Wianamatta Group

The Wianamatta Group is a geological feature of the Sydney Basin, New South Wales, Australia that directly overlies the older (but still Triassic in age) 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 (listed in stratigraphic order): 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.