateau strata, and by this study. What is the nature of the geomorphic Us Desert indicate westerly to northwesterly forming a barrier to sand transport, may contrib- boundary between the Mu Us Desert and the geomorphically effective wind directions (black ute to proximal thick loess accumulations down- Loess Plateau? How did this boundary evolve arrows in Fig. 1); these are approximately parallel surface wind vectors that were recorded dur- 1 GSA Data Repository item 2015283, figures ing modern wind-storm events (Liu et al., 2005; showing satellite images, rose diagrams, and maps *Current address: Key Laboratory of Continental of near-surface wind vectors, is available online at Collision and Plateau Uplift, Institute of Tibetan Pla- Mason et al., 2008), which are most frequent www.geosociety.org/pubs/ft2015.htm, or on request teau Research, Chinese Academy of Sciences, Beijing during spring (Roe, 2009). The northwestern from [email protected] or Documents Secre- 100101, China. Mu Us Desert locally exposes linear ridges of tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/9/835/3548979/835.pdf by University of Arizona user on 27 August 2019 within the Mu Us Desert and the incised Loess Plateau (Figs. 1 and 2), and in many places forms a drainage divide along which wind gaps are present (Fig. 3B) as a result of stream cap- ture. The Miocene Red Clay Formation, which underlies Loess Plateau strata in many places, is locally exposed in the Mu Us Desert adjacent to extensions of the escarpment (Fig. 3B; Li, 2006; our observations).

LINEAR LOESS PLATEAU TOPOGRAPHY Superimposed on the dendritic incision pattern of the Loess Plateau (Fig. 1) is a small- wavelength (<1 km) linear topographic fabric defined by kilometer-scale-long aligned ridges and parallel valleys (Fig. 3B) in the red shaded regions of Figure 1. The ridges are rilled and the valleys show evidence of fluvial incision. In places the linear topography is prominent and ubiquitous (Figs. DR2A–DR2D), whereas in others it is spatially patchy and/or more cryptic at the 10 km scale, but the overall linear orientation is still resolvable (Figs. DR2E–DR2G). Quaternary loess in the United States locally exhibits similar linear topography; although it is debated whether it is primarily of depositional or erosional origin (Flemal et al., 1972), it is accepted to be wind parallel and the role of wind erosion has been demonstrated (Sweeney and Mason, 2013). The linear topography is developed within the Malan Loess of the last glacial period, and therefore must have (or continued to have) formed since that time. To document spatial variations in linear loess topography orientation, we mapped orientations in Google Earth (n = 2869 localities), spaced across the area of the Loess Plateau where linear topography is evident. A rose diagram of all orientation measurements is shown in Figure 1C, and Figure DR3 shows rose diagrams for ~1° × 1° geographic areas. The red arrows in Figure 1 show the dominant orientation of the linear loess topography, which generally varies <5° at the scale of ~10 km (Figs. DR2A–DR2E) and deviates 5°–10° from a mean value at the scale of ~100 km (Fig. DR3). Exceptions are where Figure 1. A: Location map of Ordos Basin, China. Stippled pattern indicates sand deserts. B: Shaded relief map (www.geomapapp.org) of Ordos Basin. Yellow contours of mean annual there are spatially abrupt variations in linear to- precipitation (in mm) are from Porter et al. (2001). Red shading and arrows show the distribu- pography orientation between the two distinct tion and orientation of linear bedrock ridges in the Mu Us Desert and linear Loess Plateau azimuth populations (Fig. 1C). The linear loess topography. Black arrows show geomorphically effective wind directions, based on our in- topography is oriented 118° ± 14° (mean ± one terpretations of satellite images. The A-A’ and B-B’ dashed lines correspond to topographic standard deviation) along the windward margins profiles in Figure 2. Approximate distribution of Mesozoic and Cretaceous strata is shown. C: Rose diagram of linear loess topography orientations, plotted as unidirectional (wind of the Loess Plateau, parallel to the geomorphi- parallel). Dark gray population is representative of windward margin of Loess Plateau. Light cally effective wind directions in the adjacent gray population is representative of linear topography to the south and east. Mean azimuth Mu Us Desert (Fig. 1). Over a distance of <10 values and one standard deviations are indicated. km, the linear topography orientation rotates clockwise to a north-south azimuth (179° ± 11°; mean ± standard deviation) over the cen- wind (Mason et al., 1999). In many other places, the east (Fig. 2B). There is no indication that the tral Loess Plateau (Figs. DR2F–DR2H), and the however, the boundary is an escarpment within escarpment is a barrier to sand transport or re- eastern Loess Plateau where it abuts with the loess, hundreds of meters high along the north- lated to Quaternary faulting (Zhang et al., 1998; Luliang Mountains (Fig. 1). ern margin of the central Loess Plateau (Fig. our observations). The escarpment roughly fol- To evaluate whether the wind directions re- 2A) and less pronounced but still identifiable in lows the boundary between internal drainage solved from the geomorphology are consistent

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Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/9/835/3548979/835.pdf by University of Arizona user on 27 August 2019 A (N) (S) A’ 1800 fformerormer eextentxtent ofof LoessLoess PPlateau?lateau? 1400 Yellow River escarpment Wei graben dune fields 1000 Cretaceous bedrock Mu Us Desert Loess Plateau 600 internally drained fluvially incised A Elevation (m) 200 050 150 250 350 450 550 650 750 B (NW) (SE) B’ distance along profile (km) 1800 Otog trough escarpment Yellow River 1400 dune fields l o e s s Figure 2. Topographic profiles across Ordos Basin, China, along A-A’ and B-B’ (see Fig. 1). A: Approximately north-south profile showing the topography of the 1000 Otog Mu Us Desert loess B Mesa internally drained Loess Plateau where it is least incised, its escarpment margin, and its possible 600 incised Elevation (m) 050 100 150 200 250 300 350 former extent across the Mu Us Desert. B: Approximately east-west profile across distance along profile (km) Otog Mesa and trough, and the escarpment margin of the eastern Loess Plateau.

A wind-parallel ridge B N drainage divide Figure 3. A: Wind-parallel Cretaceous sandstone Mu Us Desert linear bedrock ridges and 37°24’N adjacent bedrock-floored de- wind gaps pression, Mu Us Desert (lo- 30 m 60 m closed cation indicated in Fig. 1B). dunes on depression B: Google Earth™, image of leeward face linear loess topography and Loess Plateau escarpment escarpment Loess Plateau (location indicated in Fig. 1). Cretaceous-floored depression The top of the escarpment is a drainage divide (blue line). linear loess topography Blue circles indicate loca- 37°12’N tions of wind gaps. Coordi- 10 km nates are 37.294°N, 107.394°E wind gaps107°E 107°20’E (image date 17 March 2013).

with the modern climatology, we compared them Hemisphere ice volume during the Quaternary at a range of spatial scales (Fig. 2). Where the to observed near-surface (10 m height) wind may have brought more frequent and farther escarpment forms a drainage divide and exhib- vectors averaged over different seasons from southeastward-reaching cold air surges that gen- its wind gaps (e.g., Fig. 3B), the late Quaternary A.D. 1979 to 2010 and over a 6 h time period erated dust storms in Central Asia (Roe, 2009) rate of escarpment retreat exceeded that of head- of maximum wind speed during spring wind- and concomitantly limited the penetration of the ward river incision, such that the upper reaches storm events (Fig. 4; Figs. DR4A–DR4G). The East Asian Monsoon into Central Asia, which in seasonal wind pattern most similar to the orien- turn would decrease the spatial extent of dust- tations of the linear topography is that of winter trapping vegetation cover (Ding et al., 1999). 106°E 112°E (Fig. DR4D). It shows the first-order clockwise Consequently, regions of former loess accumu- 41°N rotation in wind vectors over the Ordos Basin, lation transitioned into regions dominated by but with a stronger westerly component over the wind erosion. Mu Us and northern half of the central Loess We attribute the development of the linear Plateau. The four spring wind-storm events ana- bedrock ridges and closed depressions in the lyzed were all characterized by northwesterly Mu Us Desert and the Loess Plateau windward winds over the Mu Us Desert, consistent with escarpment and linear topography to wind ero- the geomorphology, but variable winds over the sion. The paleo–Yellow River provided a con- Loess Plateau, from northwesterly, to northerly, tinuous supply of sand that could be reworked and even southwesterly to southerly (Fig. 4; by wind (Stevens et al., 2013). The Loess Pla- Figs. DR4E–DR4G). The wind-storm event of teau escarpment retreated as it was sandblasted 14 March 2010, however, shows a wind pattern while loess continued to accumulate downwind. strikingly similar to that indicated by the geo- At a spatial scale greater than that of the scale of morphology (Fig. 4). Additional comparative regions of closed drainage (kilometers to tens of 35°N studies are needed, but our preliminary analysis kilometers), the Mu Us Desert exhibits a subtle implicates modern wind storms in sculpting the decrease in elevation as it approaches the Loess eolian geomorphology. Plateau escarpment (Fig. 2). Here, wind erosion 12 m/s is enhanced because of wind speed acceleration HYPOTHESES AND IMPLICATIONS associated with streamline compression over Figure 4. Near-surface (10 m height) We propose that at the onset of the Qua- the escarpment (e.g., Jackson and Hunt, 1975) wind vectors averaged over a 6 h time ternary, Loess Plateau strata extended farther in combination with the higher erodibility of period of maximum wind speed during across the Ordos Basin (Fig. 2A), consistent the loess compared to the underlying Red Clay a dust storm event over the Loess Pla- teau on 14 March 2010 (7 a.m. Green- with the subsequent ~200 km of Mu Us Desert Formation and older bedrock. We propose that wich Mean Time). Data are from Saha expansion suggested from loess grain-size stud- through localized scour, wind erosion helped et al. (2010). Black box indicates loca- ies (Ding et al., 2005). The increase in Northern generate internal drainage in the Mu Us Desert tion of Figure 1.

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Tungsheng, L., and Zhongli, D., 1993, Stepwise cou- pling of monsoon circulations to global ice vol- 2005; Stevens et al., 2006; Buylaert et al., 2008). Liu, L.Y., Skidmore, E., Hasi, E., Wagner, L., and Tatarko, J., 2005, Dune sand transport as influ- ume variations during the late Cenozoic: Global A final implication pertains to provenance enced by wind directions, speed and frequen- and Planetary Change, v. 7, p. 119–130, doi: studies, a large number of which have been con- cies in the Ordos Plateau, China: Geomorphol- 10.1016/0921-8181(93)90044-O. ducted on Loess Plateau strata with the aim of ogy, v. 67, p. 283–297, doi:10.1016/j.geomorph Xiao, J., Nakamura, T., Lu, H., and Zhang, G., 2002, reconstructing potential glacial versus intergla- .2004.10.005. 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(China), and its contribution to the extrusion Mason, J.A., Nater, E.A., Zanner, C.W., and Bell, tectonics of south China with respect to Gobi- serve to homogenize the composition of glacial- J.C., 1999, A new model of topographic effects Mongolia: Tectonophysics, v. 285, p. 41–75, age loess and dust in interglacial-age paleosol. on the distribution of loess: Geomorphology, doi:10.1016/S0040-1951(97)00170-4. Therefore, even if the loess and paleosols show v. 28, p. 223–226, doi:10.1016/S0169-555X (98) Zhou, Y., Lu, H., Zhang, J., Mason, J.A., and Zhou, an indistinguishable provenance (e.g., Che and 00112-3. L., 2009, Luminescence dating of sand-loess Li, 2013), this does not negate the possibility of Mason, J.A., Swinehart, J.B., Lu, H., Miao, X., Cha, sequences and response of Mu Us and Otindag P., and Zhou, Y., 2008, Limited change in dune sand fields (north China) to climatic changes: temporal-spatial variability in wind patterns and mobility in response to a large decrease in wind Journal of Quaternary Science, v. 24, p. 336– dust source regions. power in semi-arid northern China since the 344, doi:10.1002/jqs.1234. 1970s: Geomorphology, v. 102, p. 351–363, doi: ACKNOWLEDGMENTS 10.1016/j.geomorph.2008.04.004. This research was supported by U.S. National Science Pan, B., Hu, Z., Wang, J., Vandenberghe, J., and Hu, Manuscript received 20 February 2015 Foundation grants AGS-1203427, AGS-1203973, and X., 2011, A magnetostratigraphic record of land- Revised manuscript received 6 July 2015 EAR-1323148. Comments by J. Mason, editor J. Spo- scape development in the eastern Ordos Plateau, Manuscript accepted 8 July 2015 tila, T. Stevens, M. Sweeney, and three anonymous China: Transition from late Miocene and early reviewers helped improve this manuscript. Pliocene stacked sedimentation to late Pliocene Printed in USA

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