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Sydney, the city by the sea, where we live, work and play, sits on foundations that began around 290 million years ago. It was then a shallow marine environment. Today, Sydney is located within the centre of a major structural Permian-Triassic (i.e. 290 to 200 million years old) sedimentary basin, appropriately termed 'The Sydney Basin'. This is just one of several basins of similar age and with similar sedimentary rock sequences that extend from near Batemans Bay in the south, to south of Townsville in the north, a distance of several thousand kilometres, and collectively termed 'The Sydney-Bowen Basin'. These basins are economically important as they contain all the known large coalfields in NSW and QLD. The Sydney Basin is bound to the south and west by older, largely metamorphic and granitic rocks of the Lachlan Fold Belt. To the north, the Sydney Basin sequence passes through the Hunter Valley and northeast into the Gunnedah Basin. To the east, the Sydney Basin continues offshore to the edge of the continental shelf. The centre of the Sydney Basin is located at Fairfield, but generally only the youngest Middle Triassic sequences are exposed around Sydney. These are the Hawkesbury Sandstone and Wianamatta Group (divided into the Ashfield Shale, Minchinbury Sandstone and Bringelly Shale). In a few areas, especially on the northern beaches, the Early Triassic Narrabeen Group is exposed. Within the Sydney area, the basin-fill is a few thousand metres thick.
The Sydney Basin was initiated by crustal rifting in the Early Permian. Early deposition was mainly marine sediments which changed to largely non-marine coal measures towards the end of the Early Permian. At the end of the Permian, sedimentation changed to dominantly alluvial fan/ fluvial environments. These different environments are characterized by particular fossil assemblages and rock sequences.
Minor deformation of the sequence occurred in Permian and Triassic times. Today, with deeper and larger excavations in the main CBD, faults are becoming more widely recognized. However, most appear to have been inactive for at least 90 million years. Widespread subvertical to vertical joint sets (fractures without any offset) are common within the Hawkesbury Sandstone, as seen along Sydney's coastline.
Minor, though widespread volcanic activity occurred from ~210 million years ago up to ~47 million years ago, peaking at ~170-90 million years ago. This activity is manifest in a number of different forms including the large Prospect dolerite intrusion which has been actively quarried for Sydney's roads for over 150 years; numerous volcanic diatremes including the Hornsby diatreme in Sydney's northwest; and numerous though narrow basaltic dykes including those at Long Reef, Bondi and La Perouse. Many of these are related to the opening of the Tasman Sea from around 110 to 90 million years ago.
Uplifts since the Middle Triassic have exposed the basin beds on land, and erosion of the sequence has continued to the present-day.
Ian Graham was born in Belfast, Northern Ireland, in 1967 and immigrated to Australia with his parents, brother and sister in late 1971. He graduated from the University of Technology, Sydney in 1990 with a Bachelor of Applied Science (First Class Honours). He then took some time off away from academia before going back to the University of Technology, Sydney, where he completed his PhD in 2000 on 'The genesis and tectonic significance of chromitite-bearing serpentinites in southern NSW'. He was then a Post-Doctoral Research Fellow within the Department of Geology at the University of Pretoria, South Africa, where he conducted research on the sub-Rustenburg layered suite dioritic sills associated with the Bushveld Complex, the world's largest resource of platinum and chromite. Since 2003, he has been a research scientist within the Mineralogy and Petrology section of the Australian Museum.
Ian received the NSW Division of the Geological Society of Australia Voisey Medal in 2004 for an outstanding contribution to our understanding of the geology of NSW by an early career researcher. He is the south-western Pacific regional councilor for the International Association on the Genesis of Ore Deposits (IAGOD), Honorary Secretary of the Geological Society of Australia Specialist Group in Geochemistry, Mineralogy and Petrology (SGGMP) and member of the NSW Division committee of the Geological Society of Australia. He is an Associate Editor for Ore Geology Reviews and Records of The Australian Museum.
Copyright © Australian Museum, 2004