The Sydney Basin is an interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its eastern side now subsided beneath the Tasman Sea. The basin is named for the city of Sydney, on which it is centred.
According to NSW Primary Industries, the basin extends through approximately 350 kilometres (220 mi) of coastline from Newcastle in the north to Durras Lake (near Batemans Bay) in the south. From Durras Lake the western boundary continues in a line through Lithgow to around Ulan (near Mudgee). To the north the boundary extends 120 kilometres (75 mi) along the Liverpool Range to a point 80 kilometres (50 mi) north of Muswellbrook, and then runs 200 kilometres (120 mi) back to the coast at Newcastle. To the east the basin continues to the edge of the continental shelf.[4]
The total area of the basin is approximately 44,000 square kilometres (17,000 sq mi) onshore plus 5,000 square kilometres (1,900 sq mi) offshore.[5] The centre of the basin is located around 30 kilometres (19 mi) west of the Sydney central business district at Fairfield, though only the youngest Triassic (middle Triassic) rocks are exposed in the Sydney area.[6]
The Australian GovernmentDepartment of Climate Change, Energy, the Environment and Water classifies the basin as an interim Australian bioregion consisting of 3,629,597 hectares (8,968,930 acres).[7] Meanwhile, according to Geoscience Australia the basin covers 64,000 square kilometres (25,000 sq mi), of which 36,000 square kilometres (14,000 sq mi) is onshore and 28,000 square kilometres (11,000 sq mi) is offshore with water depths of up to 4,500 metres (14,800 ft).[2] Another Australian Government agency classifies that the basin covers approximately 25,000 square kilometres (9,700 sq mi).[1]
History and formation
The majority of Sydney Basin is raised sandstone plateau, with the exception being the Hunter Valley and the low-lying Cumberland Plain.
Minor igneous activity took place in the basin during the Early Jurassic (i.e. 210 million years ago), Late Mesozoic (i.e. 100-90 million years ago) and Cenozoic eras (i.e. 65 million years ago). The Early Jurassic activity resulted in the formation of the Prospect dolerite intrusion in Prospect Hill.[8] Although Jurassic sedimentation is not observed in the Sydney Basin, there are Jurassic volcanic breccia pipes (diatremes).[9]
The Sydney Basin is part of a major basin system that extends over 1,500 kilometres (930 mi) from the Bowen Basin in Queensland through to the Gunnedah Basin in NSW. Onshore, the basin contains 4,500 metres (14,800 ft) of Permo-Triassic clastic sediments, while the offshore basin contains 6,000 metres (20,000 ft) of sediments. The basin overlies the Lachlan Fold Belt and Late Carboniferous volcanoclastic sediments. The basin formed during extension in the Early Permian, with half-graben infilled with the Dalwood and Talaterang Groups. Foreland loading followed with the compression of the Currarong Orogen in the Early Permian.[10]
Late Permian uplift associated with the New England foreland loading phase resulted in the formation of depocentres with the northeast Sydney Basin with best preserved marine fossils. These depocentres filled with pyroclastic and alluvial-paludual sediments of the Newcastle Coal Measures. In the Triassic, uplift of the offshore basin resulted in reworking of Permian sediments in fluvial environments. The basin underwent a final phase of deformation (thrusting) in the Middle Triassic where it was uplifted to become dry land, with an erosion occurring from this time to the present.
Extension and breakup in the Tasman Sea beginning in the Late Cretaceous resulted in the current structural boundaries of the basin's eastern margin. In the south and west the Basin finishes in cliff lines formed on sandstones and conglomerates of the primary Permian sediments, with waterfalls being widespread on all escarpments.[2]
Permian: 299–252 million years ago; The settling and evolution of swamp forest, which would shape tremendous coal measures. To note, Australia's coal is younger than the Carboniferous coal of the Northern Hemisphere.
Early Triassic: 252–247 million years ago; dark, high-carbon Narrabeen shales can be viewed at Long Reef and Narrabeen. The Australian continent was part of the Gondwana supercontinent and the Sydney basin was situated within a depositional basin. The Ashfield Shale, which overlies the Sydney sandstone, indicates an alteration in river flow direction and its depositional style. That was when southeast-streaming rivers deposited fine grained sands and muds within a river delta which settled on a shallow sea's shore.[11]
Mid Triassic: 247–235 million years ago; A monolithic river with its beginnings to the south-west of Broken Hill, in what was Antarctica at that time,[12] had its delta in what was the Sydney Basin. It is around five times bigger than the Amazon river. There is predominance of silica sand with minor lenses of clay. Plant fossils are scarce, but some fish fossils are found in the clay lenses.
Late Triassic: 235–201 million years ago; As the river slowed with the erosion of the mountain range, finer shales were laid out. This strata is rich in seed fern fossils.
End of Triassic: 201 million years ago; Ascension and shifting at the Lapstone fault, with the Blue Mountains rising and the western Sydney plain descending to a flat land and Sydney CBD jousting upward.
Cainozoic (Tertiary and Quaternary): 66 million years to present; Gondwana broke up around 40 to 60 million years ago. That is when the Australian continent started to form where it drifted and rifted, where Sydney's rocks were elevated, canted and then later eroded by the weather. Sydney's sedimentary rocks were shaped into a landscape that was defined by bedrock valleys exposed into a raised plateau. Sydney's largest rivers, such as the Hawkesbury, Parramatta, Georges and Hacking Rivers eroded the region's deepest valleys. In this period, the Ashfield Shale got weathered to create a flatter landform with low, undulating topography and reasonably fertile soils, which heavily contrasted the plateaus, cliffs and gorges on the sandstone areas in the Sydney Region.[11] The Botany Bay Basin was also developed at that time, which is infilled with sand.
Late Pleistocene: 12 000 years ago; Submerging the Sydney river valleys with the post-glacial sea level rise where estuaries and deep harbours were formed.[14]
In addition, some of the rivers of the Hunter-Central Rivers catchment and the Southern Rivers catchment also lie mainly in the basin. In the Hunter-Central Rivers catchment, the Hunter River sub-catchment forms the northern boundary of the basin. In the Southern Rivers catchment, the Illawarra sub-catchment and the Shoalhaven sub-catchment forms the southern boundary.
^"Australia's bioregions (IBRA)". Department of Sustainability, Environment, Water, Population and Communities. Commonwealth of Australia. 2012. Retrieved 13 January 2013.
^Jones, I., and Verdel, C. (2015). Basalt distribution and volume estimates of Cenozoic volcanism in the Bowen Basin region of eastern Australia: Implications for a waning mantle plume. Australian Journal of Earth Sciences, 62(2), 255–263.
^Crawford, E. A., Herbert, C., Taylor, G., Helby, R., Morgan, R. & Ferguson, J., 1980 – 15. Diatremes of the Sydney Basin, p. 295-323., In Herbert, C. & Helby, R. (Editors.) – A Guide to the Sydney Basin. Geological Survey of New South Wales, Bulletin 26.20(1),25–33.
^Jones, I., Verdel, C., Crossingham, T., and Vasconcelos, P. (2017). Animated reconstructions of the Late Cretaceous to Cenozoic northward migration of Australia, and implications for the generation of east Australian mafic magmatism. Geosphere, 13(2), 460–481.
^Wilshire: Wilshire, H.G., The Prospect alkaline diabase-picrite intrusion, New South Wales, Australia in Journal of Petrology, Vol. 8 (1), pp 97–163, 1967.
^Branagan, D.F., and Packham, G.H., 2000. Field Geology of New South Wales. 3rd Edition. New South Wales Department of Mineral Resources, Sydney.
