Jump to content

Ashfield Shale

Add links
From Wikipedia, the free encyclopedia

Ashfield Shale
Stratigraphic range: Middle Triassic (Anisian) 247–241.464 Ma
Exposed Ashfield Shale opposite of Parramatta ferry wharf, Parramatta
TypeGeological formation
Unit ofWianamatta Group
Sub-unitsKellyville Laminite Member, Mulgoa Laminite Member, Regentville Siltstone Member, Rouse Hill Siltstone Member
UnderliesMinchinbury Sandstone
OverliesHawkesbury Sandstone, Mittagong Formation
ThicknessBetween 44.6 metres (150 ft) and 61.6 metres (200 ft)
Lithology
PrimaryShale
OtherSandstone, mudstone, claystone and siltstone
Location
LocationSydney Basin (Greater Sydney)
Coordinates34°05′13″S 150°45′18″E / 34.087°S 150.755°E / -34.087; 150.755 (Glenlee Railway Cutting)
CountryAustralia
Type section
Named forAshfield
Ashfield Shale is located in Australia
Ashfield Shale
Ashfield Shale (Australia)
Ashfield Shale is located in New South Wales
Ashfield Shale
Ashfield Shale (New South Wales)

Ashfield Shale is a shale formation that is part of the Wianamatta Group of sedimentary rocks in the Sydney Basin, and it lies directly on contemporaneously eroded Hawkesbury Sandstone[1] or the Mittagong Formation.[2] The Ashfield Shale consists of strata that were formed in the Middle Triassic.[2] Ashfield Shale is generally a dark, nearly black rock that should be distinguished from Bringelly Shale.[1] Ashfield Shale has outcrops over many parts of the Greater Sydney area. Many structures and deep excavations were constructed on Ashfield Shale, in addition to the shale being used as a construction material.[3]

Named after the Sydney suburb of Ashfield, this rock type is often associated with the Inner West and North Shore of the city. However, it has also been recorded at Bilpin and Mount Irvine. Ashfield Shale comprises black mudstones and silty shales with frequent sideritic ironstone bands.[1] Measured boreholes suggest a thickness at least 44.6 metres (146 ft) at the Picton area, which increases to a total of 61.6 metres (202.1 ft) around Erskine Park.[2] Ashfield Shale is associated with the critically endangered Blue Gum High Forest and Sydney Turpentine-Ironbark Forest.[4][5]

Description

[edit]
Exposed Ashfield Shale near the Pacific Highway, Chatswood

The Minchinbury Sandstone sits between the Ashfield Shale and the superimposed Bringelly Shale. The Minchinbury Sandstone serves as a buffer between the shallow water Ashfield Shale and the alluvial plain sediments of the overlying Bringelly Shale, hinting it was a strandline deposition.[2] The lower Ashfield Shale was deposited in a freshwater environment, while upper layers hold sediments that were laid in a shallow marine or brackish environment.[2] Being the basal formation of the Wianamatta group, Ashfield Shale is the only rock unit outcropping in the inner Sydney area. It has a minimum complete thickness of 44.6 metres (146 ft) in the Picton area and a maximum thickness of 61.6 metres (202 ft) near Erskine Park.[3]

The geology of the shale lenses within the Hawkesbury Sandstone is lithologically similar to the Ashfield Shale.[1] The fine grained silty sediments were laid down in a low energy, northwest flowing deltaic setting, near the shores of a shallow sea. High in the shale's formation indicate a high sand content, with ripples on the formation suggesting delta front conditions. Weathering of the shale units produces a reddish/brown podsolic soil, often with poor drainage, such as that in the Cumberland Plain. These clay soils are recognised as being reactive with an appreciable shrink-swell capacity. Because of weathering, the Ashfield shale is seldom too exposed except in quarry faces.[6]

In Lamb's Quarry, Rooty Hill, an olivine dolerite dyke intrudes Ashfield Shale.[7] The rocks found at the Ryde-interchange site usually consist of yellow brown and dark grey shale with light grey silty laminations and lenses. At Moorebank, the Ashfield shale regularly features black to dark grey shales with laminated silty bands. At Surry Hills, they are dark grey, horizontally bedded with coarse light grey and fine grained laminations and lenses.[3] Generally, Ashfield Shale samples had a consistent interbedded structure, characterised by frequent, very thin light-grey silty bands within dark-grey siltstone and with an average clay mineral content of 43%. Siderite is the main cementing agent, whereas mica acts as a subsidiary cementing agent. Microscopic studies of Ashfield Shale indicate that silt-sized quartz particles are embedded within a clay matrix, with widespread distribution of siderite. Having no microcracks, the shale is highly compacted, resulting in a pronounced alignment of the clay particles. It also exhibited a typical soft-rock behaviour, with a roughly linear stress–strain response up to a peak, followed by pronounced strain softening.[8] The Ashfield Shale is affected by faulting in parts of the Sydney region, including along the Homebush Bay Fault Zone, a major north-northeast-trending fault system extending through western and northern Sydney.[9]

Lithology

[edit]

The chemistry of the rock is typical of shales, with high iron levels, and some iron sulphide and low calcium levels. The Ashfield Shale consists of a lower sequence of dark-grey to black, sideritic claystone and siltstone lines which grade upwardly into a fine sandstone and siltstone laminite, and then into the Minchinbury Sandstone which overlies it. The lower portion mainly features dark grey claystone and siltstone laminites which grade upwards into lighter and thicker sandstone beds. Small scale bedding is abundant. The shales are sandy at the top of the sequence.[1]

Ashfield Shale exposed at Eastwood

The porosity ranges between 5% for fresh shale from Surry Hills and Moorebank sites to around 13 % for the Ashfield Shale from the Ryde-interchange site.[3] The Kellyville laminite is characterised by a clear increase in lithic-quartz sandstone laminations. The Regentville siltstone is made up of black to dark-grey siltstone. In some places, such as south-east of Moss Vale, it progressively becomes more sandy to the top where it is overlain by the Minchinbury sandstone. The Mulgoa laminite comprises dark-grey siltstone and fine, light-grey sandstone laminae.[3]

Distribution

[edit]
Light upper-level Ashfield Shale is present in the Prospect dolerite intrusion, in addition to the Sydney sandstone.

Herbert (1979) proposed the subdivision of the Ashfield Shale into four members, which from the ground upwards include: the Rouse Hill siltstone, the Kellyville laminite, the Regentville siltstone, and Mulgoa laminite.[2] The Ashfield Shale covers about 80% of the Blacktown City Council, namely north of the Great Western Highway.[7] Ashfield Shale outcrops extend to higher elevated areas and into places with higher rainfall than Bringelly Shale, such as the North Shore area where rainfall exceeds 1,000 mm (39.37 in), the Bilpin–Mountain Lagoon area and the fringes of the Illawarra escarpment where rainfall may exceed 1,500 mm (59.06 in).[10]

The stratum of Prospect quarry in Pemulwuy comprises Hawkesbury Sandstone and lacustrine Ashfield Shale.[11] Rocks of the Ashfield Shale represent the river-cliff in Mulgoa Nature Reserve, just south of Penrith which presents dark coloured Mulgoa Laminite Member, and as well as the river cliff opposite of the Parramatta ferry wharf. The Mulgoa cliff can be observed from Mulgoa Road through the riparian forest, though access by foot is challenging.[10]

There are no natural sections in which the Ashfield Shale is completely exposed.[1] 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.[1] The shale members were once partially exposed in the Thornleigh brick pit, though this has recently been filled in. Today, the most observable faces of Ashfield Shale are present along the North Shore and Northern railway lines and Bells Line of Road, and in several other road cutting exposures, like, Horace Street, St Ives and in the Picton–Campbelltown area.[10]

Engineering and construction

[edit]
Ashfield Shale visible at the bottom strata of Prospect Hill, near industrial site, Pemulwuy.

The term Ashfield shale has been used to cover siltstone, claystone and laminite.[3] In the earlier days of Sydney, the Ashfield Shale supported a number of quarries. The shale provided a suitable raw material for brickmaking.[1] 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. The Wianamatta group rocks, particularly the Ashfield Shale, and their weathering stock are important for engineering as they shape the foundations for many buildings of Sydney CBD and Western Sydney. Ashfield Shale has also been used for constructing roads. The Narellan cutting, on the abandoned Camden branch line, is cut through the Ashfield Shale.[2]

Notwithstanding, deep excavations in Ashfield Shale can be a risky procedure with the existence of many fractures. Because the main feature of the unit is the moderately to steeply immersing continuous, planar joints that are often linked with thrust faulting, and these may at times form an unstable wedge when exposed during excavation. If not contained properly, the rock's wedge may become unstable or skid into the excavation.[1] The Epping to Chatswood rail link (ECRL) involved drilling through a mixture of sandstone and highly weathered Ashfield Shale, which required advanced ground support measures like rock anchors to manage the high horizontal stresses.[12]

The adjacent Hawkesbury Sandstone is considered a safer bedrock than the (less stable and laminated) Ashfield Shale for building construction. In November 2005, the construction of the Lane Cove Tunnel was affected by the collapse of an exit ramp excavation, through weak shale with low cover and a vertical dolerite dyke in the Ashfield Shale.[13] Difficulties may be encountered where the Ashfield Shale interfaces with the Hawkesbury Sandstone and the Mittagong Formation.[14] The term Ashfield Shale has been used to cover siltstone, claystone and laminite.[3]

Vertebrates

[edit]

Actinopterygii

[edit]
Actinopterygians of the Ashfield Shale
Genus[2] Species Localities Clade Images
Cleithrolepis C. granulatus
  • St. Peters Brickpits
  • Kogarah (Blakes Quarry)
 Perleidiformes,

Cleithrolepididae
Fossil of C. granulatus.
Elonichthys E. armatus
  • St. Peters Brickpits
 Palaeonisciformes,

Palaeoniscidae
Fossil of E. armatus.
E. semilineatus
Fossil of E. semilineatus.
Elpisopholis E. dunstani
  • St. Peters Brickpits
 Palaeonisciformes,

Palaeoniscidae
Fossil of E. dunstani.
Myriolepis M. pectinata
  • St. Peters Brickpits
 Palaeonisciformes,

Palaeoniscidae
Fossil of M. pectinata.
Palaeoniscus P. antipodeus
  • Parramatta (Parsonage Hill)
  • St. Peters Brickpits
 Palaeonisciformes,

Palaeoniscidae
Fossil of P. antipodeus.
P. crassus
  • St. Peters Brickpits
Fossil of P. crassus.
Platysomus P. sp.
  • St. Peters Brickpits
 Platysomiformes,

Platysomidae
Fossil of the related P. striatus.
Promecosomina P. formosa
  • St. Peters Brickpits
 Parasemionotiformes,

Promecosominidae
Fossil of P. formosa.
Semionotus ?S. sp.
  • Gibraltar Tunnel Railway Cutting
 Semionotiformes,

Semionotidae
Life restoration of the related S. elegans.

Chondrichthyes

[edit]
Chondrichthyans of the Ashfield Shale
Genus[2] Species Localities Clade Images
Mooreodontus M. parvidens
  • St. Peters Brickpits
 Xenacanthiformes,

Xenacanthidae
Fossil of M. parvidens.
Xenacanthidae indet. Multiple sp. indet.
  • Gibraltar Tunnel Railway Cutting
  • Parramatta (Parsonage Hill)
 Xenacanthiformes,

Xenacanthidae
Fossil of indeterminate xenacanthid.

Sarcopterygii

[edit]
Sarcopterygians of the Ashfield Shale
Genus[2] Species Localities Clade Images
Archaeoceratodus A. avus
  • St. Peters Brickpits
 Ceratodontiformes,

Ceratodontidae
Sketch of A. avus specimen.

Temnospondyli

[edit]
Temnospondyls of the Ashfield Shale
Genus[2] Species Localities Clade Images
Microposaurus M. averyi
  • Picton Area
 Temnospondyli,

Trematosauridae
Life restoration of M. averyi.
Notobrachyops N. picketti
  • Mortdale (Hurstville Brick Co. Quarry)
 Temnospondyli,

Brachyopidae
Life restoration of N. picketti.
Paracyclotosaurus P. davidi
  • St. Peters Brickpits
 Temnospondyli,

Mastodonsauridae
Reconstruction of P. davidi.
Brachyopidae indet. sp. indet.
  • St. Peters Brickpits
 Temnospondyli,

Brachyopidae

Invertebrates

[edit]

Crustaceans

[edit]
Crustaceans of the Ashfield Shale
Genus[2] Species Localities Clade Images
Protamphisopus P. wianamattensis
  • St. Peters Brickpits
 Isopoda,

Amphisopodidae
Fossil of P. wianamattensis.

Insects

[edit]
Insects of the Ashfield Shale
Genus[2] Species Localities Clade Images
Ademosyne A. wianamattensis
  • Narellan Railway Cutting
 Coleoptera,

Ademosynidae
Fossil of A. wianamattensis.
Elateridium E. wianamattensis
  • St. Peters Brickpits
 Coleoptera,

Elateridae
Sketch of E. wianamattensis specimen.
Etheridgea E. petrica
  • St. Peters Brickpits
 Coleoptera,

Curculionidae
Sketch of E. petrica specimen.
Iverya I. averyi
  • Parramatta (Parsonage Hill)
 Odonata,

Triadotypidae
Fossil of I. averyi.
Mesorhynchophora M. dunstani
  • St. Peters Brickpits
 Coleoptera,

Curculionidae
Sketch of M. dunstani specimen.
Mesotitan M. giganteus
  • St. Peters Brickpits
 Titanoptera,

Mesotitanidae
Metrorhynchites M. sydneiensis
  • St. Peters Brickpits
 Coleoptera,

Schizocoleidae
Sketch of M. sydneiensis specimen.
Notoblattites N. subcostalis
  • St. Peters Brickpits
 Blattodea,

incertae sedis
Sketch of N. subcostalis specimen.

Molluscs

[edit]
Molluscs of the Ashfield Shale
Genus[2] Species Localities Clade Images
Protovirgus P. sp.
  • Bowral Brick Quarry
  • St. Peters Brickpits
 Venerida,

Glauconomidae
Fossil of P. sp. (Bowral Brick Quarry)
Unio U. sp.
  • Croydon
 Unionida,

Unionidae
Unionella P. wianamattensis
  • Gibraltar Tunnel Railway Cutting
  • Waterloo (Goodlet and Smith's Brickclay Quarry)
 Venerida,

Glauconomidae
Fossil of U. wianamattensis.

Plants

[edit]
Color key
Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon

Notes
Uncertain or tentative taxa are in small text
crossed out taxa are discredited.

Plants

[edit]
Plants of the Ashfield Shale
Genus[2] Species Localities Clade Images
Cladophlebis C. australis
  • St. Peters Brickpits
 Osmundales,

Osmundaceae
Fossil of C. australis.
Cycadopteris C.? scolopendrina
  • St. Peters Brickpits
 Cycadales,

incertae sedis
Fossil of C.? scolopendrina.
Cylostrobus C. sydneyensis
  • Brick Quarry
  • Gibraltar Tunnel Railway Cutting
 Isoetales,

Pleuromeiaceae
Fossil of C. sydneyensis.
Dicroidium D. antevsiana
  • Parramatta (Parsonage Hill)
 Corystospermales,

Corystospermaceae
Fossil of D. odontopteroides.
D. lancifolium
  • Mortdale (Hurstville Brick Co. Quarry)
D. odontopteroides
  • Mortdale (Hurstville Brick Co. Quarry)
  • St. Peters Brickpits
Ginkgo G. digitata
  • St. Peters Brickpits
 Ginkgoales,

Ginkgoaceae
Fossil of G. digitata (Sydney S.s.).
Phyllotheca P. sp.
  • Ingleburn
 Equisetales,

Phyllothecaceae
Fossil of P. sp. (Sydney S.s.)
Salisburia S. palmata
  • St. Peters Brickpits
 Ginkgoales,

Ginkgoaceae
Fossil of S. palmata.
Schizoneura S. sp.
  • Mortdale (Hurstville Brick Co. Quarry)
 Equisetales,

incertae sedis
Taeniopteris T. crassinervis
  • St. Peters Brickpits
 Tracheophytes,

incertae sedis
Fossil of the related Taeniopteris sp.

See also

[edit]

References

[edit]
  1. 1 2 3 4 5 6 7 8 9 Lovering, J. F. (1954-06-25). "The stratigraphy of the Wianamatta Group Triassic System, Sydney Basin". Records of the Australian Museum. 23 (4). The Australian Museum: 169–210. doi:10.3853/j.0067-1975.23.1954.631. ISSN 0067-1975.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 McLean, Graham (2025-07-02). "The Sydney Basin in the Triassic – a review of the geology, flora and fauna, and ecosystems. The Wianamatta Group". Technical Reports of the Australian Museum (online). 42. Australian Museum: 1–29. doi:10.3853/j.1835-4211.42.2025.1909. ISSN 1835-4211.
  3. 1 2 3 4 5 6 7 Ghafoori, Mohammad (5 Apr 2023). "Engineering Behaviour of Ashfield Shale" (PDF). School of Civil and Mining Engineering, The University of Sydney. Retrieved 4 November 2025.
  4. ↑ John Johnson (2008). "Croydon". Dictionary of Sydney. Dictionary of Sydney Trust. Retrieved 23 July 2012.
  5. ↑ "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.
  6. ↑ 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.
  7. 1 2 Adamson, C. L.; Flack, D.S. (1962). "Geological Report on Sources of Igneous Rock in the Shire of Blacktown" (PDF). Department of Mines, New South Wales. Retrieved 5 November 2025.
  8. ↑ Ezat William; David Airey (2004). "Index properties and the engineering behaviour of Bringelly Shale" (PDF). Sydney: australiangeomechanics.org. Retrieved April 8, 2025.
  9. ↑ Och, D. J.; Offler, R.; Zwingmann, H.; Braybrooke, J.; Graham, I. T. (2009). "Timing of brittle faulting and thermal events, Sydney region: Association with the early stages of extension of East Gondwana". Australian Journal of Earth Sciences. 56 (7): 873–887. doi:10.1080/08120090903005345.
  10. 1 2 3 "Wianamatta Group". Step Inc. 25 Apr 2017. Retrieved 5 November 2025.
  11. ↑ Compton, Keith (5 Apr 2023). "Prospect Quarry, New South Wales". Mindat.org. Retrieved 4 November 2025.
  12. ↑ "Rail tunnel design, Epping to Chatswood, NSW, Australia". PSM: Geotechnical & Engineering Services. Retrieved November 9, 2025.
  13. ↑ "Lane Cove Collapse". Tunnel Talk. Retrieved July 22, 2012.
  14. ↑ "Recent Experiences in Grouting Sydney Sandstone". Australian Tunnelling Society. Retrieved July 23, 2012.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Ashfield_Shale&oldid=1383796809"