DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
mm
10
8
6
4
2
0
mean daily evaporation
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
month
Walgett Bourke
Figure 2.14 Mean daily evaporation at Walgett and Bourke
(Bureau of Meteorology 2001)
2.3 GEOLOGY, TOPOGRAPHY AND SOILS
2.3.1 Geological history
The underlying lithology of the bioregion is almost entirely formed from transported
sediments. Cainozoic (Quaternary) alluvium is found in 87% of the area and the Cainozoic
lacustrine sediments and Cainozoic residual and aeolian sands are present in another 10% of
the region. The lacustrine sediments are found along the lower reaches of the Macquarie and
Castlereagh Rivers, and the aeolian sands are along the lower reaches of the Darling River
with large areas associated with the Menindee overflow lakes. The broad lithology of the
bioregion has been mapped (Map 16) and there has also been more detailed geological
mapping of some portions of the bioregion.
The Great Artesian Basin underlies the DRP bioregion, and this is divided into a series of
smaller basins including the Surat and Euromanga Basins. The Great Artesian Basin consists
of extensive Quaternary alluvial deposits associated with the Darling River. In a broader
context, the entire Barwon-Darling basin can be described as a large intercratonic Cainozoic
basin, which has been infilled with sediments derived mainly from continental alluvium
(Thoms et al. 1995).
In two areas the lithology reflects the underlying geology. In the north of the bioregion the
coarse grained Cretaceous Surat Basin sandstone has not been covered by the more recent
alluvium, forming the Lightning Ridge and the Collarenebri Interfluves. In areas of rolling
downs and lowlands between the Culgoa and Barwon Rivers there are ridges and residuals of
various rock types including conglomerate, sandstone, claystone, shale, siltstone, quartzite
and granite.
The geological history of the Walgett and Nyngan 1:250 000 map sheets, covering an area
from Walgett in the north, south to Trangie, west to Girilambone and east to Coonamble, has
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