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*
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA
1
2
ABSTRACT
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
*Current address: Key Laboratory of Continental
Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing
100101, China.
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 editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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 Geological
Society
America.
permission to copy, contact editing@geosociety.org.
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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).
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.
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
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
836www.gsapubs.org | Volume 43 | Number 9 | GEOLOGY
Elevation (m) Elevation (m)
1800
A (N)
former extent of Loess Plateau?
1400 Yellow River
rock
ous bed
1000
Cretace
600
200
1800
50
B (NW)
1400
1000
600
B
0
escarpment
dune fields
Mu Us Desert
internally drained
A
0
(S) A’
Otog trough
Otog
Mesa
50
150
250
escarpment
dune fields
Yellow River
loes
s
loess
incised
250
300
350
Mu Us Desert
internally drained
100
150
200
(SE) B’
distance along profile (km)
wind-parallel ridge
Cretaceous sandstone
60 m
30 m
A
450
550
650
distance along profile (km)
750
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
Loess Plateau where it is least incised, its escarpment margin, and its possible
former extent across the Mu Us Desert. B: Approximately east-west profile across
Otog Mesa and trough, and the escarpment margin of the eastern Loess Plateau.
B Mu Us Desert
N
drainage divide
37°24’N
wind gaps
n
pressio
d de
s-floore
eou
Loess Plateau
fluvially incised
d
se n
clo ssio
e
pr
de
dunes on
leeward face
Cretac
350
Wei graben
c
es
t
en
m
p
ar
Loess Plateau
linear loess topography
wind gaps107°E
with the modern climatology, we compared them
to observed near-surface (10 m height) wind
vectors averaged over different seasons from
A.D. 1979 to 2010 and over a 6 h time period
of maximum wind speed during spring windstorm events (Fig. 4; Figs. DR4A–DR4G). The
seasonal wind pattern most similar to the orientations of the linear topography is that of winter
(Fig. DR4D). It shows the first-order clockwise
rotation in wind vectors over the Ordos Basin,
but with a stronger westerly component over the
Mu Us and northern half of the central Loess
Plateau. The four spring wind-storm events analyzed were all characterized by northwesterly
winds over the Mu Us Desert, consistent with
the geomorphology, but variable winds over the
Loess Plateau, from northwesterly, to northerly,
and even southwesterly to southerly (Fig. 4;
Figs. DR4E–DR4G). The wind-storm event of
14 March 2010, however, shows a wind pattern
strikingly similar to that indicated by the geomorphology (Fig. 4). Additional comparative
studies are needed, but our preliminary analysis
implicates modern wind storms in sculpting the
eolian geomorphology.
HYPOTHESES AND IMPLICATIONS
We propose that at the onset of the Quaternary, Loess Plateau strata extended farther
across the Ordos Basin (Fig. 2A), consistent
with the subsequent ~200 km of Mu Us Desert
expansion suggested from loess grain-size studies (Ding et al., 2005). The increase in Northern
10 km
Hemisphere ice volume during the Quaternary
may have brought more frequent and farther
southeastward-reaching cold air surges that generated dust storms in Central Asia (Roe, 2009)
and concomitantly limited the penetration of the
East Asian Monsoon into Central Asia, which in
turn would decrease the spatial extent of dusttrapping vegetation cover (Ding et al., 1999).
Consequently, regions of former loess accumulation transitioned into regions dominated by
wind erosion.
We attribute the development of the linear
bedrock ridges and closed depressions in the
Mu Us Desert and the Loess Plateau windward
escarpment and linear topography to wind erosion. The paleo–Yellow River provided a continuous supply of sand that could be reworked
by wind (Stevens et al., 2013). The Loess Plateau escarpment retreated as it was sandblasted
while loess continued to accumulate downwind.
At a spatial scale greater than that of the scale of
regions of closed drainage (kilometers to tens of
kilometers), the Mu Us Desert exhibits a subtle
decrease in elevation as it approaches the Loess
Plateau escarpment (Fig. 2). Here, wind erosion
is enhanced because of wind speed acceleration
associated with streamline compression over
the escarpment (e.g., Jackson and Hunt, 1975)
in combination with the higher erodibility of
the loess compared to the underlying Red Clay
Formation and older bedrock. We propose that
through localized scour, wind erosion helped
generate internal drainage in the Mu Us Desert
GEOLOGY | Volume 43 | Number 9 | www.gsapubs.org
37°12’N
107°20’E
Figure 3. A: Wind-parallel
linear bedrock ridges and
adjacent bedrock-floored depression, Mu Us Desert (location indicated in Fig. 1B).
B: Google Earth™, image of
linear loess topography and
Loess Plateau escarpment
(location indicated in Fig. 1).
The top of the escarpment is
a drainage divide (blue line).
Blue circles indicate locations of wind gaps. Coordinates are 37.294°N, 107.394°E
(image date 17 March 2013).
at a range of spatial scales (Fig. 2). Where the
escarpment forms a drainage divide and exhibits wind gaps (e.g., Fig. 3B), the late Quaternary
rate of escarpment retreat exceeded that of headward river incision, such that the upper reaches
106°E
112°E
41°N
35°N
12 m/s
Figure 4. Near-surface (10 m height)
wind vectors averaged over a 6 h time
period of maximum wind speed during
a dust storm event over the Loess Plateau on 14 March 2010 (7 a.m. Greenwich Mean Time). Data are from Saha
et al. (2010). Black box indicates location of Figure 1.
837
of formerly southward- and eastward-flowing
rivers were beheaded and reversed flow direction. The envisioned process at the million-year
time scale is analogous to using a leaf blower
to generate an expanding region of wind erosion, a windward-retreating escarpment made of
leaves, and a growing pile of wind-transported
material downwind.
At the shorter time scale, the history was
more complex, as regions transitioned between
net eolian erosion and sediment accumulation
in response to glacial-interglacial and millennial time scale climate variability (e.g., Sun and
Ding, 1998; Ding et al., 1999; Xiao et al., 2002;
Lu et al., 2005; Zhou et al., 2009). This explains
local preservation of Quaternary paleosol and
loess in the Mu Us Desert, presumably where
they have been shielded from wind erosion by
topography or vegetation, and eolian sand interbedded with loess along the windward margins
of the Loess Plateau (e.g., Sun and Ding, 1998).
Given that the linear loess topography developed
since the last glacial period, and that the surface
wind pattern in the Ordos Basin resolved from
the geomorphology is similar to surface wind
patterns observed during at least some modern
spring wind storms (Fig. 4), the Loess Plateau
should be considered today as not only a sink,
but also a source of dust. We anticipate that wind
erosion of the Loess Plateau was more severe
during glacial periods when it was less vegetated
and subjected to a larger flux of windblown sand
(Ding et al., 1999). This is consistent with the
presence of deflation-attributed unconformities
within some Loess Plateau sections (Lu et al.,
2005; Stevens et al., 2006; Buylaert et al., 2008).
A final implication pertains to provenance
studies, a large number of which have been conducted on Loess Plateau strata with the aim of
reconstructing potential glacial versus interglacial atmospheric circulation pattern variations,
yet complications due to sediment reworking
are only rarely considered (e.g., Stevens et al.,
2013). Our proposal of extensive sediment reworking by wind in the Ordos Basin would
serve to homogenize the composition of glacialage loess and dust in interglacial-age paleosol.
Therefore, even if the loess and paleosols show
an indistinguishable provenance (e.g., Che and
Li, 2013), this does not negate the possibility of
temporal-spatial variability in wind patterns and
dust source regions.
ACKNOWLEDGMENTS
This research was supported by U.S. National Science
Foundation grants AGS-1203427, AGS-1203973, and
EAR-1323148. Comments by J. Mason, editor J. Spotila, T. Stevens, M. Sweeney, and three anonymous
reviewers helped improve this manuscript.
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Manuscript received 20 February 2015
Revised manuscript received 6 July 2015
Manuscript accepted 8 July 2015
Printed in USA
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