Sydney Basin - landform
The Sydney Basin Bioregion lies on the east coast and covers a large part of the catchments of the Hawkesbury-Nepean, Hunter and Shoalhaven river systems. It consists of a geological basin filled with near horizontal sandstones and shales of Permian to Triassic age that overlie older basement rocks of the Lachlan Fold Belt. The sedimentary rocks have been subject to uplift with gentle folding and minor faulting during the formation of the Great Dividing Range.
Erosion by coastal streams has created a landscape of deep cliffed gorges and remnant plateaus across which an east-west rainfall gradient and differences in soil control the vegetation of eucalypt forests, woodlands and heaths. The Sydney Basin Bioregion includes coastal landscapes of cliffs, beaches and estuaries.
Geology and geomorphology
The Sydney-Bowen Basin was formed when the earth's crust expanded, subsided and filled with sediment between the late Carboniferous and Triassic. Early stages of development were as a continental rift that filled with marine volcanic sediments, but deposition shifted to river and swamp environments in a cold climate in the early Permian.
Coal deposits accumulated and the upper parts of the basin were covered in quartz sandstone by extremely large braided rivers whose headwaters lay hundreds or even thousands of km away and flowed in from the south and the northwest to deposit the Hawkesbury Sandstone. Shallow marine sediments and later more river sediments continued to accumulate in the basin during the Jurassic but all of these younger rocks have been eroded, leaving only a thin cap of shale over the resistant sandstones.
At a late stage in the basin filling, older rocks of the New England Fold Belt were faulted across the basin along the Hunter-Mooki-Goondiwindi Thrust System that now marks the northeastern edge of the bioregion. The basin has also been subject to minor volcanic activity with more than 200 explosive vents (diatremes) and small basalt flows evident in the geology.
As in most parts of the Great Dividing Range, the most spectacular mountain landscape is found on the coastal side of the divide along the Great Escarpment where streams have eroded deep gorges and cliff faces back into the uplifted block.
The frontal slope of the Blue Mountains is formed along the Lapstone monocline. A secondary flexure and similar escarpments occur at the coast forming the Hornsby Plateau and the Illawarra escarpment. These structural features combine with different rock types and strong trends in joint patterns to control drainage patterns and the distribution of gorges and swamps.
Much of the Basin landscape is elevated sandstone plateau, with the exceptions being the Hunter Valley and the low-lying Cumberland Plain. In the south and west the Basin ends in cliff lines formed on sandstones and conglomerates of the basal Permian sediments. Waterfalls are common on all escarpments.
The post-glacial rise in sea level between 18,000 and 6,000 years ago drowned the coastal plains and river valleys to form estuaries and deep harbours now fronted by confined barrier systems of beaches, dunes and coastal lakes. Barriers have best developed at Newcastle, Kurnell and Jervis Bay and some of these areas include "cliff top" dunes that often enclose swamps and lakes formed by groundwater.
The most significant feature of the bioregion is:
- the Great Escarpment, easily the most prominent feature of the bioregion, with its reversed drainage, and entrenched meander patterns and high level terrace gravels; the Blue Mountains are part of this feature.
At a smaller scale many interesting landscapes are present, including the following:
- the Pagoda country on deeply weathered sandstones on gorge edges;
- the gorges themselves;
- contour-patterned vegetation communities on alternating sandstones and shale plateaus;
- the concentration of volcanic vents or diatremes are significant on a world scale and these features always carry locally different vegetation;
- coastal barriers, deep estuaries and spectacular cliffs with exposed "layer cake" geology and well-developed rock platforms;
- Sydney Basin coal resources are economically critical to the state; and
- several geologic features of importance to Aboriginal people, including cultural sites of prominent landscape features, stone resources in terrace gravels and basalt outcrops, stone carving and axe grinding sites on sandstone and sandstone rock shelters.
The considerable range of rock types, topography and climates in the Sydney Basin has resulted in a large variety of soils and vegetation communities.
The coastal area of the bioregion consists of frontal dunes. Dunes behind this accumulate organic matter and begin to develop coloured subsoil. The oldest dunes on the inland side of the barrier and the parabolic dunes high in the landscape, even on headlands, have well-developed podsol profiles.
Limited areas of rainforest can be found in the lower Hunter, Illawarra escarpment and on Robertson basalts, as well as in the protected gorges and on richer soil in most subregions.
Species composition and structural form are similar on sandy soils of the sandstone plateaus and the sandy soils of the dunes. Better quality shale soils form caps on sandstone and on the coastal ramps.
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Page last updated: 27 February 2011