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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.

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