Environmental Engineering Reference
In-Depth Information
FIGURE 1.16
Photograph of the outback in central Australia near Alice Springs. (Courtesy of Robert H. Webb.)
The fundamental base for much of the flat or gently undulating desert landscape is
Cretaceous; most of the macroforms such as plateaus and mesas and structural features
such as the larger lake depressions are Tertiary; while mesoforms like sand dunes, prior
stream formations, and the many small playas are Pleistocene (Figure 1.16). The Holocene
has had little bearing on the deserts of today, apart from European man's contribution to
the degeneration of ecosystems on some semiarid/arid margins. Useful general reviews of
the distinctive landscapes of the Australian Deserts are provided by Mabbutt. 90
The impact of post-Tertiary climates can be appreciated only within the context of plate
tectonics and continental drift. Around 50-60 million year BP, Australia began to drift apart
from Antarctica, and it migrated northward, drifting to within 4° of its present latitude by
the early Miocene (c. 25 million year BP). Thus, not only has Australia been subjected to
climatic changes resulting from changes in latitude, but it has also been subjected to the
climatic effects that such movement had on the nature of oceanic and climatic circulation
systems in the Southern Hemisphere and to the global climatic changes associated with
the so-called Cenozoic climatic decline.
Among the important relict Tertiary features of the Australian desert are the widespread
duricrusts 91 that include silcretes 92 and laterites with associated deep weathering profiles.
There are also widespread relict palaeodrainage systems on vast erosion surfaces, 93 and
there was a virtual inland sea in the Lake Eyre depression.
However, in the late Miocene-Pliocene, there was a gradual transition to a more arid climate.
There is, however, scant evidence about the nature of climates for much of the Pleistocene,
and it is only for the past 40,000-50,000 years that there is much information. The importance
of major Pleistocene climatic and hydrological changes is clearly evident in the riverine plains
of the east of the arid zone, which are part of the Murray system. 94 There are large spreads
of alluvium deposited by ancient “prior streams,” which were sinuous bedload channels
indicative of coarse sediment transport in flash floods. These plains are mantled by aeolian
clays, called parna , and the active floodplains are slightly entrenched beneath the prior stream
deposits and are occupied by meandering, suspended-load channels. Cooper Creek, which
flows into Lake Eyre, shows a comparably varied history in the late Pleistocene. 95
The widespread sand deserts of Australia also display the impact of Pleistocene
changes, 96 and a striking feature of the linear dune fields of the Australian deserts is that
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