Wianamatta Group
The Wianamatta Group is a geological feature of the Sydney Basin, New South Wales, Australia that directly overlies the older Hawkesbury sandstone. The formation is primarily composed of shales with interbedded lithic sandstones, resulting in generally flat topography as the shales erode. The Wianamatta Group was derived from the Aboriginal name for South Creek. Other names for the group include Wianamatta Beds, Wianamatta Formation and Wianamatta Series.
The Wianamatta Formation is the youngest geological layer member of the Sydney Basin, and therefore lies at the highest point as the highest layer member. It was deposited in connection with a large river delta, which shifted over time from west to east. The Wianamatta Group is made up of the following units : Bringelly Shale, Minchinbury Sandstone and Ashfield Shale. Based on their general characteristics and lithologic associations, the sandstones of the Wianamatta Group are described as graywacke-type sandstones. Overall, the Wianamatta Group records a regressive clastic shoreline, with depositional environments progressing upward through four main settings: seaward marine shelf, beach and barrier island, lagoonal and tidal flat complex, and fluvial coastal plain.
The post-sedimentary tectonic history of the group is closely linked to the structural evolution of the Sydney Basin as a whole. Within the present geographic extent, the Wianamatta Group is largely confined to the Cumberland Plain. Weathering of the shale units produces a rich clayey soil, often with poor drainage. Although the Wianamatta Formation received some attention from local researchers in the early 20th century, it was formally defined in 1952, comprehensively documented in 1954, and subsequently revised in 1979.
Grouping
The Wianamatta Group is divided into two subgroups: the Liverpool Subgroup – lower, approximately thick, predominantly shale, and the Camden Subgroup – upper, approximately thick, with sandstone prominent but also containing shale. The Liverpool Subgroup comprises three compositions: Ashfield Shale, Bringelly Shale and Minchinbury Sandstone. The Camden Subgroup comprises five formations: Annan Shale, Razorback Sandstone, Potts Hill Sandstone, Picton Formation, and Prudhoe Shale. The sedimentary lithology of the graywacke sandstone types and their relationship to the sedimentary setting and tectonic setting has been described. A new classification element, the depositional environment, has been proposed to complement the previously defined lithotope for characterizing lithologic sequences. Post-depositional tectonics have also been discussed, including a recent succession of tectonic features identified in the south-western suburbs of Sydney.Potts Hill Sandstone and the Razorback Sandstone are limited in extent and cannot be reliably traced beyond their type localities. Consequently, they are unsuitable as boundary markers for subdivisions between units such as the Camden and Liverpool Subgroups. Therefore, all strata over the Minchinbury Sandstone are classified as part of the Bringelly Shale, with the prominent sandstone layers treated as individual members. Naming the Wianamatta sequence, early authors used "Wianamatta Beds," "Formation," or "Shales," while Leslie Wilkinson applied "Wianamatta Series," and Carl Süssmilch used "Wianamatta Stage." Thomas Lindsay Willan separated the Wianamatta Group into these layers – upper, middle, and lower, though the criteria for these divisions were not fully described. His 1925 Sydney District map showed the stages, but the classification has not been widely adopted due to difficulties reconciling the outcrop patterns with the brief stage descriptions. The lower stage was approximately of carbonous and clay shales with several basal sandy beds. This Formation was 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 deposited 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. The Liverpool Subgroup, which lies directly above the Hawkesbury Sandstone, is dominated by shale, with two shale formations having been mapped. A centrally located sandstone formation approximately thick is present, but shale remains the principal lithology. The subgroup has a fairly consistent thickness of about. The shales of the Liverpool Subgroup define the primary rural zones of the Cumberland Basin and Camden region and provide a significant source of fresh material for brickmaking and ceramics industries. The constitutions of the Liverpool Subgroup encompass nearly the whole protrusion area of the Group. The Camden Subgroup is separated from the Liverpool Subgroup by a thick sandstone formation. It consists of a succession of interchanging sandstones and shales, with sandstone being more prominent. The subgroup exhibits variable thickness, reaching a maximum of approximately in the Razorback section. The Razorback Range serves as the type region for the Camden Subgroup. The type section is located by the Hume Highway at southern side of Razorback.
The Wianamatta Group exhibits a continuous sedimentary sequence, with no major breaks, indicating deposition as a largely uninterrupted progression of environments across the region. The sequence records a transition from marine to terrestrial settings, becoming progressively more sandy toward the top. The basal Ashfield Shale consists of shallow marine black siltstone that grades upward into laminated fine sandstone exhibiting holes and ripple traces. Overlying this, the Minchinbury Sandstone, characterized by low-angle cross-bedding, represents a beach and barrier-bar system that protected an intertidal lagoon, recorded as a laminite sequence over six metres thick, containing invertebrate burrows, desiccation fissures, and ripple structures. The basal part of the Bringelly Shale continues this lagoonal deposition and transitions upward into closely interbedded laminite and carbonaceous claystones of intertidal flats and marshes, preserving abundant fossil roots and sporadic coal layers. Upper in the Bringelly Shale, interstratified light grey and carbonaceous claystones with pervasive fossil roots grow common, suggesting incipient soil development in swampy environments. Laminites in this interval are less volumetrically significant, lack burrowing structures, and contain thick carbonaceous root sills and affected bedding, interpreted as levees associated with streams and tidal creeks. Cross-bedded sandstone channels, up to thick and with erosive contacts, represent fluvial and estuarine channels.
Research history
George Davenport Osborne highlighted the overall shortage of data on the Triassic sequence in the region, which prompted further study. The primary objective was to examine outcrops of the Wianamatta Group and determine whether consistent, mappable divisions could be established based on lithology, divisions that had previously been regarded as difficult or impossible to map. Fieldwork, combined with the interpretation of available borehole data, demonstrated that such divisions are indeed consistent and can be mapped. The work was intended to provide practical value to engineers and geologists working for public utilities and other agencies concerned with the rock types in the Sydney Basin. The first description of the rocks now recognized as the Wianamatta Formation was by geologist Joseph Jukes, who observed:"From Parramatta by Liverpool to Campbelltown the country is low, gently undulating and composed almost entirely of black and brown shales, with a few thin inter-stratified beds of sandstone in their lower portion."
Jukes identified these strata as lying over the Sydney Sandstone and estimated their thickness at at least. The following year, Clarke provided further observations and used the term "Wianamatta Rocks" for the first time. Between 1848 and 1870, Clarke occasionally referred to the "Wianamatta beds," and in 1870 he provided a more detailed description, estimating a total thickness of –. Clarke’s final description in 1878 observed that the group succeeded the Hawkesbury Sandstone and consisted primarily of shales, but also included sandstones, calcareous sandstones, ferruginous nodules, and locally cylindrical and pisolitic ironstone with profuse fossil wood, plant prints, and calcareous sandstones forming the uppermost degrees and peaks of remote hills – approximately –. Small, discontinuous patches of coal were present, but no economically valuable seams were observed. In 1883, Tenison-Woods challenged Clarke’s definition, suggesting that the shales did not overlie the Hawkesbury Sandstone but were intercalated with it, and therefore no distinct Wianamatta formation existed. This criticism has not been confirmed, and Clarke’s description is now usually accepted.
Regarding age, the Wianamatta Group was initially assigned to the Palaeozoic by Jukes. Clarke first considered it Carboniferous but later recognized a Mesozoic age, and Charles Smith Wilkinson suggested it was probably Triassic, which became generally accepted. Willan described the bottom and center levels as Upper Triassic, while suggesting the Upper Stage might be Jurassic based on Frederick Chapman's identification of foraminifera and ostracoda; however, these identifications are now considered doubtful. David F. Branagan concluded that fossil evidence indicates an Upper Triassic age for the “Wianamatta Series,” but noted that fossils are limited to a narrow zone less than above the Hawkesbury Sandstone. Therefore, the Upper Triassic designation can only be confidently applied to a small portion of the Group, while the remaining may be Jurassic, though this remains speculative. For the purposes of modern classification, the Wianamatta Group is simply considered Triassic in age. While not typically high-rank graywackes, these rocks show features approaching subgraywackes, a distinction reflected in the terminology. For field descriptions and formation names, the word “sandstone” is withheld as the general designation. Some analyses of exceptional samples indicate compositions closer to true graywackes, whereas sections of the Potts Hill Sandstone display characteristics more closely related to feldspathic quartzites.