In the late 1970s, the Bringelly Shale was redefined to include all Wianamatta Group sediments above the Minchinbury Sandstone. Consequently, substantial sandstone units—up to 30 metres (98ft) thick but of limited lateral extent, typically representing channel or point bar deposits—are now treated as members of the Bringelly Shale. These include the Potts Hill Sandstone, Razorback Sandstone, and Mt Hercules Sandstone.[3] Bringelly Shale was deposited in large swamplands and winding estuarine and alluvial channels.[4]
The average thickness is around 60 metres (200ft), though a maximum thickness of 257 metres (843ft) was recorded at Razorback, near Campbelltown. However, due to greater post-Triassic erosion in the Parramatta–Sydney area, the unit is largely confined to the synclinal structure of the Fairfield Basin, where it reaches around 60 metres (200ft) at Potts Hill.[3][5][1] Bringelly Shale was deposited on a broad, swampy alluvial plain, where fluvial channels, locally exhibiting braided characteristics, formed sporadic beds of sandstone. The shale is dark when unweathered just like the Ashfield Shale, and is usually a typical olive-green colour when weathered. Alloyed coal belts and lenses and iron oxide densities have been observed in the shale. The shale is quarried in many Sydney's western suburbs for brick and miscellaneous ceramic manufacture.[6]
Bringelly Shale is weakly cemented and exhibits lower strength and stiffness than Ashfield Shale. Although both shales have comparable unrestrained compressed strengths, typically ranging from 10 to 50 MPa, a significant proportion of the strength in Bringelly Shale is attributed to pore-water suction. The claystone units comprise several types of fine-grained sediment, including light-grey leached claystone, grey to almost black carbonaceous claystone, and non-carbonaceous medium or dark-grey claystone and siltstone. These shale types are interpreted as reflecting various sedimentary settings, with the Ashfield Shale formed under marine conditions and the Bringelly Shale deposited in an alluvial setting. Samples of Bringelly Shale have been collected from quarries at Badgerys Creek, Horsley Park, Kemps Creek, and Mulgoa, where the shale is extracted for brick manufacture. Material from all sites is characterised as a non-carbonaceous mid- to dark-grey claystone. Organic matter and minor recrystallisation of mica may contribute to cementation; however, these processes are poorly developed, and overall cementation is considered weak.[7]
Additional tests on extraordinarily weathered matter from four quarry sites indicate that weathering is affiliated with variations in mineralogy. Chlorite and some illite are broken down, accompanied by an increase in mixed-layer clay minerals and the formation of a minor montmorillonite fraction. These mineralogical changes tend to raise the receptiveness of the remaining soil relative to the parent shale. This trend is reflected in the liquid maximum of squashed shale, which rises from approximately 30 in fresh material to over 50 in extremely weathered shale. In practice, residual soils developed on the shale are considered relatively old and are commonly leached and laterised, reducing their reactivity compared with expectations based on mineralogy alone. Soils derived from Bringelly Shale may exhibit significant effects associated with expansive clays, particularly on moderate slopes where soils are younger and show reduced evidence of laterisation.[7]
Sedimentology and Lithology
Bringelly Shale shapes the topmost rock unit over an area of approximately 750 square kilometres (290sqmi) in Greater Sydney. It is typically covered by a residual soil layer 2–8 metres (6.6–26.2ft) thick. Major block samples and cores of the fresh claystone–siltstone that make up the majority of the shale have been collected from several locations, primarily from four active quarries where the shale is extracted for brickmaking.[8] The shale was laid down by a coastal alluvial river delta over the older Hawkesbury sandstone in the Triassic Period.[9] It transitions from an inlet or coastal swamp sheet at the bottom in a marshy plain deposited on the delta, to a more alluvial plain sediment at the top of the unit. It roamed through the rivers of Sydney, and accumulated sand at numerous locations, which it then solidified into sandstone.[1] Overbank sediments and channel sandstones hint that most of the Bringelly Shale was formed in sinuous streams proximate to flood basins, most likely ending in coastal lagoons.[4] There is not much evidence of induration or cementation in the shale, suggesting that its low porosity developed through substantial burial of the sediments. However, the geology of the Sydney Basin is not well comprehended, and evaluations of the depth of overlying sediments at present range from tens of metres to as much as 4,000 metres (13,000ft).[8] Bringelly Shale's durability ranges from medium in fresh, intact material to very low in extremely weathered shale. It exhibits significant microcracking along the planes of lamination.[7]
The Bringelly Shale is composed predominantly of claystone–siltstone (70%), with substantial proportions of laminite and sandstone (25%), in addition to coal and highly carbonaceous claystone (3%) and tuff (2%).[7] It is similar to Ashfield Shale in that both have low porosities, though differing in having a greater amount of calcareous, graywacke-type, lithicsandstone bands and lenses, carbonaceousclaystone, siltstone and laminite. Bringelly Shale also lacks sideriticmudstone bands that Ashfield Shale has, in addition to Ashfield Shale being a darker, nearly black, clay-mineral-rich rock. Featuring lumpy clay minerals, it swells and decays rapidly on submergence in water and is generally less durable.[10] Although classified as a rock, there is very small evidence of cementation in the claystone. The material is highly compacted and has very low porosity. Its mechanical behaviour has been investigated using triaxial, direct shear box, and ring shear tests on reconstituted specimens, which were then compared with natural samples. Results indicate that the standardized manner of the reconstituted material differs significantly after compression to stress levels required to replicate in-situ porosity, and the observed behaviour is conflicting with crucial state conceptions typically applicable at higher porosities. Comparisons between natural and reconstituted materials suggest that cementation and de-structuring have limited influence, as both exhibit similar strengths at the same void ratio, with friction angles significantly lower than those of reconstituted material at higher void ratios.[8]
Unlike the Ashfield Shale, it features sandstone lithology and lenticles that fluctuate from 2.5 centimetres (0.98in) to 150 centimetres (59in) in thickness, in addition to having a disposition for the thicker lens bands that are concentrated at the top of the rock. These thicker sandstone intervals are generally concentrated near the top of the formation, but their lateral extent is sparse, and the majority taper off rapidly. Petrographically, these sandstones are comparable to the graywacke-type sandstones found in other, more massive Wianamatta Group formations. The sandstones within the shale are filled sediments in channels that were created by the braided rivers in the area, which wind across the marshy lowlands.[2] In unweathered sections, the shale is black and it looks like the Ashfield Shale, though it lacks the sideritic mudstone bands typical of that constitution. Weathered shale typically exhibits an olive-green color. Impure coal seams, lenses, and iron oxide encrustations were observed within the shales.[5]
The lower 30 metres of the Bringelly Shale are thinly bedded and contain the highest carbonaceous content within the Wianamatta Group. Above this basal zone, claystone, siltstone, and sandstone beds increase in thickness. The Bringelly Shale is a primary source of brickmaking material in the Sydney region. Its variable siderite content produces a range of fired colours from cream to red, although economically significant deposits of light-firing material are rare. Approximately two million tonnes of Bringelly Shale are extracted annually in the Sydney area.[3]
Claystone typically grades into siltstone in sequences up to 15 metres (49ft) thick. These beds are composed primarily of quartz, kaolinite, and micaceous clays, with a higher proportion of expandable mixed-layer illite/smectite than the underlying Ashfield Shale. Siderite occurs commonly, though less uniformly than in the Ashfield Shale.[3]
Laminite units, generally less than 5 metres thick, are distributed throughout the Bringelly Shale. They comprise varying proportions of quartz-lithic, light grey, fine, micro-cross-bedded sandstone and dark grey siltstone, with laminae typically 10–20 mm thick. Siderite grains and nodules, as well as carbonaceous plant remains along bedding planes, are common.[3]
Channel sandstones in the basal 30 metres (98ft) are typically less than 2 metres (6.6ft), increasing to more than 6 metres above this zone and rarely reaching 16 metres (52ft). An exception is the Mt. Hercules Sandstone in the Razorback Range, which attains a thickness of up to 44 metres (144ft). Angular shale fragments frequently occur above the basal contacts in massive, medium-grained sandstone. Published petrology data indicate that these sandstones generally contain less than 40% quartz, 5–30% feldspar, and the remainder as clay pellets and a matrix of clay, calcite, chlorite, and siderite.[3]
Distribution
The Bringelly Shale formation exhibits extensive outcrops across the Sydney, Liverpool, and Camden sheets, particularly in the Greater Western Sydney area. Smaller, isolated exposures occur atop Box Hill, Rogans Hill and Moss Vale.[5] The disjointed sandstone lenses become thicker and more conspicuous from Western Sydney Regional Park and to the south of it, whereby shaping the hilly landscape between Campbelltown and Picton.[2] Namely found on Old Hume Highway, approaching Picton, the sandstone cliffs that become thicker are roughly around 200 metres (660ft) in height. The steep banks of the sandstone lentils influence the flora of the Cumberland Plain Woodland, with such escarpments being observed in Western Sydney Dry Rainforest areas.[7] The shale runs into the Potts Hill Sandstone through a minor transitional zone of vacillating sandstone and shale bands.[11]Cladophlebis australis, an extinct fern, and Lycopod cone scales have been found in the formation.[4]
Razorback sandstone
The Razorback Sandstone, named after the Razorback Range in the parish of Picton, County Camden, is a prominent geological formation traversed by the Hume Highway approximately 11 kilometres (6.8mi) south of Camden. The type area for this formation is the Razorback Range itself, with the type section exposed in road cuttings through the northern rise of the Hume Highway, where approximately 18 metres (59ft) of massive graywacke-type sandstone can be observed.[12]
The Razorback Sandstone also occurs at a distinct second physiographic bench level, recognizable beyond the type area. In the Liverpool Sheet, isolated outcrops appear near Cecil Hills and Bringelly, while in the Camden Sheet, scattered exposures cap geographic features including north of Narellan, Gregory Hills, near Currans Hill (formerly Kenny Hill), Mount Annan and Sugarloaf Hill (in what is now Spring Farm and Menangle). Lithologically, the formation consists predominantly of massive graywacke-type sandstone, typically around 18 metres (59ft) thick, with minor thin, transversely restricted lenses of dark shale generally less than 18 centimetres (7.1in) in thickness. The sandstone exhibits southerly-dipping bedding, and features such as iron-oxide densities and calcite streaks are common throughout the formation.[12]
Potts Hill sandstone
The Potts Hill Sandstone is named after a low ridge at Potts Hill, around 1.6 kilometres (0.99mi) north of Bankstown, west of Sydney. The ridge exists due to the unyielding outcrop of the sandstone. The formation's type area is located at the Water Board quarry, where the quarry face, ranging from 6–7.5 metres (20–25ft), provides an excellent illustration of lateral variation from massive sandstone layers at least 75 centimetres (30in) thick to sections extensively interrupted by silty dark shale bands and lenses.[12]
The lowest of the three formations, the sandstone represents the major sandstone formations of the Wianamatta Group's Upper Division. It is exposed in road cuttings through the Hume Highway at the Razorback Range and in numerous small quarries across the Liverpool and Camden Sheets, including Hoxton Park and Cecil Hills, the Bringelly district, around Narellan Vale, the western flank of Mount Annan, and Razorback Range. Lithologically, the Potts Hill Sandstone is predominantly a massive graywacke-type sandstone, though the type section at Potts Hill is unusually rich in quartz, approaching a feldspathic composition. The sandstone is commonly calcareous, with almost-vertical junctions loaded with subsidiary calcite. Dark, greenish shale lenses are intermittently present, as well as iron oxide and sideritic nodules, present-bedded layers usually orientated from the north, and globular weathering in the larger exposures.[12]
The formation reaches a maximum thickness of about 12 metres (39ft), at Razorback Range, with an average thickness of approximately 9 metres (30ft). Fossil plants described by McCoy in 1847—such as Gleichenites odontopteroides, Odontopteris microphylla, Pecopteris? tenuifolia, and Phyllotheca hookeri—may originate from Potts Hill Sandstone outcrops near Cobbitty, though the precise locality is uncertain. Stratigraphically, the Potts Hill Sandstone overlies the Bringelly Shale, with the transition occurring through a narrow zone of interchanging sandstone and shale bands, completing the passage from shale-dominated strata to massive sandstone within less than 25 centimetres (9.8in).[12]
↑William, E and Airey, DW. A Review of the Engineering Properties of the Wianamatta Group Shales [online]. In: Vitharana, Nihal Dhamsiri (Editor); Colman, Randal (Editor). Proceedings 8th Australia New Zealand Conference on Geomechanics: Consolidating Knowledge. Barton, ACT: Australian Geomechanics Society, 1999: 641-647. Availability: <https://search.informit.com.au/documentSummary;dn=736896154066797;res=IELENG> ISBN1864450029.
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