Geoscience Reference
In-Depth Information
infiltration and reduce run-off, leading to an increase in base flow along the valley
floors. Weaker summer monsoon precipitation would mean fewer extreme erosive
events, allowing fine-grained dust mantles to be gradually removed from the slopes
and to accumulate along the valley floors. A lower winter cloud base and gentle
winter rains would facilitate progressive deposition of the reworked eolian clays and
development of the late Pleistocene valley fills. The LGMwas not uniformly arid, and
at one locality in the central Flinders Ranges, up to twenty individual flood events are
preserved as slack-water deposits (Haberlah et al.,
2010a
; Haberlah et al.,
2010b
).
There was a rapid change in environmental conditions at around 17-16 ka, with
cessation of valley-fill formation and widespread vertical incision down to bedrock,
followed by gully erosion that continues to this day. Singh and Luly (
1991
) analysed
pollen fromLake Frome. They found that the treeless conditions of the LGMgave way
to a flora dominated by
Eucalyptus
and
Callitris
, followed after 17 ka by a decline in
Callitris
and a more gradual decline of Chenopodiaceae. That these events were part
of a global change in climate is suggested by the apparently synchronous retreat of
mid-latitude mountain glaciers in both hemispheres from 17 ka onwards, which have
been attributed to synchronous warming in those latitudes (Schaefer et al.,
2006
). Blue
Lake in the Snowy Mountains was ice-free by 15.8 ka, indicating rapid deglaciation
(Barrows et al.,
2001
).
22.6.5 Rivers and source-bordering dunes
The longest rivers flowing in Australia today are those like the Murray-Darling that
rise in the Eastern Highlands and flow west or south-west across semi-arid plains,
where much of their flow is lost in seepage and to evaporation during floods. In the
south, snow-melt is an important contributor to these rivers, but in the north, they
depend on summer rainfall. In the aptly named Riverine Plains in semi-arid south-
east Australia, there is a series of large former channels that range from large and
sinuous to large and straight (
Chapter 10
,
Figure 10.9
). In a classic study of the
former channels in the lower Murrumbidgee Valley, Schumm (
1968
) concluded that
the large, sinuous suspension-load channels had formed under a wetter climate with
bank-full discharges several times those of today. The linear channels contained a
coarse channel fill and fell into the category of bed-load channels. Schumm (
1968
)
concluded that they were active under a more seasonal flow regime characterised by
episodically very high discharge from more sparsely vegetated headwaters.
Bowler (
1978a
) expanded on this work and obtained a detailed radiocarbon chro-
nology from charcoal within channel fill sediments. He found that during the LGM
(21
2 ka), the rivers flowing from the Eastern Highlands were transporting and
depositing a coarse load of sand and gravel across the Riverine Plains until around
15 ka. With the advent of luminescence dating, it proved possible to obtain reli-
able ages for discrete phases of widespread fluvial aggradation at around 35-25 and
±
