Ashfield Shale

Ashfield Shale Stratigraphic range: Middle Triassic PreꞒ Ꞓ O S D C P T J K Pg N
Exposed Ashfield Shale of the Wianamatta group, near the Pacific Highway, Chatswood, Australia
Type	Geological formation
Unit of	Wiannamatta group
Sub-units	Kellyville Laminite Member, Mulgoa Laminite Member, Regentville Siltstone Member, Rouse Hill Siltstone Member
Overlies	Mittagong Formation
Thickness	up to 64 metres (210 ft)
Lithology
Primary	Shale
Location
Location	Sydney Basin
Country	Australia
Type section
Named for	Ashfield

Ashfield Shale is part of the Wiannamatta group of sedimentary rocks in the Sydney Basin. It lies directly on contemporaneously eroded Hawkesbury sandstone or the Mittagong formation. These rock types were formed in the Triassic Period.^[1] It is named after the Sydney suburb of Ashfield. Some of the early research was performed at the old Ashfield Brickworks Quarry. This rock type is often associated with the Inner West and North Shore of the city. However, it has also been recorded at Penrith, Revesby, Bilpin and Mount Irvine.

Description[]

Ashfield Shale comprises black mudstones and grey shales with frequent sideritic clay ironstone bands. The thickness ranges between 45 and 64 metres.^[1] It is 20 metres thick at the Sydney Olympic Site.^[2] The chemistry of the rock is typical of shales, with high iron levels, and some iron sulphide and low calcium levels. The geology of the shale lenses within the Hawkesbury Sandstone is chemically similar to the Ashfield Shale.^[1] At Turramurra 33 metres remains and formerly there was a good deep exposure of it at the Railway Station until this suffered shotcreting.^{[citation needed]}

Small scale bedding is abundant. The shales are sandy at the top of the sequence. There are up to ten bands in a fifteen-metre section. The fine grained silty sediments were laid down in a low energy, south-east flowing deltaic setting, near the shores of a shallow sea.

There is no natural selection in which the Ashfield Shale is completely exposed. However, it can be seen at railway and roadside cuttings, as well as old quarries. With weathering and exposure, the shale becomes a paler colour.

Weathering of the shale units produces a reddish/brown podsolic soil, often with poor drainage, such as that in the Cumberland Plain.^[3] These clay soils are recognised as being reactive with an appreciable shrink-swell capacity.

Engineering and construction[]

The Epping to Chatswood railway line was partially drilled through Ashfield Shale

The adjacent Hawkesbury Sandstone is considered a safer bedrock than the (less stable and laminated) Ashfield Shale for building construction. In 2005 the construction of the Lane Cove Tunnel was affected by the collapse of an exit ramp excavation, through Ashfield Shale.^[4] Difficulties may be encountered where the Ashfield Shale interfaces with the Hawkesbury Sandstone and the Mittagong Formation.^[5]

Flora[]

Ashfield Shale is associated with the critically endangered Blue Gum High Forest and Sydney Turpentine-Ironbark Forest.^[6][7]

Industry and agriculture[]

In the earlier days of Sydney, the Ashfield Shale supported a number of quarries. The shale provided a suitable raw material for brickmaking.^[6] Sydney soils based on shale are not particularly fertile. But at Parramatta they proved more suitable to agriculture than those at Farm Cove, in the early days of the First Fleet.

Fossils[]

Ashfield Shale is considered a freshwater lacustrine paleoenvironment. It was gradually inundated by brackish water, then shallow marine waters over a long period of time.

Fossils are not common in this stratum, however, fossil bivalves, plants, isopods, insects and amphibians have been recorded. One outstanding example being of a Paracyclotosaurus^[1] at St Peters, 2.25 metres long. It is one of the most complete mastodonsaurid skeletons ever recovered. Notobrachyops is a genus of brachyopid temnospondyl amphibian. It is known from a skull roof impression found in the Ashfield Shale at the old Hurstville Brick Company quarry at Mortdale.

The Ashfield Shale has also yielded a shark species, a lungfish species, six species of paleoniscid fish, a species of holostean fish, and a subholostean fish.^[8]

Fossil fauna[]

Sharks of the Ashfield Shale
Taxa	Presence	Description	Images
Family: Xenacanthidae Pleurocanthus decheni			Pleurocanthus

Fish of the Ashfield Shale
Taxa	Presence	Description	Images
Family: Semionotiformes Semionotus sp.			Semionotus
Family: Semionotiformes Cleithrolepis sp.			Cleithrolepis
Family: Semionotiformes Acentrophorus sp.
Family: Palaeoniscidae sp.
Family: Palaeoniscidae Palaeoniscus sp.
Family: Platysomus sp.
Order: Dipnoi Sagenodus sp.

Amphibians of the Ashfield Shale
Taxa	Presence	Description	Images
Family: Capitosauridae Paracyclotosaurus davidi			Paracyclotosaurus
Family: Trematosaurinae Microposaurus averyi			Microposaurus averyi
Family: Brachyopidae Notobrachyops picketti

References[]