Cobar Peneplain - landform
The Cobar Peneplain Bioregion is a subdued bedrock-controlled landscape in the centre of semi-arid NSW. Described as a low undulating plain, the Cobar Peneplain is easily distinguished from most of the surrounding bioregions which are relatively flatter landscapes of floodplains (Riverina and Darling Riverine Plains bioregions) and sandplains and dunefields (Murray Darling Depression Bioregion).
The Cobar Peneplain is a prominent topographical landscape of rolling downs and flat plains punctuated by stony ridges and ranges and is formed on the northwesterly extension of the Lachlan Fold Belt. The more elevated areas of the Cobar Peneplain are characterised by shallow, red soils and aeolian sands associated with the Darling River and the Murray Basin mantle in the lower areas in the west and south, while alluvial deposits from the Bogan River fringe the Peneplain in the east.
Geology and geomorphology
The Cobar Peneplain bioregion is based on Palaeozoic rocks largely within the Lachlan Fold Belt. It is lapped by the Murray Basin and the Great Australian Basin and although it is described as a peneplain, the implications attached to this word concerning tectonic stability, landscape and soil genesis should not be uncritically accepted.
The region contains a wide range of bedrock types that exert a strong influence on topography. Rock outcrops form low ranges or lines of residual hills controlled by structure (bedding, folds and faults). Rocks in the eastern half of the bioregion are older (Ordovician), more deformed and more highly mineralised than those in the west (Devonian), although the dominant structural trends in both are northwest.
Quartz sandstones, conglomerates and siltstones with low angle folds are typical of the younger rocks, and these form prominent multiple ridges like the ranges at Mt Grenfell up to 300m high, or the more complex folds seen in the Cocoparra Ranges near Griffith. Topography on the older rocks around Cobar is more subdued as residual hills, low rounded ridges, and stony slopes formed on softer, more weathered shales, phyllites and cherts, with only occasional features such as Mt Boppy standing as much as 100 m above the plain.
Igneous rocks are more common in the southern part of the region and granites north of Nymagee make attractive landscapes of rugged peaks and tors. Very small areas of basalt lava are found from Griffith to north of Cobar, with the most interesting being the rare example of 10-16 million year old leucitite lava capping the tabletop peak of El' Capitan.
During the Tertiary and possibly as recently as 5 million years ago, marine sediments were deposited in the Murray Basin with the coastline being the southwestern edge of the Cobar Peneplain. In the Quaternary, after these shallow seas receded, sands were mobilised by wind to form dunes and sandplains that advanced onto the peneplain. A drainage system of wide shallow valleys with a few lakes also developed despite the low rainfall and low gradients.
Today the creeks respond to local rainfall but only occasionally deliver water to the Bogan or Darling Rivers.
Significant landscape features include the following:
- the Gunderbooka Range and the Mt Grenfell Ranges are good examples of structural and lithological control of topography. They are also important as sites of cultural and archaeological significance to Aboriginal people;
- Gunderbooka and other peaks stand in isolation on the plains and are likely to have high local endemism in the biota;
- the downs and plains are ecologically distinctive with their apparent uniformity, absence of surface water and well-developed biotic patterning of mulga groves caused by interactions between topography, runoff, soil and vegetation; and
- important mining heritage is present at a number of locations, particularly Cobar, which has been the state's largest copper producer, as well as smaller mines at Nymagee, Mt Hope, Canbelego, Mineral Hill, Tottenham, Ardlethan and elsewhere.
Special geological features include:
- leucitite lavas of El Capitan;
- columnar lavas of the Ambone Volcanics;
- Devonian freshwater fish fossils in the Mt Grenfell area; and
- archaeology and palaeo-environmental potential of Barnato Lakes.
Soils across the bioregion are reasonably uniform and relate closely to topographic position and local geology. On ridge crests they are thin, stony, well-drained red loams. Downslope the soil thickens as a colluvial mantle, usually with a large proportion of stones and with an increasing texture contrast between topsoil and subsoil.
On lower slopes the stoniness decreases, red subsoils give way to yellow subsoils, carbonate levels increase and soil drainage is more impeded. Brown clays are more common than grey clays in drainage lines, red sands and earthy sands are widespread but there are only a few areas of sandplain and dunefield.
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Page last updated: 27 February 2011