Geoscience Reference
In-Depth Information
20-14 ka, during times of late Pleistocene flood discharges that were at least season-
ally much higher than they are today (Page et al.,
1991
; Page and Nanson,
1996
;Page
et al.,
1996
;Ogdenetal.,
2001
;Pageetal.,
2001
;Bowleretal.,
2006
).
We saw in
Chapter 10
that there were close links between late Pleistocene snow-
melt and river discharge and sediment deposition in south-east Australia (Williams
et al.,
2009b
). Barrows et al. (
2001
) obtained
10
Be cosmogenic nuclide ages of 32
±
1.4 ka for the three youngest glacial advances in the
semi-arid Snowy Mountains of south-east Australia and ages between 23 and 16 ka
for periglacial deposits in that region. A combination of sparse vegetation in late
glacial times, together with extensive slope mantles formed by periglacial solifluc-
tion, would have meant that there was an abundant supply of coarse debris to rivers
during the spring snow-melt. Further north beyond the limits of glacial and periglacial
processes, the sparse plant cover would have contributed to high rates of run-off and
a coarse sediment load, as evident in the large channels out on the alluvial plains
(Williams,
1984e
; Williams,
2000
b; Williams,
2001a
). An additional factor contrib-
uting to high LGM run-off was the influence of lower temperatures in the uplands,
which could have at least doubled the run-off coefficient in the Snowy Mountains
(Reinfelds et al.,
2014
). Once the climate became warmer and wetter during the very
late Pleistocene and early Holocene, the plant cover became denser and soils began to
form, leading to a change from bed-load to suspension-load channels. The decline in
precipitation over the past 5 ka led to progressively smaller channels (Williams et al.,
2009b
).
There is also a direct association between Pleistocene river activity and source-
bordering dune formation in central and south-east Australia (
Chapter 8
). Source-
bordering dunes are common in Australian deserts wherever there is a regular supply
of alluvial sand transported by seasonally flowing streams (Wasson,
1976
;Bowler,
1978a
;Bowler,
1978b
; Williams et al.,
1991a
; Nanson et al.,
1995
;Pageetal.,
2001
;
Maroulis et al.,
2007
; Cohen et al.,
2010a
). There appear to be three prerequisites for
the formation of fluvial source-bordering dunes. A regular supply of bed-load sands
brought in by rivers that dry out seasonally, leaving their sandy point-bars and sandy
channel beds exposed to deflation, is the first requirement. Strong, unidirectional
winds are needed to move the channel sands and form a linear, or parabolic, dune.
Sparse or absent riparian vegetation is needed for unimpeded sand movement out
of the channel through deflation. The first condition is the most important, because
without regular replenishment of the alluvial sand supply, the dunes will be unable to
develop and propagate downwind.
Cohen et al. (
2010a
) have established a comprehensive chronology of fluvial and
eolian sediments in the lower Cooper Creek region of central Australia based on fifty-
seven TL and six OSL age estimates. Cooper Creek originates from the confluence
of the Thompson and Barcoo rivers, both of which rise in the Eastern Highlands of
Australia, where they are fed by tropical summer rainfall. The Cooper then flows
2.5, 19.1
±
1.6 and 16.8
±
