Geoscience Reference
In-Depth Information
Table 17.4
Sand sea wind environments.
that even today tend towards arid-semi-arid conditions or
at least seasonal dryness - parameters that in many cases
would have been enhanced at glacial times by enhanced
continentality.
The extensive blanket loess deposits of Central Asia
(particularly in the Karakum, in Tajikistan and in Uzbek-
istan) extend to over 200 m and in the Huangtu Gaoyuan,
or Loess Plateau, of China deposits attain a thickness of
up to 500 m (Huang, Pang and Zhao, 2000), although 150
m is more representative of consistent maximum depths
(Derbyshire and Goudie, 1998; Goudie, 2002). In both
regions these deposits date back to the Pliocene, and the
loess sequences in fact comprise alternating loess and
palaeosol units, demonstrating that periods of deposi-
tion (during cold phases) alternated with warmer periods
of greater landscape stability. While these mid-latiutude
loess deposits are commonly regarded as 'cold' loessic de-
posits, their distribution actually shows close affinity to the
aeolian processes that have affected the desert areas that
they border. In both regions, the loess deposits occur at the
downwind end of a sequence of aeolian depositional units.
Thus in China (Figure 17.10) the sandy desert of the Mu
Us grades southeastwards to sandy loess and then to the
extensive loess plateau. This in effect represents chang-
ing aeolian transport efficiency southeastwards away from
sediment source areas.
Sand sea (m
3
/m width per year)
Rate of sand drift
1.
High-energy environments
25-40
Northern Arabian sand seas
Northwestern Libya
2.
Intermediate-energy environments
15-25
Simpson Desert, Australia
Western Mauritania
Peski Karakumy, Kazakhstan
Peski Kyzylkum, Kazakhstan
Erg Oriental, Algeria
Erg Occidental, Algeria
Namib Sand Sea, Namibia
Rub'al Khali, Saudi Arabia
3.
Low-energy environments
<
15
SW Kalahari, southern Africa
Sahelian zone sand seas
Gobi Desert, China
Thar Desert, India
Taklimakan Desert, China
17.4
The global distribution of loess
Figure 17.2 shows the global distribution of loess de-
posits, which cover up to 10 % of the Earth's land sur-
face (Pecsi, 1968). Despite a complex definitional history,
where the role of water in loess deposition and the role of
post-depositional processes has been debated, loess can
principally be defined as a terrestrial accumulation of ae-
olian silt (dust) (e.g. Pye, 1987) or as a deposit, primarily
aeolian in origin, that is principally comprised of silt-size
sediment that is dominated by quartz. Fine sand and clay
can also be present. Because of its fine-grained nature, silt
is readily transported in suspension and is prone to long-
distance transport (see Chapter 18). Much dust therefore
ends up being deposited in the oceans (e.g. Hesse and
McTainsh, 1999) and not on the land surface.
17.4.2
Peridesert loess
In lower latitudes, dust deposits are less widespread but
are clearly associated with desert conditions. It is to these
deposits that the term 'peridesert loess' is more usually ap-
plied. These are the loess deposits comprised of material
attributed to generation by abrasion during aeolian sand
saltation (e.g. Smith, Wright and Whalley, 2002). Inter-
estingly, most of the major desert dust sources today that
contribute material to long-distance atmospheric transport
are associated with deflation from dry lake basins and not
directly from aeolian abrasion. However, the distinction
between glacial and peridesert loess is not as clear-cut
as sometimes presented, as implied above. Derbyshire
(1983) demonstrated, both mineralogically and through
mapping the limits of Quaternary glaciations in China,
that the extensive Chinese loess deposits were not solely
comprised of glacial erosion material, but of sediments de-
rived from desert contexts. Indeed, several authors have
now identified a range of regularly occurring dryland pro-
cesses that can produce ample silt that is available for
dust generation and ultimately the deposition of loess on
desert margins. These include a range of weathering pro-
17.4.1
Loess production and distribution
In the mid-latitudes, loess deposits are strongly associ-
ated with the generation, transport and deposition of silt
from glacial erosion systems during cold phases in the
Quaternary (Smalley and Vita-Finzi, 1968; Smalley and
Krinsley, 1978). This is not to say that conditions dur-
ing deposition were not rather arid, since the continental
location of many of these deposits, in central Europe, in-
