The Wianamatta Group, of Middle Triassic age, is the topmost Mesozoic unit in the Sydney Basin. Three formations are recognized. The Ashfield Shale, 45 to 60 m thick, comprises a dark grey to black siltstone which grades upwards into a laminite. The overlying, thin, quartz lithic Minchinbury Sandstone is less than 4 m thick. The Bringelly Shale comprises 257 m of mainly claystone and siltstone with laminite, sandstone, carbonaceous claystone, and rare coal. The Wianamatta Group was deposited during a single regressive episode. The sequence grades upwards from a lacustrine to a marine or brackish facies, (Ashfield Shale), through a shoreline facies comprising a beach and barrier bar complex (Minchinbury Sandstone), into an over-lying lagoonal marsh facies which grades up into an alluvial or estuarine coastal plain facies (Bringelly Shale). The Wianamatta Group is the chief source of raw material for the production of structural bricks in the Sydney Region. Both the Ashfield Shale and the Bringelly Shale possess the ability to bloat and have potential for the production of lightweight expanded aggregate.
Herbert C. 1979. The Geology and Resource Potential of the Wianamatta Group. Geological Survey of New South Wales - Bulletin 25