Geoscience Reference
In-Depth Information
Table 3.2
Some possible differences between active and ancient aeolian sands.
Active sands
Ancient sands
95 % of particles are sand-sized
Bimodal or unimodal size distribution
Larger particles more rounded
Bedding structures are often present
Low carbonate content
Higher components of 'fines' due to subsequent
inputs or weathering
May be altered by post-depositional inputs/losses
Rounding may be reduced by post-depositional
chemical weathering
Structure destroyed by burrowing animals and plant
root growth
Carbonate contents increased through organic inputs
sedimentology' relies on the same uniformitarian princi-
ples that are applied in the interpretation of relict land-
forms, and suffers the same problems. Particularly im-
portant are the effects of post-depositional modification
(diagenesis), which can alter or masque important diag-
nostic characteristics.
Aeolian deposits in both marine and terrestrial contexts
can be identified by a range of sedimentary characteris-
tics (Table 3.2), though these may be altered significantly
after deposition. At Didwana in the Thar Desert, India,
for example, a
(Thomas, 1987c). This is because sedimentary character-
istics can be inherited from preceding phases of deposition
and reworking, or even from parent sediments. The same
applies to reddened dune sands, which have sometimes
been assumed to indicate aeolian stability and humidity
(see Gardner and Pye, 1981) or other aspects of dune
landscape history (Bullard and White, 2005). Therefore
aeolian attributes in a sediment do not necessarily indi-
cate that aeolian processes were the last mechanism of
deposition.
Evaporite deposits are an important component of many
arid and semi-arid basin environments (see Chapter 9) and
the coastal sebkhas of some arid regions (Glennie, 1987).
Lowestein and Hardie (1985) identified a three-stage cy-
cle of ephemeral-playa sedimentation that results in the
modification of a salt-crystal structure through dissolu-
tion and redeposition and the alternation of mud and salt
layers. The sequences they identify in modern playa sed-
iments are also identifiable in the sedimentary record and
have assisted in the environmental interpretation of playa
deposits preserved in the Quaternary record (see, for ex-
ample, Hunt and Mabey, 1966; Smith
et al.
, 1983).
Although the minerals within playa salt deposits are
somewhat dependent upon the chemistry of inflowing wa-
ters (Lowestein and Hardie, 1985), differences in evapor-
ite deposits can yield information about the degree of
salinity and hence evaporation rates during past peri-
ods of playa sedimentation (Ullman and McLoed, 1986;
Risacher and Fritz, 2000). Bromide and chloride con-
centrations may be particularly valuable in this respect
(Allison and Barnes, 1985; Ullman, 1985). Oxygen and
carbon isotope ratios from lacustrine sediments may also
yield useful information though applications may be
more restricted than from ocean sediments (Stuiver, 1970;
Gasse
et al.
, 1987; Liu
et al.
, 2009). Lowenstein
et al.
(1994) have analysed the fluid inclusions in halite from
a 15 m lake sediment core extracted from the Qaidam
20 m high section in aeolian sands, di-
agnosed by their sedimentary structures and sedimento-
logical characteristics, exposed in a canal cutting indicates
that multiple periods of aridity have occurred since 190ka,
the basal luminescence date Singhvi
et al.
, (2010). In the
United Arab Emirates, quarrying of mega sand dunes has
exposed the complex internal structure of dunes of over
60 m thickness. When subject to OSL dating, these reveal
the long, and punctuated, records of aeolian accumulation
that have occurred in the region during the late Quater-
nary (Figure 3.2). In other contexts, road cuttings (Figure
3.3) and occasional natural exposures reveal dune inter-
nal structures that aid record interpretations. However, in
many studies, lack of internal exposures means that sam-
pling for dating and analysis, via drilling (Figure 3.4),
is done 'blind', thus limiting interpretation purely to the
ages that are obtained by applying luminescence dating
(see Chapter 17) and the properties of the sediments them-
selves. The use of ground penetrating radar may, however,
in some contexts assist in overcoming this problem, as
it may reveal the structures with dune bodies (Bristow,
Lancaster and Duller, 2005).
Interpreting the environmental significance of sand
with aeolian attributes is not necessarily simple (e.g.
Fitzsimmons, Magee and Amos, 2009). In the Kalahari,
the sands found not only in conjunction with relict aeo-
lian landforms but also with those of lacustrine and fluvial
∼
