Bringelly Shale
Bringelly Shale is a component of the Wianamatta group of sedimentary rocks in the Sydney Basin of eastern Australia. Formed in the Triassic Period, it has an extensive outcrop in the western parts of Sydney. The shale has its greatest geographical extent at Bringelly, near the suburb of Liverpool.
Lithology
The shale is the topmost layer of sedimentary rock laid down by a river delta over the older Hawkesbury sandstone in the Triassic Period. Overlying the Ashfield Shale, the Bringelly Shale is a more complex formation that consists of different lithologies, which include claystone-siltstone (70%), laminite and sandstone (25%), coal and highly carbonaceous claystone (3%) and tuff (2%). The claystone units comprise many different types of fine-grained sediments, usually light-grey leached claystone, dark-grey to black carbonaceous claystone and non-carbonaceous mid to dark-grey claystone and siltstone.
It is similar to Ashfield Shale in that both have low porosities, though differing in having a greater amount of calcareous, graywacke-type, lithic sandstone bands and lenses, carbonaceous claystone, siltstone, laminite, but would lack sideritic mudstone bands that Ashfield Shale has. Bringelly Shale has lumpy clay minerals, and it swells and decays rapidly on submergence in water and is generally less durable.
Description
The average thickness is around 60 metres. It was deposited in a swampy alluvial plain with winding streams that formed sporadic beds of sandstone. The shale is dark when unweathered just like the Ashfield Shale. The shale is usually a typical olive-green colour when weathered. Alloyed coal bands and lenses and iron oxide concretions have been observed in the shale. The shale is quarried in many western suburbs of Sydney for brick and miscellaneous ceramic manufacture.
Samples of Bringelly Shale were acquired from quarries that were used to extract shale for brick production at Kemps Creek, Badgerys Creek, Horsley Park and Mulgoa, and the shale from all these areas was described as a non-carbonaceous mid to dark-grey claystone. There is substantial debate about the post-depositional history of the shales in the Sydney Basin, with evaluations for the depth of over-lying sediments ranging between tens of metres to 4 km.
