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Broken Hill Complex - landform

Topography

The Broken Hill Complex Bioregion in western NSW is geologically unique in the state. The western half is composed of ancient basement rocks of the Adelaide Fold Belt, and the eastern half is the edge of the much younger rocks of the Tasman Fold Belt.

Many of the rocks and minerals found in the region are of considerable interest and economic importance, and geology exerts strong controls on the landscape.

Geology and geomorphology

The Barrier Ranges are a triangular block of metamorphic and deformed sedimentary rocks forming a series of northeast and northwest trending ridges rising up to 300 m above the surrounding plains. Bedrock includes schist and gneiss, intrusive granites, amphibolites and very coarse pegmatites.

Some of these rocks contain inherited minerals up to 2,600 million years old and the whole sequence was once part of the land mass of Rodinia that included parts of present-day North America and China.

Basement rocks are partly overlain by Cambrian and Devonian conglomerates, quartz sandstones, shales and thin beds of limestone. Scopes, Mootwingee and Wonnaminta Ranges are mainly Ordovician to Devonian conglomerates - sandstones and shales that have been only gently folded.

On the eastern margin of the Mootwingee and Wonnaminta Ranges, horizontally bedded sandstones of Cretaceous age form flat-topped mesas and tablelands that extend northeast into the Mulga Lands Bioregion.

The geomorphology of the ranges is controlled by the different rock types and their structure. Faults up to 60 km long form prominent scarps between hill country and plains. Hard rocks outcrop as strike ridges and remnant pinnacles standing above long shallow slopes, with thin soils developed on the softer rocks.

These rock cut slopes pass to wide footslopes and alluvial plains. Hills on granite are more rounded and subdued. Limestone and dolomites occur in limited areas as linear outcrops.

Streams have cut steep-sided gorges containing sheltered waterholes through the ranges. Beyond the footslopes the streams expand as alluvial fans, distributing sediment into sandy floodouts and clay playas.

Much of the drainage from the Barrier, Mootwingee and Wonnaminta ranges ends in the Bancannia trough, where shallow lakes and swamps have formed and alluvial sand has been blown into sandplains and dune fields.

Geodiversity

Important features include the following:

  • prominent landscapes such as the Mundi Mundi, Nundooka and Koonenberry fault scarps and adjacent plains, the Pinnacles, and the dip-slope escarpments and deep gorges of the Mootwingee Range;
  • the region contains more than 2,000 mineral locations, the most famous being the Broken Hill main lode, one of the world's largest silver, lead and zinc deposits. Sites include the remnants of the Broken Hill ore body and its mining history; silver deposits and mining history at Thackaringa; tin mines at Euriowie; and numerous rare rock types and mineral deposits such as the Triple Chance and Egebek pegmatites;
  • other heritage features are associated with particular geological features. The best examples are the important Aboriginal sites at Mootwingee, Sturts Meadows and Euriowie, where occupation was facilitated by water holes in protected gorges, and Aboriginal rock engravings have been made on selected sandstone faces;
  • In 1845 Charles Sturt erected a stone cairn on the top of Mt Poole when trapped by drought at Depot Glen. The boulders gathered from the mountain top are quartzite and silcrete that had also been used for thousands of years by Aboriginal people as raw material for artefacts.

Smaller features of note include:

  • contour banding of soils and vegetation on the stony downs country;
  • giant desiccation polygons on the plains south of Topar; and
  • palaeo-drainage patterns of lakes and former streams between Mootwingee and the Barrier Ranges.

Soils

Rock-weathering processes have been operating continuously in the region for more than 90 million years and a deep weathered mantle has formed across most of the landscape. Many slopes are mantled by gibber (rounded, silica-rich boulders) derived from the breakdown of silicified sediments (silcrete duricrusts).

Soils vary with parent materials. Shallow, stony soils are found between outcrops in the ranges where the runoff from rock surfaces is important in maintaining plants and animals through drought. Stony desert pavements are widespread and often form contoured patterns of soil and vegetation in downs country.

Soils in the depositional basin are deep red sands with uniform sandy profiles under dunes, and gradational profiles in the sandplains. Most soils have a moderate to high level of calcium carbonate in the profile. Heavy cracking clays in floodouts and on lake beds are often unvegetated because they contain high levels of gypsum and sometimes salt.

Abandoned shorelines and low lunettes attached to the eastern side of playas and lakes such as Lake Bancannia provide evidence of climate change over the past 30,000 to 100,000 years.

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Page last updated: 26 April 2016