Geoscience Reference
In-Depth Information
become possible to extend age ranges back beyond that
of
14
C. In the arid zone context, the main challenges lie
in the dating of exposure age of surfaces and of fluvial,
lacustrine and aeolian sediments. Taking these in turn, the
exposure age of surfaces can be approached via:
to boreholes. Seismic profiling may add considerably to
the understanding of sedimentary basin architecture, but
such remotely sensed data yield only relative chronologi-
cal information.
1. Dating of varnish coatings (AMS
14
C) (Dorn, 1994).
2.5
Existing erosional and depositional
records in arid environments
2. Dating the exposure age of rock surfaces using cosmo-
genic isotopes (Gosse and Phillips, 2001; Cockburn
and Summerfield, 2004).
Given the issues associated with the application of the
range of dating techniques outlined above to arid zone
sequences and landforms, it is difficult to substantiate any
assumptions about the lengths of record of erosion or sed-
imentation in different tectonic settings. Longer records
are not necessarily associated with stable tectonic areas;
instead tectonic activity may play a key role in ensur-
ing continuity of sediment supply (erosion) and deposi-
tion. Data on existing lengths of record are summarised in
Table 2.4.
At the global level, pediments tend to be concentrated
on granitic terrains. The highest spatial concentration of
pediments is reported from the southwest USA, from
a basin and range area of interorogenic rifts (Cooke,
Warren and Goudie, 1993). The more stable cratonic ar-
eas of southern Africa and central Australia also feature
pediments, but it is the association with active volcanism
that has allowed the development rate of the pediments in
the Mojave Desert to be established (Dohrenwend, 1994).
At a more local level, a limited amount of information
exists on the slope angles of pediments in Arizona and
California in relation to proximity to faults. Data from
Cooke, Warren and Goudie (1993, Table 13.1c) indicate
that fault-associated pediments have slopes of 2
◦
55
±
1
◦
12
, whereas those not associated with faults had slopes
of 2
◦
10
±
3. Dating of palaeosurfaces - sealed by lava flows (dated
using K/Ar or
40
Ar/
39
Ar) (Dohrenwend, 1994).
The dating of sedimentary deposits (fluvial, lacustrine,
aeolian), which is also critical in understanding the role of
climate change in the development of dryland landscapes
(Chapter 3), may involve the application of the following
techniques:
14
C dating of organic carbon or inorganic carbonates.
1.
2. Uranium-series dating of inorganic carbonates.
36
Cl dating of evaporites (within lacustrine sequences)
(e.g. Jannick
et al.
, 1991).
3.
36
Cl for
4. Cosmogenic dating of sediment clasts using
10
Be for quartz or chert (e.g. Matmon
carbonates and
et al.
, 2009).
5. Dating of component sediment grains using lumines-
cence techniques, provided the sediment grains were
exposed to light, prior to deposition and burial (e.g.
Bristow, Duller and Lancaster, 2007; Sohn
et al.
, 2007),
or have been heated to high temperature, e.g. in hearths
or by proximity to lava flows (see Box 17.1 in Chapter
17).
48
. Although difference in the means is statis-
tically significant at the 0.01 % level, the problem is that,
in physical terms, mean slope angles are very similar and
there is significant overlap of the standard deviations.
6. Conventional K/Ar,
40
Ar/
39
Ar or fission-track dating
can be used in contexts in which volcanic ashes are
intercalated with fluvial or lacustrine sequences.
2.5.1
Drainage patterns and fluvial systems
There is considerable evidence of the long-term impact
of tectonic activity over time periods of 10
6
-10
7
yr on
patterns of drainage in contemporary drylands. Frostick
and Steel (1993b) discuss the development of patterns of
radial drainage in southern Africa in response to doming
of the crust at a regional scale. The development of rift
systems may also result in the reorganisation of drainage
patterns, with some surprising results. In their model for
In addition to the techniques listed above, fission-track
thermochronology opens up the possibility of determining
long-term rates of uplift and denudation (Gleadow and
Brown, 2000).
Access to potentially datable material may be diffi-
cult and, depending on the depositional environment, it is
often limited to natural cuttings and sections, sometimes
