The Sydney Basin

Boundaries

The basin is bounded to the south and west by an older sequence of largely low-grade metamorphic and granitic rocks of the Lachlan Fold Belt. To the north, the rocks of the Sydney Basin pass into the Hunter Valley sequence that is transitional between the Sydney Basin and New England Fold Belt. To the north-west, the Sydney Basin sequence passes into the Gunnedah Basin. To the east, the Sydney Basin sequence continues offshore to the edge of the continental shelf. The centre of the basin is located at Fairfield, near Liverpool in Sydney, but only the youngest Triassic (middle Triassic) rocks are exposed in the Sydney area.

Thickness

The total maximum thickness of the Sydney Basin is in the range of some 5000 m, though a complete sequence has not been found at any single locality. Minor deformation occurred in both Permian and Triassic times and its effects are noticeable in some parts of the basin.

Initiation and uplift

The basin was initiated by crustal rifting in the Early Permian. Early deposition consisted of dominantly marine sediments which changed to mainly non-marine coal measures towards the end of the Early Permian. At the end of the Permian, sedimentation changed again to dominantly alluvial fan/fluvial environments. The best preserved Permian sequences occur in the Hunter Valley and marine fossils are abundant in some places. On the south coast of New South Wales, the Permian succession is very thin.

About the middle of the Triassic Period, the basin was uplifted to become dry land, and erosion has occurred from this time to the present.

Stratigraphy

Permian

• Dalwood Group and lower Shoalhaven Group: calcareous sandstone, conglomerate, shale, limestone, lava flows and tuff.
  Greta Coal Measures: sandstone, shale, conglomerate, coal seams
• Maitland and Shoalhaven Groups: siltstone, sandstone, shale, conglomerate (with abundant volcanics - the Gerringong Volcanics on the south coast; and glacial sediments in the Hunter Valley)
• Illawarra, Tomago and Newcastle Coal Measures: abundant thick coal seams, sandstone, shale, conglomerate (with abundant plant fossils)

Triassic

• Narrabeen Group: lithic sandstone, quartz sandstone, claystones, siltstones, conglomerate (with plant, fish and amphibian fossils)
• Hawkesbury Sandstone: quartz-rich sandstone (with abundant cross-bedding) with interbedded shale
• Mittagong Formation: thin sandstone beds
• Wianamatta Group: consists of three main formations called the Ashfield Shale (shale, siltstone, claystone), Minchinbury Sandstone (sandstone) and Bringelly Shale (shale, sandstone)

Post-Triassic sedimentation and structures

Since uplift of the basin in the Triassic, up to 600 m of sediment has been deposited on parts of the continental shelf north of Sydney. Significant thicknesses of sediment (up to 80 m) have accumulated in coastal depressions such as Botany Bay and Lake Macquarie. Most of this unconsolidated material is Cenozoic (i.e. less than 60 million years) or Quaternary (i.e. less than 2 million years) in age.

The Cumberland Plain was probably formed about 80 million years ago, associated with the opening of the Tasman Rift off eastern Australia, and this disrupted old stream patterns and largely resulted in the geography that we see today.

At different times during the Cenozoic, more humid conditions existed than today, resulting in the formation of lateritic soils (i.e. iron and aluminium rich soils). These laterites are well developed on the Hawkesbury Sandstone, Narrabeen Group and Wianamatta Group. Present-day weathering has resulted in new profiles being developed on these much older fossil soil profiles.

The rock sequences of the Sydney Basin have undergone only minor folding and faulting since they first formed. The only large-scale folds are those of the Lapstone Monocline (which extends in a north-south direction for over 150 km) at the eastern foothills of the Blue Mountains, and further west, the Tomah Monocline.

With more intensive and larger-scale excavations in the Sydney region in recent times, faults are becoming more widely recognised, though most appear to have been inactive for at least the last 90 million years. Widely-spaced subvertical to vertical joint sets (fractures without any offset) are common within the Hawkesbury sandstone and prominent along the coastline.

Igneous activity

Minor igneous activity occurred in the Early Jurassic (i.e. 210 million years ago), Late Mesozoic (i.e. 100-90 million years ago) and Cenozoic times (i.e. 65 million years ago). The Early Jurassic activity resulted in the formation of the Prospect dolerite intrusion near Parramatta.

Jurassic-age igneous activity also resulted in the formation of diatremes (volcanic breccia pipes representing the root-zones of short-lived volcanoes called maars) composed of varying amounts of sedimentary breccia, volcanic breccia and basalt. Over 30 of these are known in the Sydney region but most are poorly exposed. An exception to this is the Hornsby diatreme in north-western Sydney which has been well-exposed by quarrying. This diatreme is composed of volcanic breccia containing numerous xenoliths.

Another well-exposed diatreme is the Bondi diatreme, occurring along the coastline, just north of the famous Bondi Beach. This is composed of sedimentary breccia injected with basalt. There are also numerous basaltic dykes of probable Jurassic age within the Sydney region, most likely related to these diatremes. These are well exposed along the coastal cliffs and rock platforms, predominantly trending in an east-south-east direction. A number of mainly Jurassic basaltic and doleritic dykes also occur at Long Reef, La Perouse, Hunters Hill and Port Kembla.

Late Mesozoic activity resulted in the formation of numerous coastal basaltic dykes such as those along the New South Wales central coast, and Newcastle. Cenozoic activity was mainly in the form of isolated basalt flows that now form cappings to Mount Banks and Mount Wilson in the Blue Mountains, and Peats Ridge on the central coast of New South Wales.