Wianamatta Shale is the uppermost outcropping unit in the Sydney Basin

Sydney Basin

The Sydney Basin is a sedimentary basin on the east coast of New South Wales, Australia consisting of Permian and Triassic sedimentary rocks...

 in New South Wales

New South Wales

New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

, Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. It dates from the Triassic Period and generally comprises fine grained sedimentary rocks such as shales and laminites with less common sandstone units. The Wianamatta Group is made up of the following units (listed in stratigraphic order):

• Bringelly Shale
• Minchinbury Sandstone
• Ashfield Shale

The Wianamatta Group directly overlies the older (but still Triassic in age) Hawkesbury sandstone

Sydney sandstone

Sydney sandstone is the common name for Sydney Basin Hawkesbury Sandstone, historically known as Yellowblock, is a sedimentary rock named after the Hawkesbury River north of Sydney, where this sandstone is particularly common....

.

Depositional environment

The Wianamatta Group has been inferred to represent a cycle of basin infilling associated with the migration of a large delta front from west to east. The Ashfield Shale was deposisted in a low energy marine environment and preserves laminated silty sediments. The Minchinbury Sandstone comprises a set of sandy barrier islands at the former shoreline. The Bringelly Shale was deposited in a swampy alluvial plain with meandering streams flowing from the west forming discontinuous beds of sandstone.

Weathering

Weathering of the shale units produces a rich clayey soil, often with poor drainage, such as that in the Cumberland Plain

Cumberland Plain

The Cumberland Plain is a region in the Sydney Basin of New South Wales, Australia. The plain extends from 10 kilometres north of Windsor in the north, to Picton in the south; and...

. These clay soils are recognised as being reactive with appreciable Shrink-Swell Capacity

Shrink-swell capacity

The Shrink-swell capacity of clay refers to the extent to which a clay will expand when wet and retract when dry. Soil that is problematic due to high capacity is known as shrink-swell soil, or expansive soil.-Description:...

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Low lying areas where groundwater is close to the surface are also susceptible to dryland salinity. Groundwater quality can range from fresh to highly saline, with the deeper groundwater generally less saline.

Other rock types found in Sydney the Sydney Basin include the Narrabeen shale, Mittagong formation

Mittagong Formation

The Mittagong Formation is a type of sedimentary rock occurring in the Sydney Basin in eastern Australia. Formed in the triassic period. It may be seen as a narrow band of fine grained sandstone between the shale and the Hawkesbury sandstone . The maximum thickness is ten metres.In northern Sydney...

, Illawarra Coal Measures, Newcastle Coal Measures, and Shoalhaven Group.