Geoscience Reference
In-Depth Information
Table 3.1
Criteria for the identification of arid zone sediments.
Aeolian deposits
(a) Deposits may vary considerably in thickness
(b) Laminae dips range from 0 to 34
◦
(repose angle for sand) unless affected by post-depositional Earth movements. Angles
may be reduced by post-depositional compression
(c) Laminae bedding identifiable with structures in modern dunes (e.g. Ahlbrandt and Fryberger, 1982)
(d) Grain sizes range from coarse silt to coarse sand (c. 60-2000
µ
µ
m, majority in 125-300
m range
(e) Low silt and clay content (25 %, but see Table 3.2)
(f) Large particles often rounded, smaller are subrounded to subangular (see also Thomas, 1987a)
(g) May be cemented (aeolianite: Gardner, 1983) by haematite or calcium carbonate
(h) May be reddened (Gardner and Pye, 1981)
(i) Distinctive surface micromorphology when viewed using a scanning electron microscope (e.g. Krinsley and Doornkamp,
1973).
Water-lain deposits
(a) Commonly calcite cemented. Locally cemented by gypsum or anhydrite
(b) Conglomerates may be common
(c) Sand fraction may be absent - removed by deflation
(d) Mudflow conglomerates present
(e) Sharp upward decrease in grain size. Indicates rapid water-level fall
(f) Clay pellets, pebbles or flakes common. Due to effects of salt efflorescence (e.g. Bowler, 1986)
(g) Mud cracks common
(h) Often interbedded with aeolian deposits
Source:
Modified after Glennie (1987).
3.2.3
The Quaternary Period
these processes can be effectively parameterised to ensure
that the arid attribution is correct. Third, it cannot always
be assumed that all the features or sediments are not a
result of extreme events due to natural short-term climatic
variability rather than longer-term climatic change.
While geomorphological evidence is confined to the
land surface (though 'drowned' features have been iden-
tified in some coastal locations: see Fairbridge, 1964;
Sarnthein, 1972; Jennings, 1975), sedimentary data are
more wide-ranging. The great advances in Quaternary
science that have been forthcoming from the analysis of
ocean sedimentary cores (see, for example, discussions
in Imbrie and Imbrie, 1979) have also yielded valuable
information on fluvial and aeolian inputs into the oceans
from arid areas. On land, pollen and other biogenic ev-
idence contained within sediments suitable for preserva-
tion can provide key evidence on vegetation community
changes, including subtle shifts from arid to semi-arid
regimes (Thompson and Anderson, 2000; Scott, 2002).
Deserts have been described as 'remarkable repositories of
[Quaternary] palaeoclimatic information' (Williams
et al.
,
1993, p. 171). For the Quaternary Period, the sedimen-
tological record of arid zone dynamics is often comple-
mented by morphological evidence, in the form of features
such as degraded or fossilised sand dunes, inactive rock
varnished alluvial fans and palaeolake shorelines. The key,
therefore, for identifying past arid zones is the identifica-
tion of landforms/sediments diagnostic of arid conditions
during their formation/deposition in areas that are not arid
today. The converse applies for evidence of more humid
conditions in drylands during the past: non-arid features
found in today's arid zones. In recent decades, the identifi-
cation of such landforms, often poorly defined or masked
by vegetation on the ground, has been greatly enhanced
by the availability of high-quality remotely sensed data.
However, while in principle this approach to environmen-
tal reconstruction sounds seductively simple, in practice
this is far from the case, for three main reasons. First,
it relies on the assumption that process domains that are
distinctively arid can be identified, and that this can be
recognised from the resultant landforms and sediments.
3.2.4
Sedimentary records
