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cross-beds with only a thin veneer of overlying sediments
(see, for example, Reading, 1996). Although erosional in
nature, Stokes surfaces also act as important controls upon
deposition and are commonly associated with thin layers
of sediment, indicating the role of the water table in ae-
olian sedimentation. The erosional topography of Stokes
surfaces is not always completely planar, partly due to
the water table mimicking the dune field topography, and
may also exhibit surface irregularities (Reading, 1996).
The capillary tension within damp sand can assist in the
formation of a variety of sedimentary adhesion structures
due to wind sculpturing, including adhesion ripples, lam-
inations, warts and other steep-sided, irregular bedforms
(Kocurek, 1981a).
The second influence of near-surface groundwater as a
limit to wind scour is through the cementation of sediment
in the vicinity of the water table. High levels of evapora-
tion combined with a high water table may promote the
precipitation of evaporite cements in the phreatic zone or
of salcretes nearer the surface at the top of the capillary
fringe zone (see Chapter 8). Schenk and Fryberger (1988)
and Fryberger, Schenk and Krystinik (1988), for exam-
ple, record massive phreatic gypsum cementation in the
White Sands dune field, New Mexico, which currently
acts as a basal limit to aeolian deflation and has created
extensive planar surfaces in interdune areas. Wet-damp
surface interdune deposits consist predominantly of gyp-
sum salt ridges and mats of microorganisms and algae,
which form within the capillary fringe. Sediment accu-
mulation in the interdune areas at White Sands follows a
distinctive seasonal cycle. Typically, interdune floors are
at their dampest during the late autumn to early spring,
with aeolian sediment trapping occurring due to the rise
of the capillary fringe and the associated development of
salt ridges and biogenic mats. During the late spring to
early autumn when interdune surfaces are at their driest,
salt ridges and biogenic mats are typically dry and brittle
and consist of an admixture of wind-blown sand (Kocurek
et al. , 2007). Cementation in proximity to the water table
may also produce an irregular surface topography, with
Fryberger, Schenk and Krystinik (1988) recording 'mini-
yardangs', nonstreamlined eroded bumps, salt ridges and
scour pits in halite-cemented sediments in the Jadurah
Sand Sea, Saudi Arabia. In addition to cementing the sand
surface, the presence of salts also raises the threshold ve-
locity of sand (Pye, 1980).
The third link between groundwater and aeolian pro-
cesses in dryland regions occurs where shallow water
tables promote the establishment of a vegetation cover
as opposed to acting as an erosional base level. Fixed
dunes in central Niger have had their surfaces stabilised
action of cyanophytes and fungi acting as binding agents
(Talbot, 1980). Former water tables are suggested by Tal-
bot (1985) to have influenced vegetation colonisation of
the dunefield and hence stabilised the dune surface. Resid-
ual dune ridges may develop as a result of the estab-
lishment of dune vegetation along a line partway up the
lower stoss slope of dunes during periods of higher wa-
ter table (Levin et al. , 2009). The causes of such water
table fluctuations could include the inundation of inter-
dune areas during fluvial flood events (Langford, 1989;
Purvis, 1991) or by seawater from lagoons at high tide
(Inman, Ewing and Corliss, 1966). Surface vegetation
may act locally to protect sediments from wind erosion,
while plant roots and intergranular surface tension due
to pore water may act to bind deeper sediments, result-
ing in the development of an arcuate ridge once adja-
cent dune deposits are deflated. The impacts of varia-
tions in groundwater salinity upon vegetation cover may
also influence dune field morphometry. Studies in White
Sands dune field have shown that barchans dunes are
(a)
(b)
Figure 16.10 Example of a contemporary wet aeolian system:
(a) linear dunes crossing a sabkha surface in the southern Rub
Al Khali close to the Oman/Saudi Arabia border, (b) close-up
of the interface between linear dune and sabkha sediments
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