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Geology of the Sydney Basin


The following is an text extract from the "Sydney Region North West Sector - Regional Environment Study" outlining the geological aspects of the Sydney Basin. Although the study area concentrates on the top half of the proposed Sydney G.I., the geological characteristics of the southern half are similar to the characteristics mentioned in the extract.

1.1 Physiographic units

The study area lies within the Sydney Basin which is a fairly simple asymetrical structural basin which has its centre at about Fairfield and extends from Port Stephens to Bateman's Bay. The general character of the Basin is shown in Figure 6. Within the Basin, the principal physiographic units relating to the study area are the Cumberland Plain in the centre, the Hornsby Plateau to the north and north east, and the Blue Mountains Plateau to the west. In parts of the study area there is a sharp transition between the plain and plateaux while in others the change is marked only by gently rolling hills with elevations ranging from 40-80 metres. These characteristics are shown on Figure 7.

The Cumberland Plain is drained by meandering creeks flowing to the Hawkesbury and Parramatta Rivers. The whole Plain, with the exception of a few rounded hills, typically has an elevation of less than 30 metres and a range of 10-60 metres. The surrounding plateaux have been formed by warping and have been deeply incised by their antecedent streams.

The Blue Mountains Plateau rises abruptly from the Cumberland Plain along the north-south line of the Lapstone monocline to a height of over 150 metres above sea-level near Glenbrook and, with associated faulting, to over 570 metres near Kurrajong Heights.

Along the north western boundary of the study area, the Hornsby Plateau, with an elevation less than 300 metres, is arbitrarily separated from the Blue Mountains Plateau by the gorge of the Colo River. In the south east, this plateau rises from the Cumberland Plain along a warpline extending from the vicinity of Cattai to Botany Bay. In general the plateaux have an elevation of about 200-220 metres with gullies down to 80-100 metres along drainage lines. Steep slopes with elevation changes of over 100 metres are not uncommon.

Insert from Figure 6 showing geographical characteristic of the Sydney Basin

1.2 Geological history

The geology of the Sydney Basin and the study area is predominantly the result of sedimentation and phases of earth movements. The underlying structure of the Basin was laid down during the Permian and earlier geological periods under marine and marshy conditions which, with major earth movements, produced the sandstone and siltstone formations and intervening coal measures lying at considerable depths under Sydney.

The geological formations that outcrop in the study area are mainly Triassic sediments which were deposited in lakes in the unfolded parts of the geosyncline of the Sydney Basin that had developed by the end of the Permian. The first group of sediments, which only occurs on the fringes of the study area, was the Narrabeen Group which has a wide range of sediments. This was followed by the Hawkesbury Sandstone and lastly the Wianamatta Group containing, in the study area, Ashfield and Bringelly Shales which were deposited in a series of isolated depressions. Since Triassic times the surface of the Sydney basin has been above sea level and consequently any further deposition of sediments have been terrestrial.

During the Jurassic period the sediments of the Sydney Basin were intruded by small bodies of magma, the best known in Sydney being the Prospect dolerite intrusion but represented in the study area by the volcanic breccia outcrop at Marsden Park.

The next depositions in the Basin were Tertiary freshwater sediments. Very little surface evidence remains but a major outcrop occurs in the study area in the sands and clay of Londonderry-Windsor area and also the sands at Maroota. The tertiary deposits were typically a coarse gravelly layer up to eight metres in thickness underlying silt and sand of an average depth of about six metres. Subsequent weathering has turned much of the sandy silt into siliceous clay. At the close of the Tertiary period earth movements occurred which produced an uplift of 600 metres in a large area of eastern Australia. These movements produced the Blue Mountains and Hornsby Plateaux described above, with the lagging behind of part of the peneplain becoming the Cumberland Plain. This movement produced the entrenching of existing rivers including the Hawkesbury-Nepean and the Grose and the development of new rivers of which the Colo is one.

The Quaternary period saw a rise in sea level of about 60 metres which drowned the lower Hawkesbury River together with the great part of the eastern Australian coast. The Quaternary deposits were formed by flood or wind and are well in evidence in the study area in the flood plain of the Hawkesbury River and its tributaries. These deposits consist of varying depths of estuarine and river sands and gravels, typically six to eight metres of gravel overlaid by about six metres of sandy silt.

The geology of the area that now appears as a result of the above history is shown on Figure 6. It can be summarised as Hawkesbury Sandstone in the east and around the northern and western fringe, with Ashfield Shales of the Wianamatta Group occurring in the Glossodia area and in a band from Pitt Town to Kellyville. Bringelly Shales, also of the Wianamatta Group, are found further west along a line from Vineyard to Quakers Hill and at Marsden Park. The south western part of the study area is dominated by the more recent deposits containing a variety of sand, silt, gravel and clay formations.

Soil types relate to parent geological material and subsequent weathering. The soils derived from the sandstone are generally poor but the repeating ridge-slope-gully formations produce a number of types ranging from barely structured sandy soils on the ridges and steep slopes to deep, strongly structured sandy clays in the gullies. The rolling terrain of the shale areas with slopes rarely exceeding 20 per cent and the different parent material produces heavy textured red podsolic soils on upper and mid slopes and yellow podsolics on lower slopes and flat drainage depressions. On the alluvials the soil type depends on the age of the sediment. The tertiary related soils cover a wide range from sand deposits to gravel, silt soils and, at their most developed, duplex soils with distinct clay subsoils. The soils on the younger alluvials range from sandy loams to clays and often show little structure.

Insert from Figure 7 showing the topography of the north-west sector of the Sydney Basin

1.3 Landscape systems

The physiographic units of the study area translate into three major landscape systems - the River Plain, Dissected Plateaux and Shale Slopes which are illustrated on Figure 7.

1.3.1 River Plain

The River Plain system has an area of 25,600 hectares and features the Hawkesbury River along its western edge and the Eastern, South and Ropes Creek tributaries flowing through its centre. Despite the reasonable fertility of the Plain's alluvial soils, the type of vegetation it supports is influenced primarily by the low rainfall resulting from the effect of the surrounding uplands. The characteristic vegetation community is a sparse eucalypt woodland with a slightly more dense open forest developing where the water table is higher and the water holding capacity of the soil is increased.

Soil type is the next most important factor influencing vegetation type on the Plain. The rich agricultural soils of the Quaternary alluvium support one type of canopy community, plus swamp communities and riverbank trees and shrub thickets. Because of the fertility and higher permanent water table of these soils the tree community is usually quite dense (i.e. a forest).

The vegetation on the Tertiary alluvium relates to the degree of soil development. Areas with clay subsoils support forests while the other, less clayey, soils all support sparser woodlands. The tree species of the woodlands vary in response to small changes in the proportion of sand-to-silt-to-gravel in the soil.

Along the Hawkesbury River floodplain are a number of small areas of perennially high water table. Natural vegetation here is predominantly sedgeland and reed swamp with the occasional occurrence of swamp forest. All are classified in this survey as wetlands.

1.3.2 Dissected Plateaux

The Dissected Plateaux landscape system covers an area of 74,770 hectares. The Plateaux to the east and west experience a higher rainfall than the River Plain mainly because of their orographic effect. Due in large part to the higher rainfall, the vegetation is more dense than on the Plain. The soil, however, is very poor and drains freely. This results in an open forest formation on the upper slopes and sparse woodlands on ridgetops are not uncommon.

On the Plateaux there are also variations in the vegetation in response to differing aspects and soil type and structure. The sclerophytic adaptation vegetation for growth on poor soils with good rains means that the vegetation reacts strongly to the extra drying effects of full exposure to sunlight. Thus, a different type of vegetation grows on north facing slopes than on south facing slopes.

It is because of this variety of environmental conditions and the vegetation's response to them that the flora of the Hawkesbury Sandstone is often called a complex of types.

On parts of the Plateaux the vegetation, wh11e having much in common with the other Plateaux types, is more dense and has a more varied understorey. This occurs where shale caps on the sandstone and active weathering of the sandstone have produced soils that are richer and have higher waterholding capacity.

The Dissected Plateau in the north of the study area is different from the others in that it does not have a higher rainfall than the River Plain. The lower rainfall reduces the complexity of the vegetation and results in a wider occurrence of woodland formation which is less responsive to soil type and aspect as well.

The Dissected Plateaux also have wetland communities. These are mainly mangrove/saltmarsh areas in tidal re-entrants along the lower Hawkesbury and Berowra Creek.

1.3.3 Shale Slopes

The Shale Slopes landscape system occurs where the Ashfield and Bringelly Shales remain in the north and east of the study area. The two major areas of this system total approximately 25.670 hectares. The vegetation of the Shale Slopes reflect, the gradual change from the Plateaux to the River Plain and rainfall similar to the latter. However, it should not be considered a transition vegetation type since outside the study area to the south this zone occurs without culminating in a plateau. In fact it constitutes the major vegetation type of the Cumberland Plain.

References

1984, Sydney Region North West Sector - Regional Environment Study Volume 1, Department of Environment and Planning, Sydney, pp 19-22.

1984, Sydney Region North West Sector - Regional Environment Study Volume 2 Map Atlas, Department of Environment and Planning, Sydney, Figures 6 & 7. 

 

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