North Coast - landform
The North Coast Bioregion covers northern NSW from the shoreline to the Great Escarpment. Typically, there is a sequence from coastal sand barrier, through low foothills and ranges, to the steep slopes and gorges of the Escarpment itself, with rainfall increasing inland along this transect.
Geology and geomorphology
The North Coast Bioregion is one of the most diverse in NSW. It has Devonian and Permian bedrocks that are part of the New England Fold Belt and have been closely faulted as they were thrust over the northern margin of the Sydney Basin. Small bodies of granite and granodiorite have intruded the sedimentary rocks and there are three centres of Tertiary basalt eruption.
At the time of the opening of the Tasman Sea by plate movements 80 to 100 million years ago during the break up of Gondwana, the coast of the Australian continent was uplifted and warped. As the ocean widened the uplifted block subsided at the coast and river systems developed that eroded back toward the inland flexure along the warp. Rapid headward erosion of these streams formed the Great Escarpment and cut deep gorges back into the plateau areas of the adjacent New England Tablelands Bioregion. The Great Escarpment is very prominent in this bioregion.
The largest volcanic centre, resulting from Tertiary basalt eruption, is the Tweed volcano and the associated Mt Warning caldera (exploded crater) near the NSW/Qld border. This complex is dated between 20 and 24 million years old, and at the time of eruption was a shield volcano with low slopes that covered an area 80 by 100 km. Mt Warning itself is the remains of a large central feeding chamber, filled with coarser rock than the basalts, rhyolites and tuff on the caldera rim.
The bioregion covers that part of the Late Triassic to Early Cretaceous Clarence-Moreton Basin in NSW. Sediments in this sequence are similar to those in the Sydney Basin and include some minor coal seams. The Clarence-Moreton Basin, however, is in a warmer, wetter environment than most of the Sydney Basin and relatively few plants are common to both environments.
During the cold periods of the Quaternary, the sea level was more than 100 m lower than at present and in the past 18,000 years it has risen to its present position, sweeping sand from the continental shelf before it. This sand has accumulated in the coastal barrier systems that reach their maximum degree of development in the Myall Lakes system, with high foredunes, low inner barrier ridges, wide lake basins and high parabolic dunes blown onto bedrock hills.
Important features include the following:
- the Tweed volcano complex and Mt Warning are one of the best exposed in Australia;
- Ebor Plateau and Comboyne Plateau are central volcanoes about 18 million years old;
- a large number of economic mineral deposits, including extended deposits of heavy minerals in beach and dune sands, most of which have associated heritage items;
- the Great Escarpment is well developed in this bioregion with deep gorges on every major river; and
- areas of serpentinite are evidence of deep-sea sediment accretion on the Australian mainland through time - these rocks also weather to soils with toxic levels of some metals, which affects the vegetation growing on them.
The soil and vegetation patterns in the bioregion are very complex because of the different substrates, the topographic variation and the climatic differences encountered across and along the bioregion. In general, only the most fertile soils (normally from basalts) support rainforests, but exceptions to this are found in numerous protected pockets where plant nutrients have accumulated through organic cycling in litter.
On the basalts the soils are typically red, friable loams or clay loams with high fertility, good structure and excellent water-holding capacity. On granites and most of the quartz rich sedimentary rocks, shallow yellow earths are found on hillcrests, yellow and brown texture contrast profiles are found on the slopes, and organic loams or sandy loams are found on the alluvial plains. In the coastal dunes, deep siliceous sands and very well developed podsols can be found.
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Page last updated: 27 February 2011