Geoscience Reference
In-Depth Information
attention. Until quite recently it was not known when dunes first appeared in Australia
and when and under what conditions they had been active since then. Pollen evidence
shows that the onset of climatic desiccation in central Australia dates back to the
mid-Miocene some 15 million years ago (15 Ma) (Martin,
2006
). The pollen data
are entirely consistent with the evidence from dated molecular phylogenies of diverse
Australian taxa indicating the greatest divergence of arid-adapted taxa around 15 Ma
ago (Byrne et al.,
2008
).
The oldest stony deserts (known in Australia as 'gibber plains') have yielded
cosmogenic nuclide ages of 4-2 Ma, possibly reflecting the onset of major Northern
Hemisphere cooling and ice cap development over North America around 2.6 Ma
(Fujioka et al.,
2005
). The earliest dunes did not appear until about a million years ago
(Fujioka et al.,
2009
), reflecting an accentuation of the trend towards extreme aridity
that was underway during the very late Pliocene and early Pleistocene (Fujioka and
Chappell,
2010
).
In the western Murray Basin of South Australia, Lomax et al. (
2011
) obtained
98 OSL ages from thirteen dune sections dating back to at least 380 ka. The ages
showed two major phases of dune sand deposition at 72-63 ka and 38-18 ka, with
some accretion at 14.5-13.5 ka, 12-11 ka and 8-5 ka. They concluded that although
the dune records were discontinuous and often hard to interpret in terms of climate,
high rates of dune deposition in this region tended to coincide with drier conditions,
and breaks in the dune depositional record with wetter conditions inferred from other
sources of evidence.
The distribution and orientation of the Australian desert dunes (
Figure 8.14
)have
been mapped with increasing accuracy over the past few decades using a combination
of ground surveys and air photos in earlier years (King,
1960
; Jennings,
1968
; Sprigg,
1979
; Wasson et al.,
1988
) and satellite imagery and large scale topographic maps
more recently (Hesse,
2010
). Hesse's (
2010
) remapping of desert dunes throughout
Australia has convincingly demonstrated that there is a strong topographic control
over the distribution of dune fields in Australia, with dunes preferentially occupying
major depositional centres, much as in the Sahara. For example, the Lake Eyre Basin,
which covers about one-seventh of the continent, is host to three major sand deserts:
the Simpson, Tirari and Strzelecki deserts.
The very striking linear dunes of the Simpson Desert have been the subject of
considerable fieldwork, and there has been a great deal of speculation as to how and
when they developed. Several models have been proposed to account for linear dune
formation in this region. Twidale (
1972
) concluded that the linear dunes of the Simpson
Desert originated from transverse sand mounds or lunettes located on the downwind
margin of widespread alluvial plains and playas. These source-bordering dunes then
progressed downwind as linear dunes aligned parallel to the dominant sand-moving
winds. He postulated that the dunes moved forwards by a process of downwind sand
accretion and that they were able to move across many hundreds of kilometres of
desert, given an adequate supply of sand. He observed that in many places the dunes
