Mulga Lands - landform
Only a few areas of Palaeozoic bedrock are found in the bioregion, where resistant quartz sandstones emerge from the Cretaceous and Quaternary blankets of sediment. These form low rounded isolated ranges and hills such as Mt Pleasant and could be considered as outliers of the Cobar Peneplain Bioregion.
Geology and geomorphology
The Mulga Lands Bioregion is dominated by horizontal Cretaceous sandstones and claystones deposited in an inland sea about 100 million years ago. These sediments vary in thickness across the basement rocks and they form the main water-bearing strata of the Great Australian Basin.
Artesian mound springs from these aquifers have forced through overlying strata but most are now inactive, an important exception being Perry springs. Many parts of the surface sandstones and more recent sands were silicified during the Tertiary to form silcrete, a tough fine-grained quartzite. These rocks often contain plant fossil remains indicating much wetter environments than occur at present.
Silcrete pebbles and boulders mantle the landscape as gibber and massive silcrete often forms the low cliff lines on plateau and tableland margins. The Cretaceous sediments have been gently folded and have small offset faults that are probably important in controlling the distribution of plateaus.
Alluvial fans of the Paroo and Warrego river systems have deposited sands and clays between the plateau areas of sandstones. Under today's rainfall these streams do not often flow into the Darling, and the clay pans toward the end of the fans are often saline, suggesting that groundwater may have an influence on them.
Sand has been blown from the alluvial sediments and probably from Cretaceous sandstones to form extensive sandplains with limited areas of dunefield that encroach on the edges of the high ground. Elsewhere, the alluvial systems are dominated by grey and brown cracking clays in channels and claypans. Lakes and swamps are normally dry but all wetland systems are very important ecologically in wet years.
Important features of the bioregion include the following:
- extensive ephemeral wetland systems;
- fans of both the Warrego and Paroo - excellent examples of Quaternary riverine landforms preserved in a semi-arid zone;
- Peery Springs, the last flowing mound spring in the NSW section of the Great Australian Basin;
- Lake Burkanoko and Lake Nichebulka, east of Wanaaring, both good examples of inland hypersaline lakes;
- the Paroo River - the last free-flowing tributary of the Darling, with no water control structures and no catchment cultivation;
- the White Cliffs and Gemville opal fields, the first commercial fields in the world which contain numerous heritage elements associated with mining and are still producing opal; and
- silcrete, which was widely used as a stone tool resource by Aboriginal people, so it is likely that the region contains numerous quarry sites.
The plateau and tablelands typically have shallow, stony, red-brown loams. These merge downslope, often through patterns of contour banding into brown texture contrast soils. Silcrete gibber is widespread as a surface lag gravel. In the sandplain areas, red earths and red siliceous sands are the norm but the colour is not as bright as in the larger dunes of the Simpson-Strzelecki Bioregion.
Grey and brown cracking clays dominate the alluvial sequences with limited areas of sand deposition often reworked into source bordering sand sheets or low dunes. Most soils contain calcium carbonate in their sub-soil and clays in the claypans. Lake beds at the far end of the alluvial fans are often saline and contain gypsum.
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Page last updated: 27 February 2011