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Physical Description

Two units of the Late Permian Gerringong volcanic faces are exposed on Bombo Headland. The Kiama Sandstone member forms a narrow wave-cut platform and adjacent vertical cliff face around the south-eastern extremity of the quarry. To the north the sandstone dips below sea level and is overlain by about 20m of porphyritic basalt, termed Bombo Latite member. The contact between the two units is well-exposed in the cliff section at the eastern end of the section of the two points comprising the headland. The red-brown colour (due to oxidisation of haematite) of the sandstone contrasts markedly with the grey-black latite, which displays spectacular columnar jointing elsewhere in the quarry. Isolated columns 5-5 meters in height stand adjacent to the coast between the north and south parts of the quarry; just to the north the sea wall exposes cross sections of the columns 1-2.5 metres in diameter, resulting in a 'Giants Causeway' appearance.

Petrographic descriptions of the latite emphasise its conspicuous porphyritic texture; large labradorite to andesine phenocrysts with minor clinppyroxene are set in a groundmass of feldspar microlites with interstital chlorite and iron oxide. The latite is commonly vesicular. Agglomerates or volcanic beccias are developed in some areas of the quarry. Eg in the south-western portion near the access road. This lithology is readily distinguished from the latite by its chaotic appearance and light-coloured matrix.

A capping of cream-coloured weathered latite, still retaining the characteristic porphyritic texture, may be studied at the top of the northern and western quarry faces. This sharpley-defined zone of surface weathering and soil formation overlies relatively fresh rock exhibiting columnar jointing.

The Bombo Latite Member was subsequently intruded by at least 5 basaltic (monchiquite) dykes of probable tertiary age which flowed around and between the columns of latite often taking 90 degree changes in direction. Early workers (Jaquet 1905 & Harper 1915) mapped and described these dykes but subsequent development of the quarry provided further exposures and obliterated others. Today dykes are mainly in the northern half of the quarry but at least one extends across the floor of the excavation in the vicinity of the isolated columnar stacks. They are of interest due to their inclusions of xenoliths and xenocrysts, which are believed to represent fragments of the earth's mantle incorporated in magmas oribinating from within that zone. Sussmilch (1905) described xenoliths of hypersthene gabbro, augite peridotite, enstatite peridotite and pyroxenite occurring as rounded fragments and boulders embedded in the monchiquite. From a deeper level of what was probably the same dyke, Wilshire & Binne (1961) recorded hornbledite and glimmerite as the dominant xenolithes. Present exposures of most of the other dykes appear to lack macroscopically visible xenolithes.

(Percival)

History

In 1979 a nomination was received from the Geological Society of Australia (NSW Division) for the Bombo Quarry. The importance of the geological features was brought to the attention Heritage Council's by Dr Suzanne Wass of Macquarie University's School of Earth Sciences. The quarry was owned by the Metropolitan Water, Sewerage and Drainage Board and it was proposed that a pollution control plant be constructed on the floor of the disused quarry.

Following site inspections and lengthy consultations between the Metropolitan Water, Sewerage and Drainage Board, NSW Heritage Council, NSW Planning Commission and other key agencies a Permanent Conservation Order was placed over the site in 1983. It was transferred onto the State Heritage Register in 1999.

Internet links

Webpage	Source
Bombo Headland Quarry Geological Site
Bombo Headland Quarry Geological Site	Attraction Homepage

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