South Eastern Highlands - landform
The South Eastern Highlands Bioregion covers the dissected ranges and plateau of the Great Dividing Range that are topographically lower than the Australian Alps, which lie to the southwest. It extends to the Great Escarpment in the east and to the western slopes of the inland drainage basins. The bioregion continues into Victoria. The substrate is formed of Palaeozoic granites, metamorphosed sedimentary rocks and Tertiary basalts.
Geology and geomorphology
The highlands are part of the Lachlan fold belt that runs through the eastern states as a complex series of metamorphosed Ordovician to Devonian sandstones, shales and volcanic rocks intruded by numerous granite bodies and deformed by four episodes of folding, faulting and uplift. The general structural trend in this bioregion is north-south and the topography strongly reflects this. There are four centres of Tertiary basalt flows.
The oldest rocks are a small sliver of the Early Ordovician serpentinite running from Gundagai past Tumut into the lower Snowy Mountains. These unusual rocks were formed in deep marine conditions and were plastered against the edge of Australia when an area of sea floor and an island arc closed up. A similar sequence is found at Lucknow, about 9km south-east of Orange.
The largest island arc environment is the late Ordovician Molong Volcanic Arc that extends from the northern end of the bioregion to Kiandra. This contains mixed sediment deposited from massive submarine landslides (turbidites) interbedded with quartz sandstone and basaltic tuffs. The sequence is intruded by gabbro, dolerites and later granites. Most granite bodies are oriented parallel to the general north-south structural trend, but the youngest bodies, like the Bathurst granite (about 325 million years old) cut across this trend.
In the Devonian, the region was open sea accumulating fine sediment now represented by shales, sandstone and volcanic sediments in a series of parallel troughs such as at Tumut, Hill End, and from Captains Flat to Goulburn. The whole rock sequence is highly mineralised and contains many large base metal and gold deposits of economic importance.
In the Tertiary, volcanic activity was widespread and there are large areas of associated river sands and gravels in the mid-Shoalhaven valley. Canobolas was a central volcano 50 km in diameter now eroded to reveal more than 50 remnant vents, plugs, dykes, and trachyte domes.
The largest lava fields are found on the Monaro where 65 eruption centres have been identified. These flows are very thin and are interbedded with river and lake sediments. They have been dated as 34-55 million years old. In the Snowy Mountains an inverted relief pattern of 18-20 million year old hill top flows burying river gravels was worked for gold at Kiandra. Similar flows preserving old valleys are found at Crookwell, Abercrombie, Nerriga and in the Macquarie Valley.
Topographically, the dominant features of the bioregion are plateau remnants, granite basins with prominent ridges formed on contact metamorphic rocks and the western ramp grading to the South Western Slopes. Streams cutting through the bioregion are deeply entrenched with only a few terrace features. Valleys are narrow and there is little Quaternary sediment except in the numerous lake basins of the Monaro province.
There are numerous localities exposing structural features in the bedrock, such as the following:
- state circle faults in Canberra;
- complex folds in a railway cutting at Captains Flat;
- the mid-Devonian cauldron subsidence of the Bindook porphyry complex;
- mega debris flow blocks up to 1 km wide of Nubrigyn limestones at Molong; and
- the entire Canobolas volcanic field with numerous plugs and dykes.
The bioregion also holds important examples of rare rock types, such as:
- serpentinites in the Gundagai - Tumut belt and Lucknow; and
- a suite of metamorphic rocks associated with complex dioritic intrusions at Hartley.
Other significant features include the following:
- mine sites such as Adelong, Captains Flat, Burraga, Cadia, Sunny Corner and Hill End and their associated heritage;
- karst systems, deep limestone gorges and fossil sites in limestone are also significant, for example, Yass, Abercrombie, Bungonia, Wombeyan, Jenolan, Borenore, Molong and elsewhere;
- important Quaternary vertebrate fossil sites occur in swamps on the Monaro plateau; and
- numerous shallow or dry lake basins with lunettes and fossil high water strand lines occur in the bioregion; the best known examples are Lake George and Lake Bathurst, but similar features are found all over the Monaro and as far north as Bathurst; research at Lake George has revealed a story of climate and environmental change well into the Quaternary and all such lakes have potential for similar research.
Soils vary across the bioregion in relation to altitude, temperature and rainfall. On the Palaeozoic slates, sandstones and volcanics, mottled red and yellow texture contrast soils, with red earths are found. On the granites, shallow red earths occur on ridges, yellow texture contrast soils on all slopes and deep coarse sands in alluvium. On Tertiary basalts, shallow red-brown to black stony loams exist, with alluvial loams and black clays in swampy valley floors. Limited areas of shallow organic loams are present at high altitude on Canobolas. Some of the tertiary sands in the mid-Shoalhaven deep have been worked into low dunes under a past climate and now have deep siliceous sand or yellow earth profiles.
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Page last updated: 18 April 2016