Brigalow Belt South - landform
The bioregion forms the southern extremity of the Qld Brigalow Belt but is not dominated by brigalow (Acacia harpophylla). It consists of landscapes derived from both extensive basalt flows and quartz sandstones and consequently has very variable soils and vegetation depending on the local rock type or sediment source.
Geology and geomorphology
The bioregion's bedrock comprises horizontally bedded Jurassic and Triassic quartz sandstone and shale with limited areas of conglomerate or basalts. Some of the sandstone at the heads of streams forms a low but rugged topography of cliffs and small plateau features. Streams follow the direction of major joint planes in the narrow sandstone gorges, depositing colluvial fans of coarse sands and gravels in the wider valleys.
Even further down valley the topography is more subdued, partly buried in alluvial debris and largely eroded to rolling plains. Evidence of larger stream courses of Quaternary age occur in the long, sand-filled channels and clay plains with gilgai, or shallow depressions between ridges in which rainwater collects.
These sedimentary rocks are the fingers' edge of the Surat Basin and the alluvial plains derived from them are important water intake beds for the Great Australian Basin, a large Jurassic-Cretaceous basin covering a large part of eastern Australia, of which the Surat Basin is a part. Some of the Jurassic sediments contain interbedded volcanics that are locally important in affecting soils and vegetation. The more important volcanics are the extensive basalt flows of the Liverpool Range and Warrumbungles (which represents the eroded core of an ancient shield volcano), and flow remnants of the Inverell Basalts at Croppa Creek.
The Liverpool Range is the largest lava field province in NSW, dated between 32 and 40 million years, with up to 400m thickness of basalt covering an area of over 6,000 km2. The lava fields did not have a central volcanic vent but erupted from multiple fissures.
All the volcanic flows covered a pre-existing topography that is now being exposed as a result of erosion, revealing buried river gravels and lake sediments that contain well-preserved plant and fish fossils and a long record of climate change through those slices of geologic time.
Today's landscape is dominated by Quaternary sediments in the form of alluvial fans and outwash slopes that resemble the larger fans of the adjacent Darling Riverine Plains Bioregion to the west but are composed of coarser sediment and fan out at slightly steeper angles. The relative distribution of sediment from basalt or sandstone has a major impact on soil quality and vegetation.
The main features of interest are the landscapes themselves, especially:
- the numerous volcanic attributes of the Warrumbungles; and
- the major lava field of the Liverpool Range with its important grassland ecosystems.
Other features of significance are:
- the diatomite deposits formed in large freshwater lakes during the volcanic times;
- several springs and bogs such as Cuddies Spring, where Pleistocene animal fauna and a pollen record have been recovered in association with early human stone tools;
- excellent Jurassic fish fossils known from a limited outcrop in the Talbragar valley; and
- several important ephemeral wetlands which have a probable record of Quaternary climate change.
Soils vary greatly across this topography, as do microclimate and aspect, so it is necessary to differentiate areas of hill tops and plateau from slopes and valley floors in both sandstone and basalt areas as all of these factors affect the vegetation.
The sandstone ridge tops carry thin, discontinuous soils with stony, sandy profiles and low nutrient status. Downslope, texture contrast soils (soils that have a sharp increase in texture, ie. increase in clay content, on passing from surface soil layers to subsoil) are more common and are typically found with harsh clay sub-soils, while in the valley floors sediments tend to be sorted into deep sands with yellow earthy profiles, harsh grey clays, or more texture contrast soils with a greater concentration of soluble salts.
In basalt country the hill tops have stony, red or brown, well-structured clays with high nutrient values. Similar but often thicker soils are found on the slopes and the valley floors where they too accumulate clay materials.
Documents to download
Page last updated: 27 February 2011