Dealing with salinity in Wheatbelt Valleys - Department of Water

Commander, Schoknecht, Verboom and Caccetta
Figure 3: Solid geology of the Agricultural Area (after Myers and Hocking 1998)
Finkl & Fairbridge (1979) have suggested that
peneplanation of the Yilgarn Craton was completed
in the Proterozoic, and that denudation rates were
minimal ever since. They argued for a western
source for the Permian-Cretaceous Perth Basin
sediments (Fairbridge & Finkl, 1978), though this was
challenged by van de Graaff (1981) who argued for
removal of some 500 m of rock from the Yilgarn
Craton since the Jurassic. However, it has recently
been found that the Archaean provides only a minor
source of detrital zircons to Quaternary mineral sand
deposits in the Perth Basin (Sircombe & Freeman
1999). This strongly implies a western source for
Perth Basin sediments from Proterozoic rocks, and
supports the suggestion of minimal erosion of the
Yilgarn Craton at least since the Jurassic/Cretaceous.
Thus the landscape appears to have been moderately
stable since the age of the dinosaurs.
Most authors now agree that peneplanation was
complete by the end of the Creatceous. Twidale
– 5 –
(1994) postulates that the ‘new plateau’ of Jutson
(1934) in the eastern Yilgarn must also be of Eocene
age, on the basis of the Eocene sediments there, and
that the ‘older plateau’ is even older.
Drainage
The oldest drainage systems now preserved are
likely to be in the zone of ancient drainage (Figure 1)
containing the Pingrup, Lockhart and Camm valleys,
between the median and the central watersheds,
recently recognised by Beard (1998; 1999). The
region may have been traversed by palaeoriver
systems draining Antarctica when the continents
were joined (Figure 4), and Beard (1999) has
suggested that these rivers may have originally
drained southwards (Figure 5), following the opening
of a seaway between Australia and Antarctica in the
Jurassic 150 million years ago (Table 2). It may be
significant that these valleys follow the NNW
structural trend in the basement rocks.