Dealing with salinity in Wheatbelt Valleys - Department of Water
Dealing with salinity in Wheatbelt Valleys - Department of Water
Dealing with salinity in Wheatbelt Valleys - Department of Water
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Commander, Schoknecht, Verboom and Caccetta<br />
Figure 3: Solid geology <strong>of</strong> the Agricultural Area (after Myers and Hock<strong>in</strong>g 1998)<br />
F<strong>in</strong>kl & Fairbridge (1979) have suggested that<br />
peneplanation <strong>of</strong> the Yilgarn Craton was completed<br />
<strong>in</strong> the Proterozoic, and that denudation rates were<br />
m<strong>in</strong>imal ever s<strong>in</strong>ce. They argued for a western<br />
source for the Permian-Cretaceous Perth Bas<strong>in</strong><br />
sediments (Fairbridge & F<strong>in</strong>kl, 1978), though this was<br />
challenged by van de Graaff (1981) who argued for<br />
removal <strong>of</strong> some 500 m <strong>of</strong> rock from the Yilgarn<br />
Craton s<strong>in</strong>ce the Jurassic. However, it has recently<br />
been found that the Archaean provides only a m<strong>in</strong>or<br />
source <strong>of</strong> detrital zircons to Quaternary m<strong>in</strong>eral sand<br />
deposits <strong>in</strong> the Perth Bas<strong>in</strong> (Sircombe & Freeman<br />
1999). This strongly implies a western source for<br />
Perth Bas<strong>in</strong> sediments from Proterozoic rocks, and<br />
supports the suggestion <strong>of</strong> m<strong>in</strong>imal erosion <strong>of</strong> the<br />
Yilgarn Craton at least s<strong>in</strong>ce the Jurassic/Cretaceous.<br />
Thus the landscape appears to have been moderately<br />
stable s<strong>in</strong>ce the age <strong>of</strong> the d<strong>in</strong>osaurs.<br />
Most authors now agree that peneplanation was<br />
complete by the end <strong>of</strong> the Creatceous. Twidale<br />
– 5 –<br />
(1994) postulates that the ‘new plateau’ <strong>of</strong> Jutson<br />
(1934) <strong>in</strong> the eastern Yilgarn must also be <strong>of</strong> Eocene<br />
age, on the basis <strong>of</strong> the Eocene sediments there, and<br />
that the ‘older plateau’ is even older.<br />
Dra<strong>in</strong>age<br />
The oldest dra<strong>in</strong>age systems now preserved are<br />
likely to be <strong>in</strong> the zone <strong>of</strong> ancient dra<strong>in</strong>age (Figure 1)<br />
conta<strong>in</strong><strong>in</strong>g the P<strong>in</strong>grup, Lockhart and Camm valleys,<br />
between the median and the central watersheds,<br />
recently recognised by Beard (1998; 1999). The<br />
region may have been traversed by palaeoriver<br />
systems dra<strong>in</strong><strong>in</strong>g Antarctica when the cont<strong>in</strong>ents<br />
were jo<strong>in</strong>ed (Figure 4), and Beard (1999) has<br />
suggested that these rivers may have orig<strong>in</strong>ally<br />
dra<strong>in</strong>ed southwards (Figure 5), follow<strong>in</strong>g the open<strong>in</strong>g<br />
<strong>of</strong> a seaway between Australia and Antarctica <strong>in</strong> the<br />
Jurassic 150 million years ago (Table 2). It may be<br />
significant that these valleys follow the NNW<br />
structural trend <strong>in</strong> the basement rocks.