Sydney Basin
Region
Map of the Sydney Basin
Theinterim Australian bioregions, with the Sydney Basin in red
Coordinates:33°56′53″S150°45′36″E / 33.94806°S 150.76000°E /-33.94806; 150.76000
Country	Australia
State	New South Wales
Area [2]
• Total	36,295.97 km2 (14,013.95 sq mi)
Population
• Total	6,300,000[1]
• Density	173.6/km2 (450/sq mi)
Annual rainfall	650–1,300 mm (26–51 in)[1]
Regions around Sydney Basin South Eastern Highlands North Coast Tasman Sea South Eastern Highlands Sydney Basin Tasman Sea Australian Alps South Coast Tasman Sea

TheSydney Basin is aninterim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast ofNew South Wales,Australia and with some of its eastern side now subsided beneath theTasman Sea. The basin is named for the city ofSydney, on which it is centred.

Around 5,000 metres (16,000 ft) thick, the Sydney Basin consists ofPermian andTriassicsedimentary rocks, which stretches fromNewcastle in the north toBatemans Bay in the south, and west to theGreat Dividing Range. The basin is also home to the major centres ofNewcastle,Gosford, andWollongong, as well as the state capital ofSydney, and contains economically significant reserves ofcoal.

Sydney's famousharbour and the sculptured cliffs of theBlue Mountains are signature formations of relatively hard upper strata ofsandstone. The basin contains theUNESCO World Heritage-listedGreater Blue Mountains Area.^[3]

According toNSW Primary Industries, the basin extends through approximately 350 kilometres (220 mi) of coastline from Newcastle in the north to Durras Lake (nearBatemans Bay) in the south. From Durras Lake the western boundary continues in a line throughLithgow to around Ulan (nearMudgee). To the north the boundary extends 120 kilometres (75 mi) along theLiverpool Range to a point 80 kilometres (50 mi) north ofMuswellbrook, and then runs 200 kilometres (120 mi) back to the coast at Newcastle. To the east the basin continues to the edge of thecontinental 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 theSydney central business district atFairfield, though only the youngest Triassic (middle Triassic) rocks are exposed in the Sydney area.^[6]

TheAustralian GovernmentDepartment of Climate Change, Energy, the Environment and Water classifies the basin as aninterim Australian bioregion consisting of 3,629,597 hectares (8,968,930 acres).^[7] Meanwhile, according toGeoscience 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]

Minor igneous activity took place in the basin during theEarly Jurassic (i.e. 210 million years ago), LateMesozoic (i.e. 100–90 million years ago) andCenozoic eras (i.e. 65 million years ago). The Early Jurassic activity resulted in the formation of theProspect dolerite intrusion inProspect Hill.^[8] Although Jurassic sedimentation is not observed in the Sydney Basin, there are Jurassic volcanicbreccia pipes (diatremes).^[9]

The Sydney Basin is part of a major basin system that extends over 1,500 kilometres (930 mi) from theBowen Basin inQueensland through to theGunnedah 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 theLachlan Fold Belt and Late Carboniferous volcanoclastic sediments. The basin formed during extension in theEarly Permian, withhalf-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 theNew 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 theMiddle 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 andconglomerates 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 tremendouscoal measures. To note, Australia's coal is younger than theCarboniferous coal of theNorthern Hemisphere.
• Early Triassic: 252–247 million years ago; dark, high-carbonNarrabeen shales can be viewed atLong Reef andNarrabeen. TheAustralian continent was part of theGondwana supercontinent and the Sydney basin was situated within a depositional basin. TheAshfield 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 ofBroken Hill, in what wasAntarctica at that time,^[12] had its delta in what was the Sydney Basin. It is around five times bigger than theAmazon River. There is predominance ofsilica sand with minor lenses ofclay.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 inseed fern fossils.
• End of Triassic: 201 million years ago; Ascension and shifting at the Lapstone fault, with the Blue Mountains rising and thewestern Sydney plain descending to a flat land andSydney CBD jousting upward.
• Jurassic: 201–145 million years ago; Erosion, withAshfield Shales remaining on top. Deep V-shaped valleys in theHawkesbury sandstone. Fracturing, volcanic intrusions formProspect Quarry,Mount Tomah,Mount Wilson andHornsby Quarry.^[13]
• Cainozoic (Tertiary andQuaternary): 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'ssedimentary rocks were shaped into a landscape that was defined bybedrock valleys exposed into a raised plateau. Sydney's largest rivers, such as theHawkesbury,Parramatta,Georges andHacking 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] TheBotany Bay Basin was also developed at that time, which is infilled with sand.
• LatePleistocene: 12 000 years ago; Submerging the Sydney river valleys with the post-glacialsea level rise where estuaries and deep harbours were formed.^[14]

Thehydrology of the basin comprises three maindrainage basins as defined by the New South Wales Office of Water that lie entirely or mainly within the geography of the basin; namely theCentral Coast catchment, theHawkesbury-Nepean catchment, and theSydney Metropolitan catchment.

In addition, some of the rivers of theHunter-Central Rivers catchment and the Southern Rivers catchment also lie mainly in the basin. In the Hunter-Central Rivers catchment, theHunter River sub-catchment forms the northern boundary of the basin. In the Southern Rivers catchment, theIllawarra sub-catchment and theShoalhaven sub-catchment forms the southern boundary.

• Cumberland Plain
• Geography of Sydney
• Geology of Australia
• Illawarra Coal Measures
• Mittagong Formation
• Narrabeen group
• Sydney sandstone
• Wianamatta Shale
• Tasman Abyssal Plain

 This article incorporatestext byCommonwealth of Australia (Geoscience Australia) 2018., available under theCC BY 4.0 license. (accessed on 25 March 2018).

• NSW National Parks and Wildlife Service – Sydney Basin Bioregion
• NSW Department of Primary Industries – Sydney Basin – Geological Overview
• NSW Office of Water – Basins and catchments

• Biogeography of New South Wales
• Eastern Australian temperate forests
• Geography of New South Wales
• Geology of New South Wales
• IBRA regions
• Sedimentary basins of Australia

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