Sydney Basin

Sydney Basin New South Wales
Map of the Sydney Basin
The interim Australian bioregions, with the Sydney Basin in red
Coordinates	33°56′53″S 150°45′36″E / 33.94806°S 150.76000°E / -33.94806; 150.76000
Population	6,300,000[1]
• Density	173.6/km2 (450/sq mi)
Area	36,295.97 km2 (14,014.0 sq mi)[2]
Mean max temp Mean min temp Annual rainfall[1]  ? ? 650−1,300 mm 598.8 in
Localities around Sydney Basin: South Eastern Highlands North Coast Tasman Sea South Eastern Highlands Sydney Basin Tasman Sea Australian Alps South Coast Tasman Sea

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.

Around 5,000 metres (16,000 ft) thick, the Sydney Basin consists of

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

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

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]

Timeline

• 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.
• 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
  Plant fossils
  are scarce, but some fish fossils are found in the clay lenses.
• seed fern
  fossils.
• End of Triassic: 201 million years ago; Ascension and shifting at the Lapstone fault, with the Blue Mountains rising and the
  Sydney CBD
  jousting upward.
• Jurassic: 201–145 million years ago; Erosion, with
  Mount Tomah, Mount Wilson and Hornsby Quarry.^[13]
• 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]