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Nandewar - landform

Topography

The Nandewar Bioregion is formed on Palaeozoic sedimentary rocks on the western edge of the New England Tablelands and includes the Tertiary basalts of Inverell and Kaputar. The hilly landscapes are warmer but drier than the tablelands and carry vegetation communities more typical of the western slopes, with some tableland species.

Geology and geomorphology

The New England Fold Belt is the youngest structural feature in NSW and is separated from the Lachlan Fold Belt by the Sydney-Bowen Basin that is filled with Mesozoic sediments. The oldest rocks in the sequence are Devonian sedimentary and volcanic rocks, formed in an island arc environment. The youngest are Triassic sandstones and shales deposited by rivers on the edge of the Gunnedah Basin, about 250 million years ago, at a time when New England was being lifted by intrusions of granite.

Major volcanic eruptions occurred in two phases: in the lava field flood basalts of the Inverell area (34-32 and 22-19 millions of years ago), and in a central volcano similar to that in the Nandewar Ranges (21-17 million years ago). The maximum preserved thickness of the flows is 800m in the variety of lavas present. Only the core of the Nandewar volcano remains as exposed plugs and dykes. Flows from the New England centres buried river gravels and lake sediments that are now being exposed and contain deposits of tin, sapphires and diamonds.

A narrow strip of ultrabasic rocks, including serpentinites that are derived from a deep ocean floor, marks the suture where a former island arc complex was linked to the Australian mainland. These rocks pass through Woodsreef and Tamworth where they are associated with limestones in which karst landscapes are formed. The composition of these rocks is so unusual that they always have distinct soils and vegetation.

Geomorphically, the western slopes can be seen as a dissected ramp that links the uplifted highlands with the western plains. Western rivers pass across the ramp without depositing large volumes of sediment and the Darling Riverine Plains alluvial fans begin at the base of the ramp.

Geodiversity

The broad geologic features of these environments can be seen in other parts of the western slopes and Great Dividing Range but particular features of note include the following:

  • the volcanic landforms of Mt Kaputar and the cold-tolerant vegetation found on it;
  • the karst landscape features at Ashford Caves and near Tamworth;
  • rare geology of the serpentinites with unusual mineralisation, soils and vegetation, including heritage elements of a former asbestos mine at Woods Reef;
  • sub-basaltic drainage patterns in the Inverell basalts, the occurrence of leaf fossils, gemstones and tin and the associated mining heritage; and
  • the granite tor topography and deep gorge of the Severn River near Ashford.

Soils

The bioregion is characterised by clay or loam soils, but siliceous soils derived from acid volcanic rocks are also found.

On the sedimentary rocks, shallow stony soils occur on ridges passing to texture contrast soils on almost all slopes. These change in colour from red brown subsoils on upper slopes to yellow subsoils on lower slopes. They support diverse vegetation communities that are also affected by altitude.

The granites develop gritty shallow profiles between outcrops and tors on the crests, grading to harsh texture contrast soils with yellow clay subsoils that are prone to gully development on the lower slopes.

Basalt areas on Kaputar have frequent rock outcrops interspersed with shallow, stony, brown loams. Black earths are found on lower slopes and in valleys.

In the Inverell area the basalts develop black earth profiles that thicken downslope and, where the underlying sands and gravels are exposed, the coarse sandy soils may develop podsol pans and support different vegetation. Alluvial loams and clays with moderate to high fertility are found in the valleys.

Dark, alkaline, pedal clays develop on limestone, and the serpentinites have shallow stony profiles with concentrations of elements that are toxic to many plants.

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Page last updated: 26 April 2016