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

The Ordos Basin of China encompasses the Mu Us Desert in the northwest and the Chinese Loess Plateau to the south and east. The boundary between the mostly internally drained Mu Us Desert and fluvially incised Loess Plateau is an erosional escarpment, up to 400 m in relief, composed of Quaternary loess. Linear ridges, with lengths of ~102–103 m, are formed in Cretaceous–Quaternary strata throughout the basin. Ridge orientations are generally parallel to near-surface wind vectors in the Ordos Basin during modern winter and spring dust storms. Our observations suggest that the Loess Plateau previously extended farther to the north and west of its modern windward escarpment margin and has been partially reworked by eolian processes. The linear topography, Mu Us Desert internal drainage, and escarpment retreat are all attributed to wind erosion, the aerial extent of which expanded southeastward in China in response to Quaternary amplification of Northern Hemisphere glaciation. INTRODUCTION The ~750 km (north-south) by ~450 km (east-west) Ordos Basin in China is bound by late Cenozoic rift-flank mountain ranges (Zhang et al., 1998) and encompasses the Mu Us Desert in the northwest and a large portion of the Chinese Loess Plateau in the south and east (Fig. 1). The Yellow River flows northward into the basin along the Yinchuan graben, eastward along the Hetao graben, southward through the eastern Loess Plateau to where it joins the Wei River, and then exits the basin to the east (Fig. 1); it may have followed this course since at least 2 Ma (Craddock et al., 2010; Pan et al., 2011). Much of the deflationary Mu Us Desert exhibits internal drainage and closed topographic depressions (Fig. 2). It exposes mostly Mesozoic bedrock in its western part and variably active and stabilized dune fields above Mesozoic bedrock in its eastern and southern parts (Fig. 1; Li, 2006). In stark contrast, the Chinese Loess Plateau is composed of Earth’s largest accumulation of Quaternary loess and is strongly incised by the Yellow River and its tributaries (Figs. 1 and 2). The loess strata are as much as several hundreds of meters thick and commonly interlayered with paleosols. Loess accumulation occurred primarily during glacial periods when Central Asia was colder and drier, whereas paleosols developed during interglacial periods when the East Asian Monsoon penetrated farther inland (Tungsheng and Zhongli, 1993; Porter, 2007). There are several motivating questions for this study. What is the nature of the geomorphic boundary between the Mu Us Desert and the Loess Plateau? How did this boundary evolve during the Quaternary? How was the Loess Plateau built? Central Asia became more arid during the Quaternary, concomitant with the increase in Northern Hemisphere ice volume (Tungsheng and Zhongli, 1993), and likely resulted in net desert expansion. Based on an upsection increase in the size and amount of sand in loess along the northern margin of the central Loess Plateau, Ding et al. (2005) proposed that the desert region was located ~200 km farther windward (inland) at the onset of the Quaternary compared to its position during the last glacial period. If correct, this implies a spatial migration in regions characterized by net eolian erosion versus accumulation and a retreating windward margin of the Loess Plateau. To test this hypothesis, we investigated the geology and geomorphology of the Ordos Basin (Fig. 1), with emphasis on mapping landforms in the field and with satellite imagery. We also compared wind patterns resolved from the geomorphology with modern near-surface wind vectors observed seasonally and during dust storms to evaluate our observations within a climatologic context. Our findings demonstrate the importance of wind erosion in sculpting local and regional topography, generating internal drainage, and simultaneously building and reworking a loess plateau. BEDROCK WIND EROSION IN THE MU US DESERT Wind streaks and dune geometries in the Mu Us Desert indicate westerly to northwesterly geomorphically effective wind directions (black arrows in Fig. 1); these are approximately parallel surface wind vectors that were recorded during modern wind-storm events (Liu et al., 2005; Mason et al., 2008), which are most frequent during spring (Roe, 2009). The northwestern Mu Us Desert locally exposes linear ridges of weakly cemented Cretaceous strata with meters to several tens of meters of relief, and lengths ranging from hundreds of meters to several kilometers (Fig. 3A). The distribution and orientations of the ridges are indicated by the red shading and red arrows in Figure 1; the red arrows show mean orientations of multiple individual ridges, the number of which scale inversely with their size. Many of the ridges exhibit steep windward faces and/or show evidence of being streamlined in map view, and thus can be classified as yardangs, whereas other linear bedrock ridges are not obviously streamlined. Where present, elongate troughs between the ridges are variably floored by bedrock (Fig. 3A), vegetation, or dunes (both active and vegetation stabilized). Linear bedrock ridges and locally welldeveloped fields of yardangs are also present along the windward and leeward margins of an approximately north-south–aligned, ~60-kmlong, ~50-m-high, and to 8-km-wide mesa of Cretaceous bedrock west of Otog city (Fig. 1; Figs. DR1A and DR1B in the GSA Data Repository1). East of Otog Mesa is an ~8-km-wide mesa-parallel trough, presumably wind excavated, and then dunes of sand at higher elevation (Fig. 2B; Fig. DR1A). These observations indicate the importance of eolian processes in sculpting the landscape of the Mu Us Desert. LOESS PLATEAU WINDWARD ESCARPMENT The spatial transition in the Ordos Basin from bedrock erosion to dunes to loess in the windward (and increased-precipitation gradient) direction (Fig. 1) is observed globally and is an intuitive pattern. Remaining enigmatic, but also widely documented, are abrupt transitions between regions of wind erosion and thick loess accumulation (e.g., Mason et al., 1999). The boundary between the Mu Us Desert and Loess Plateau provides an impressive example. Locally along the western margin of the eastern Loess Plateau, tributaries of the Yellow River mark boundaries between sand dunes of the Mu Us Desert and Loess Plateau strata, and by forming a barrier to sand transport, may contribute to proximal thick loess accumulations down1 GSA Data Repository item 2015283, figures showing satellite images, rose diagrams, and maps of near-surface wind vectors, is available online at www .geosociety .org /pubs/ft2015.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. *Current address: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. GEOLOGY, September 2015; v. 43; no. 9; p. 835–838 | Data Repository item 2015283 | doi:10.1130/G36724.1 | Published online 28 July 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. 836 www.gsapubs.org | Volume 43 | Number 9 | GEOLOGY wind (Mason et al., 1999). In many other places, however, the boundary is an escarpment within loess, hundreds of meters high along the northern margin of the central Loess Plateau (Fig. 2A) and less pronounced but still identifiable in the east (Fig. 2B). There is no indication that the escarpment is a barrier to sand transport or related to Quaternary faulting (Zhang et al., 1998; our observations). The escarpment roughly follows the boundary between internal drainage 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 capture. 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 smallwavelength (<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 there are spatially abrupt variations in linear topography orientation between the two distinct azimuth populations (Fig. 1C). The linear loess topography is oriented 118° ± 14° (mean ± one standard deviation) along the windward margins of the Loess Plateau, parallel to the geomorphically effective wind directions in the adjacent Mu Us Desert (Fig. 1). Over a distance of <10 km, the linear topography orientation rotates clockwise to a north-south azimuth (179° ± 11°; mean ± standard deviation) over the central Loess Plateau (Figs. DR2F–DR2H), and the eastern Loess Plateau where it abuts with the Luliang Mountains (Fig. 1). To evaluate whether the wind directions resolved from the geomorphology are consistent 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 precipitation (in mm) are from Porter et al. (2001). Red shading and arrows show the distribution and orientation of linear bedrock ridges in the Mu Us Desert and linear Loess Plateau topography. Black arrows show geomorphically effective wind directions, based on our interpretations of satellite images. The A-A’ and B-B’ dashed lines correspond to topographic 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 parallel). Dark gray population is representative of windward margin of Loess Plateau. Light gray population is representative of linear topography to the south and east. Mean azimuth values and one standard deviations are indicated.