Geoscience Reference
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of South Australia. These compare with mean rates of 35 m Ma
−
1
from soil-mantled,
transport-limited spurs in the humid south-east of the continent.
Directly measured annual rates of slopewash on undisturbed granite and sandstone
slopes in the seasonally wet tropics of northern Australia were 54
±
40 m
3
km
−
2
on
30 m
3
km
−
2
on sandstone (Williams,
1973a
). These rates would
amount to an average rate of about 55
±
granite and 56
35 mMa
−
1
for both rock types if extrapolated
over longer time scales, but the climate and plant cover would of course have varied
during that time, as would rates of slope erosion. Slope lowering by slopewash was
on average five times more rapid than that effected by soil creep, and slope lowering
was twice as fast on the colluvial-alluvial sandstone foot-slopes than it was on the
rocky, weathering-limited hill slopes (Williams,
1973a
; Williams,
1976b
).
The key lesson to be drawn from studies of erosion rates is that even resistant
uplands in the arid interior are unlikely to retain their original form, unless they have
been buried beneath thick layers of sedimentary rocks and have only recently been
exhumed (Belton et al.,
2004
). A case in point is the recent emergence of the Lower
Proterozoic hills and valleys from beneath the Upper Proterozoic sandstones of the
west Arnhem Land escarpment (Williams,
1991
).
Other widespread erosional landforms include the extensive plateaux developed
on horizontal Mesozoic sandstones and siltstones, many of which have resistant
duricrust cappings of silcrete or ferricrete (see
Chapter 15
). Because the surfaces of
these plateaux are covered in a thin layer of silcrete or ferricrete pebbles, they are
known locally as stony tablelands, and they are the geomorphic equivalent of the
hamada
of the Sahara and Near East and the
gobi
plains of Mongolia.
The depositional landforms are a great deal younger and consist of dunes and
sand plains, lakes and playas, and drainage systems ranging from long-defunct to
ephemeral or seasonal. Associated with the lakes or former lakes are crescent-shaped
clay dunes, or lunettes, and source-bordering dunes are a feature of many seasonal
river channels.
±
22.4 Mesozoic and Cenozoic tectonism and volcanism
The desiccation of the Australian continent is closely related to its tectonic history
(Veevers,
1984
; Williams,
1984
d; Veevers,
2000a
;Veevers,
2000b
;McGowranetal.,
2004
; Fujioka and Chappell,
2010
; Quigley et al.,
2010a
; Blewett,
2012
). Until about
100 million years ago, in the early Cretaceous, Australia was largely submerged
beneath a warm, shallow ocean, and it mainly consisted of three large islands in the
west, north and east. The western island included the Archaean and Precambrian
shield areas of western Australia. The rocks forming the northern island and a few
central uplands ranged in age from Proterozoic to Palaeozoic, and the eastern island
consisted primarily of Palaeozoic rocks. The hills and valleys of these uplands have