Geoscience Reference
In-Depth Information
Aeolian
sediment
availability
Aeolian
transport
capacity
Cumulative
system
response
Aeolian
system
response
Sediment
production
Climate
Destructional
Sand-starved
conditions
Arid
Lagged
transport-
limited influx
Lagged
availability-
limited influx
Weath-
Weath-
ering
Weath-
ering
ering
Stabilization
Stored
sediment
Humid
Humid
0 --> +
0 --> +
0 --> +
Figure 17.5
Theoretical model of aeolian system response to climatic changes (after Kocurek, 1998). The model aims to show
how sediment production, availability and aeolian transport interact to leave (or not) a sedimentary record of past conditions in
sand seas.
and Haynes, 2001) and occurs through the movement
of extremely low-amplitude giant ripples (500-1000 m
wavelength; see Breed, McCauley and Davis, 1987) or
even higher amplitude 'chevrons' (Maxwell and Haynes,
1989). This may appear counterintuitive to earlier state-
ments that sand sheets do not possess bedforms; however,
these transport features are not perceptible on the ground
and are only visible via remote sensing (Maxwell and
Haynes, 2001). When vegetation is present, it is not so
dense as to significantly inhibit aeolian sand transport.
Mobile barchan dunes represent bulk transport rates of
up to 3.49 m
3
/m width per year. Substantial quantities
of sand may be transported huge distances across sand
sheets before accumulating into dunes, as in the case of
the Makteir erg in Mauritania (Fryberger and Ahlbrandt,
1979). Sand transport pathways may be sinuous, showing
sensitivity to local topography and being analogous to
'rivers of sand' (Zimbelman, Williams and Tchakerian,
1994).
Dune sediments can reach sand seas along dominant
non-aeolian pathways. In the Taklamakan Desert, China
(Zhu, Zou and Yang, 1987), central Australia (Twidale,
1972; Nanson
et al.
, 2008) and other areas, so-called
valleys or seasonally inundated channel systems. Wadi
sediments may be particularly important local aeolian
forms as they dry out through drainage rather than evapo-
ration and are therefore uncemented (Glennie, 1970). It is
in fact likely that alluvial sources have made direct or indi-
rect contributions to the sediments in parts of many sand
seas, especially in low relief situations (Cooke, Warren
and Goudie, 1993).
Namib Desert dune sands have arrived at their des-
tination through the combined effects of river, sea and
wind transport (Lancaster and Ollier, 1983). In the Wahiba
Sands of Oman, hyperaridity has precluded fluvial con-
tributions to Holocene sand sea accumulation (Warren,
1988). Instead, sediment supply is attributed to coastal
erosion of soft sandy materials (Figure 17.6). The ex-
tensive dune systems of the Kalahari represent periods
of aeolian reworking of pre-existing continental basin
sediments, which have accumulated by various means
since the Mid-Jurassic (Thomas, 1987). These have been
supplemented in the southwestern Kalahari by aeolian
inputs operating along thin sand sheets (Fryberger and
Ahlbrandt, 1979) (Figure 17.7). Such
intraergal
and
extraergal
sands can sometimes be distinguished by
