Geoscience Reference
In-Depth Information
(a)
Wind
(b)
Groundwater
table
Groundwater table
deflation surface
Unconformities created
by earlier deflationary
episodes
Figure 16.11
The formation of flat bedding planes by aeolian deflation to the level of the groundwater table. (a) Bedform
climbing leads to the accumulation of sand in trough cross-beds. (b) Subsequent deflation to the water table occurs due to either
a change in wind regime or a decrease in sand supply, with eroded sand deposited far downwind (after Loope, 1984).
most prevalent above areas of saline groundwater, while
parabolic dunes typify topographic highs that accumulate
a lens of fresher precipitation-derived water and hence
develop a partial vegetation cover (Langford, Rose and
White, 2009).
In addition to impacting upon contemporary sediment
erosion and deposition, the presence of a high water table
may also exert a major influence upon the longer-term
accumulation and preservation of aeolian sediments. In
so-called
wet aeolian systems
(e.g. Figure 16.10), the ef-
fect of a high water table is to reduce erosion in interdune
areas and near the base of dunes, where they lie within
the capillary fringe. Long periods of water table stability
may lead to the formation of a first-order bounding sur-
face (see Figure 16.11). However, if the water table rises
over time, e.g. due to increased precipitation, reduced
evaporation, land/basin subsidence or sea-level rise, both
dune and interdune sediments will progressively accu-
mulate, with interdune flats growing at the expense of
dunes. In general, the vertical sediment accumulation rate
will equal the rate of water table rise (Reading, 1996),
to deposition being dominated by water- rather than wind-
laid deposits (Paim and Scherer (2007). There is some
debate over whether extensive planar first-order bound-
ing surfaces (Brookfield, 1977) forming bedding planes
in ancient aeolian sandstones were all generated in this
way (e.g. Rubin and Hunter, 1982, 1984; Loope, 1984;
Kocurek, 1981a, 1981b, 1984, 1988, 1991). The original
concept of a water table acting as a control of deflation as
put forward by Stokes (1968) is now viewed as one pos-
sible scenario for the creation of bounding surfaces, with
interdune migration and bedform climbing (e.g. Rubin
and Hunter, 1982) viewed as less site-specific alternatives
(Mader and Yardley, 1985).
Despite these reservations, there is widespread evi-
dence for sediment cohesion, cementation and salt en-
crustation associated with former water tables preserved
within the sedimentary architecture of ancient aeolian
sandstones (e.g. Bromley, 1992; Chan and Kocurek, 1988;
Gaylord, 1990; Crabaugh and Kocurek, 1993; Loope,
1985, 1988). Units of the Permian age Cedar Mesa Sand-
stone in southeast Utah, USA, for example, represent an
