Bald Hill Claystone

Bald Hill Claystone is a sedimentary rock found in the Sydney Basin in eastern Australia. It is part of the Clifton sub-group of the Narrabeen Group of sedimentary rocks. It was formed by weathering of the Gerringong Volcanics in the early Triassic.^[1] Named after Bald Hill, in the northern Illawarra, where it is 15 metres thick. The claystone is easily noticed at Long Reef, where it is 18 metres thick.

Bald Hill Claystone at Long Reef (New South Wales), Australia

Mineralogy

The Bald Hill Claystone is a redbed containing laterite. Primarily a red shale or fine to medium grained sandstone.^[2] Kaolinite is found in proportions of 50% to over 75%. Iron rich haematite is also present. Felspar and quartz may be present. This mineralogy indicates that Bald Hill Claystone is unlikely to swell. Typically this rock type is a chocolate brown to red brown colour, with bands of silty grey, or sandy greenish grey.^[3]

Fossils

Fossils of lycopod tree roots may be seen in this strata.^[4] Gymnosperm spore pollen from Protohaploxypinus samoilovichii has also been recorded.^[5]

See also

• Sydney Basin
• Hawkesbury sandstone
• Ashfield Shale
• Narrabeen group
• Garie Formation
• Bulgo Sandstone

References

1. "Sydney Basin – Geological Overview". Retrieved 21 November 2010.
2. "Australian Stratigraphic Units Database, Geoscience Australia". dbforms.ga.gov.au. Retrieved 20 October 2015.
3. "Geology of the Triassic Rocks of the Sydney Area - Pells Consulting" (PDF). Pells Consulting. Retrieved 21 October 2015.
4. Late Eocene and Oligocene Paleosols from Badlands National Park, Issue 193, page 9, author Greg John Retallack
5. Retallack, Greg (1977). "Paleosols in the upper Narrabeen Group of New South Wales as evidence of Early Triassic paleoenvironments without modern analogues" (PDF). blogs.uoregon.edu. Australian Journal of Earth Sciences. Retrieved 21 October 2015.