Geoscience Reference
In-Depth Information
Wasson, 1984). Finally, it can be noted that for many sand
seas there is evidence for multiple phases of accumulation
(e.g. Figure 17.5). One reason for hiatuses in accumula-
tion is climate change. It has been suggested that during
the last 30 000 years active accumulation of sediment in
sand seas may have been restricted to less than a third of
the time (Lancaster, 1990), with episodicity of accumula-
tion even affecting sand seas in the driest regions (Haney
and Grolier, 1991), because even with aridity present,
sufficient wind energy is required for sand transport and
deposition. Other reasons for disruptions in accumulation
are tectonic activity and, in coastal situations, base level
changes. The overall duration of sand sea accumulation
may possibly be established by examining the mineralogi-
cal maturity of the materials they comprise (Muhs, 2004),
since a dominance of quartz suggests long-establishment
or sand seas that they have experienced long periods of
stability, as other weaker minerals have been removed by
weathering (Muhs, 2004).
seas experience less than 27 m
3
/m width per year poten-
tial sandflow (Table 17.4); however, estimating sandflow
rates over large areas using limited meteorological data
can be problematic. Many low-energy sand seas occur
near the centre of semi-permanent high- and low-pressure
cells, while the high-energy sand seas fall in the Trade
Wind zones near the margins of these pressure systems.
The transference of sand from high- to low-energy loca-
tions within sand seas has been demonstrated by stud-
ies in the Jafurah erg, Saudi Arabia (Fryberger
et al.
,
1984), the Namib Desert (Lancaster, 1985) and the Gran
Desierto, Mexico (Lancaster, Greeley and Christensen,
1987). Clearly, therefore, the general classification of
wind environments by Fryberger and Ahlbrandt (1979)
masks considerable intraregional variations in sandflow
potential. This may account for the difference between
their classification of the Simpson Desert (Table 17.3)
and Ash and Wasson's (1983) assertion that the major
limitation on sand movement there today is the low wind
speeds.
17.3.3
Sandflow conditions and sand
sea development
17.3.4
Sand sheets
It has been noted that sand sea development requires both
favourable sediment budgets and wind conditions. Where
net sand transport rates are high, throughflow exceeds de-
position and bedform development is limited to sheets or
stringers with ripples or highly mobile dune types such as
barchans (Wilson, 1971). In such throughflow-dominated
locations, sandflow is usually unsaturated and ground sand
cover incomplete, because unsaturated throughflow has
the potential to erode pre-existing deposits (Lancaster,
1999). If surface conditions are favourable, however, iso-
lated bedforms can develop so long as the sand received
by the bedform is equal to that which is lost downwind.
Wilson (1971) termed this situation
metasaturated flow
.
Sand seas proper accrue where sandflow is saturated and
accumulation exceeds net transport, with consequential
bedform development and the vertical or lateral accumu-
lation of sand.
Wilson (1971) provided theoretical models of erg
development in terms of wind regime and sandflow
variations, demonstrating how sand seas should grow
in locations of convergent windflow and wind speed
deceleration, e.g. in intermontane basins (Figure 17.8).
An empirical study (Fryberger and Ahlbrandt, 1979) has
supported Wilson's major assertions and identified the
synoptic and topographical conditions favouring sand sea
development (Figure 17.8(b)).
Fryberger
Sand sheets can develop in aeolian environments where
conditions do not favour dune development, though they
may exhibit low-relief aeolian features such as ripples
and
zibar
(see Chapter 18). They can be small, local, fea-
tures of a few square kilometres, often on the margins of
dunefields (Kocurek, 1986) and associated with the previ-
ously referred-to movement of sand from high- to lower-
energy locations. Sand sheets can, however, also represent
major regional landscape components (Breed, McCauley,
and Davis, 1987). For example, the Selima Sand Sheet of
southern Egypt is a 'single utterly flat sheet of firm sand,
known to cover an area of nearly 100 000 square miles'
(Maxwell and Haynes, 2001, p.1623) (Figure 17.9).
Kocurek and Nielson (1986) recognised five major con-
trols on aeolian sand sheet development.
Vegetation
,es-
pecially grasses, may encourage the accretion of low-
angle laminae while limiting the construction of dunes.
Coarse sand
, which is not readily formed into dunes, can
characterise sand sheets, sometimes upwind of a dune-
field, where it remains as a surface lag deposit, as at the
Algodone dunefield, California (Kocurek and Nielson,
1986). A coarse-sand sheet can therefore sometimes be
regarded as a deflational remnant, rather like some desert
pavement or
regs
.A
near-surface groundwater table
(Stokes, 1968) or 'Stokes surface' (Fryberger, Schenk and
Krystinik, 1988) can be an important control on sand sheet
development, acting as a base level to the action of wind
