It is intended that this Plan be implemented over a ten-year period



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2.2Topography


The topography of the region ranges from coastal floodplains to mountainous ranges as shown in Figure 3. The highest point in the Border Ranges region is Mount Superbus (1375 m) in south-east Queensland. The eroded calderas of Mt Warning and Focal Peak form a series of radiating ranges that connect to each other through the McPherson Range that runs along the NSW–Queensland border and joins the Great Dividing Range at Wilsons Peak in the west of the Planning Area. The Great Dividing Range turns north from Wilsons Peak along the high altitude Main Range that drops in elevation as it heads inland in the far north-west of the Planning Area. The McPherson Range and Main Range form steep east and north facing scarps over the Bremer, Logan and Albert River Valleys that drain north to Moreton Bay. To the west of the Main Range, the tributaries of the Condamine River form deeply incised valleys that drain west into the Murray–Darling Basin. These valleys become less incised as the Great Dividing Range loses altitude in the north-west of the area.

In the east of the Border Ranges region the escarpments of the Lamington and Springbrook Plateaus, together with the Tweed Range and Nightcap Range, form high cliffs over the eroded Mt Warning caldera which is centred on Mt Warning itself. In the north-east, the Beechmont, Canungra and Darlington Ranges form long fingers of elevated land that drop in altitude to the north onto the Logan River floodplain. The exception to this is Mount Tamborine, an elevated plateau on the northern end of Darlington Range, which forms a prominent backdrop to the Coomera and Nerang Valleys.

Along the coastal strip, the Nimmel, Tallai, Wunburra and Burringbar Ranges form a series of steep-sided valleys that drain into the smaller coastal catchments of the Tallebudgera and Currumbin Creek Valleys and the Coomera, Nerang and Brunswick River Valleys, all of which have narrow floodplains. In the south-east, the low basaltic plateau centred on Alstonville and bounded by the Mackellar Range to the west, is dissected by numerous gently sloping valleys that lead south to the broad floodplain of the Richmond Valley.

Further west, the Richmond River floodplain expands towards Casino and Kyogle. It is bounded in the west by the Richmond Range, which forms the divide between the Richmond and upper Clarence Valleys. West of the Richmond Range, the Tooloom and Koreelah Ranges drop abruptly into the narrow upper Clarence Valley and are separated by the broad valleys of Duck and Tooloom Creeks. Further west, Koreelah Creek runs to the east of the Great Dividing Range escarpment along the high elevation Acacia Plateau.


2.3Geology


The following section has been derived from discussions in Stevens (1977) and interpretation of the various geological 1:250 000 mapsheets covering the Border Ranges region (Brunker et al. 1972; Olgers et al. 1972; Geological Survey of Queensland 1973, 1974). Figure 4 shows the distribution of broad geological groups in the region.

The predominant features of the Border Ranges region are the two eroded shield volcano calderas of Mt Warning in the Tweed Valley and Focal Peak to the west near Mt Barney, both of which were formed about 20–30 million years ago. The Tweed, Nightcap, Darlington, Beechmont and eastern McPherson Ranges are the eroded remnants of the Mt Warning volcano, while the Richmond, Tooloom, Koreelah, Main and western McPherson Ranges are eroded remnants of the Focal Peak volcano. These two volcanoes underwent a series of eruptions creating two significantly different types of lava flows. The most extensive flows were of Tertiary basalt that weathered to form the deep fertile red soils typical of the area around Lismore and Springbrook. These soils usually support subtropical rainforest in wetter areas and dry rainforest in areas where the rainfall is lower. Much of these once-extensive basalt plateaus have eroded to expose the older underlying geologies and form the coastal floodplain of the major river valleys. The other type of lava flow was of rhyolite, which weathers slowly to form low-nutrient, free-draining soils. A third lava type present in these volcanoes, trachyte, generally did not occur as a flow but formed volcanic plugs and dykes. Rhyolites and trachytes are particularly resistant to erosion and can be seen as prominent cliffs, mountains and outcrops such as Mt Warning, Mt Lindesay, Nimbin Rocks and the cliffs of the Tweed caldera. Soils derived from trachyte and rhyolite typically support drier eucalypt forest, but at higher elevation where rainfall is greater, these soils can support tracts of warm temperate rainforest.

Underlying these volcanic rocks are older, Triassic sedimentary rocks of the Clarence–Moreton Basin (135–200 million years old), the Triassic volcanics (200 million years old) and Silurian metamorphosed sediments (meta-sediments) of the Neranleigh–Fernvale Series (250–500 million years old). The Clarence–Moreton Basin typically comprises sandstones, claystones, mudstones and conglomerate which erode to form low-nutrient, free-draining soils. However, in higher rainfall areas along elevated ranges, along with enrichment from the overlying basalt outcrops, soils often support areas of dry rainforest, as occurs along the Richmond Range. The Neranleigh–Fernvale meta-sediments include greywackes and phyllites that outcrop along coastal headlands, the Condong and Burringbar Ranges and the hills of the Gold Coast hinterland. Because of their elevation and proximity to the coast, these areas attract higher rainfall and often support subtropical rainforest in protected gullies. In the south-west of the region there are small areas of the New England Fold Permian granites (200–300 million years old) that form low-nutrient, free-draining soils supporting drier sclerophyll vegetation.

The most recently laid down geological strata, the Quaternary sediments, occur in valleys, lowlands and floodplains. These sediments have been laid down in the last 2.6 million years.




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