Wiki: Ashfield Shale - upcScavenger

Ashfield Shale is part of the Wianamatta shale of in the Sydney Basin. It lies directly on contemporaneously eroded Hawkesbury sandstone or the Mittagong formation. These rocks were formed in the Triassic Period.

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 and grey shales with frequent Siderite clay ironstone bands. The thickness ranges between 45 and 64 metres. It is 20 metres thick at the Sydney Olympic Site. 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. At Turramurra 33 metres remains and formerly there was a good deep exposure of it at the Railway Station until this suffered shotcreting.

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 river delta setting, near the shores of a shallow sea. Ashfield Shale underlies the Prospect dolerite intrusion in Pemulwuy.

Natural selection in which the Ashfield Shale is completely exposed is rare. 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 Podzol, often with poor drainage, such as that in the Cumberland Plain.

Fairley A, Moore P (2025). 9780731810314, Kangaroo Press. ISBN 9780731810314

These clay soils are recognised as being reactive with an appreciable shrink-swell capacity.

Engineering and construction

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. Difficulties may be encountered where the Ashfield Shale interfaces with the Hawkesbury Sandstone and the Mittagong Formation.

Flora

Ashfield Shale is associated with the critically endangered Blue Gum High Forest and Sydney Turpentine-Ironbark Forest.

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. 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 delta Paleoecology. 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 , plants, , insects and have been recorded. One outstanding example being of a Paracyclotosaurus at St Peters, 2.25 metres long. It is one of the most complete Mastodonsauridae skeletons ever recovered. Notobrachyops is a genus of brachyopidae temnospondyli 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 palaeoniscidae fish, a species of holostei, and a subholostean fish.

Fossil fauna

Order: Coleoptera Indeterminate
Family: Curculionidae Indeterminate
Family: Curculionidae Etheridgea petrica
Family: Elateridae Elateridium wianamattense
Family: Schizocoleidae Metrorhynchites sydneiensis
Family: Schizocoleidae Metrorhynchites dunstani
Order: Orthoptera Mesotitan giganteus
Order: Protorthoptera Notoblattites subcostalis
Unranked taxon: Dicondylia Iverya averyi

Family:

Unionidae
Unio sp.

Family:

Xenacanthidae
Xenacanthus decheni

Family: Semionotiformes Semionotus sp.
Family: Semionotiformes Cleithrolepis sp.
Family: Semionotiformes Acentrophorus sp.
Family: Palaeoniscidae Myriolepis sp.
Family: Palaeoniscidae Palaeoniscus sp.
Family: Platysomidae Platysomus sp.
Order: Dipnoi Sagenodus sp.

Family: Capitosauridae Paracyclotosaurus davidi
Family: Trematosaurinae Microposaurus averyi
Family: Brachyopidae Notobrachyops picketti

Family: Eosuchia Kudnu mackinlayi
Family: Rhynchosauria Rhynchosauria indet.
Family: Theropoda Theropoda indet.

Categories: Geologic Formations Of Australia, Triassic Australia, Shale Groups, Fossiliferous Stratigraphic Units Of Oceania, Paleontology In New South Wales, Geology Of New South Wales

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Ashfield Shale

( Geologic Formations Of Australia )

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( Geologic Formations Of Australia )