Ashfield Shale
Stratigraphic range: Middle Triassic
Exposed Ashfield Shale of the Wianamatta group, near the Pacific Highway, Chatswood, Australia
TypeGeological formation
Unit ofWiannamatta group
Sub-unitsKellyville Laminite Member, Mulgoa Laminite Member, Regentville Siltstone Member, Rouse Hill Siltstone Member
OverliesMittagong Formation
Thicknessup to 64 metres (210 ft)
Lithology
PrimaryShale
Location
LocationSydney Basin
CountryAustralia
Type section
Named forAshfield

Ashfield Shale is part of the Wianamatta 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. 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 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

Insects of the Ashfield Shale
Taxa Presence Description Images

Order:

  1. Indeterminate

Family:

  1. Indeterminate

Family:

  1. Etheridgea petrica

Family:

  1. Elateridium wianamattense

Family:

  • Schizocoleidae
  1. Metrorhynchites sydneiensis

Family:

  • Schizocoleidae
  1. Metrorhynchites dunstani

Order:

  1. Mesotitan giganteus

Order:

  1. Notoblattites subcostalis

Unranked taxon:

  1. Iverya averyi
Bivalvia of the Ashfield Shale
Taxa Presence Description Images

Family:

  1. Unio sp.
Sharks of the Ashfield Shale
Taxa Presence Description Images

Family:

  1. Pleurocanthus decheni
Pleurocanthus
Fish of the Ashfield Shale
Taxa Presence Description Images

Family:

  1. Semionotus sp.
Semionotus

Family:

  1. Cleithrolepis sp.
Cleithrolepis

Family:

  1. Acentrophorus sp.

Family:

  1. Myriolepis sp.

Family:

  1. Palaeoniscus sp.

Family:

  • Platysomidae
  1. Platysomus sp.

Order:

  1. Sagenodus sp.
Amphibians of the Ashfield Shale
Taxa Presence Description Images

Family:

  1. Paracyclotosaurus davidi
Paracyclotosaurus

Family:

  1. Microposaurus averyi
Microposaurus averyi

Family:

  1. Notobrachyops picketti
Reptiles of the Ashfield Shale
Taxa Presence Description Images

Family:

  1. Kudnu mackinlayi

Family:

  1. Rhynchosauria sp.

Family:

  1. Theropoda sp.

References

  1. ^ a b c d Packham, Gordon Howard, ed. (November 1969). The Geology of New South Wales. Sydney: Geological Society of Australia. pp. 417–421.
  2. ^ "Geological History". Sydney Olympic Park Authority. NSW Government. Retrieved 23 July 2012.
  3. ^ Fairley A, Moore P (2000). Native Plants of the Sydney District:An Identification Guide (2nd ed.). Kenthurst, NSW: Kangaroo Press. p. 19. ISBN 0-7318-1031-7.
  4. ^ "Lane Cove Collapse". Tunnel Talk. Retrieved July 22, 2012.
  5. ^ "Recent Experiences in Grouting Sydney Sandstone". Australian Tunnelling Society. ats.org.au. Retrieved July 23, 2012.
  6. ^ a b John Johnson (2008). "Croydon". Dictionary of Sydney. Dictionary of Sydney Trust. Retrieved 23 July 2012.
  7. ^ "Blue Gum High Forest - Typical Form" (PDF). Mapping and Assessment of Key Vegetation Communities Across the Ku-ring-gai Local Government Area. Ku-Ring-Gai Council. Archived from the original (PDF) on 18 March 2011. Retrieved 23 July 2012.
  8. ^ Cosgriff, J. W. (1973). "Notobrachyops picketti, a brachyopid from the Ashfield Shale, Wiannamatta Group, New South Wales". Journal of Paleontology. 47 (6): 1094–1101.
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