Bringelly Shale

, Pemulwuy The average thickness is around , though a maximum thickness of 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 at Potts Hill. 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. 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. 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 . 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. Unlike the Ashfield Shale, it features sandstone lithology and lenticles that fluctuate from to 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. 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. Stratigraphy sandstone in Smithfield 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. • Claystone typically grades into siltstone in sequences up to 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. • 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. • Channel sandstones in the basal are typically less than , increasing to more than 6 metres above this zone and rarely reaching . An exception is the Mt. Hercules Sandstone in the Razorback Range, which attains a thickness of up to . 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. ==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. The shale runs into the Potts Hill Sandstone through a minor transitional zone of vacillating sandstone and shale bands. Cladophlebis australis, an extinct fern, and Lycopod cone scales have been found in the formation. ==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 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 of massive graywacke-type sandstone can be observed. 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 thick, with minor thin, transversely restricted lenses of dark shale generally less than in thickness. The sandstone exhibits southerly-dipping bedding, and features such as iron-oxide densities and calcite streaks are common throughout the formation. ==Potts Hill sandstone==

The Potts Hill Sandstone is named after a low ridge at Potts Hill, around 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 , provides an excellent illustration of lateral variation from massive sandstone layers at least thick to sections extensively interrupted by silty dark shale bands and lenses. 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. The formation reaches a maximum thickness of about , at Razorback Range, with an average thickness of approximately . 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 . ==See also==