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Hornsby Quarry contains a “perfect” cross-section of a diatreme.
Hornsby Quarry contains a “perfect” cross-section of a diatreme.

Geologists lobby to preserve quarry rock formation

Geologists are lobbying to modify a spoil management plan that could reduce access to a scientifically significant rock formation in an old quarry.

The Geological Society of Australia (GSA) has raised a number of concerns about the New South Wales Roads and Maritime Services’ (RMS) plans to fill Sydney’s disused Hornsby Quarry with spoil from the nearby NorthConnex motorway tunnelling project.

According to Dr Ian Percival, co-convenor of the GSA’s NSW geoheritage subcommittee, excavation at the quarry in the 1960s and 1970s revealed a geological phenomenon known as a diatreme. This feature, which is believed to date back to the Early Jurassic period about 200 million years ago, formed from an unusual type of volcano known as a maar-diatreme.

Percival explained that maar-diatreme volcanoes are created when ascending magma intersects the water table. This results in a stream pressure-driven explosion that ejects rock from below the Earth’s crust upwards, with the fragments subsequently falling into a conical or basin-like cavity within a compact area.

Hornsby Quarry is situated within the Sydney Basin, a geological structure that spans 64,000km2 extending from Australia’s east coast inland to the Blue Mountains and Hunter Valley. While 95 diatremes have been mapped within the basin, Percival said Hornsby Quarry’s diatreme was “distinctly different”.

“It contains relatively fresh rocks compared to others [diatremes] that have weathered into depressions. It is the largest, best preserved example in the entire Sydney Basin.

“Quarrying at the site has also exposed a perfect cross-section of the diatreme,” Percival added. “It’s quite rare to actually see the disc-shaped layers of rock as they’ve fallen back into the vent.”

Percival said researchers had not been able to study the diatreme since public access was prohibited after the quarry’s closure in 2002.

GSA concerns

Despite the fact that Hornsby Quarry was acknowledged as a geoheritage site by the Federal Government in 1979, Percival said the environmental impact statement (EIS) for the Hornsby Quarry spoil management project indicated the RMS did not fully appreciate the diatreme’s scientific significance.

In a submission to the RMS, the GSA pointed out that plans to raise the quarry void floor an additional 64m would obscure the unique disc-shaped bedding exposed on the quarry’s eastern face. The GSA also expressed concern that the positioning of the project’s conveyor belt system, which would result in spoil being dumped on the quarry’s eastern benches, could potentially impact the diatreme.

The RMS responded to the GSA submission in a Preferred Infrastructure report released in October, stating it would assess the feasibility of contouring the spoil to a height of 54m on the eastern side of the quarry void. The RMS also claimed that partially filling the quarry would not physically damage the diatreme.

Percival said, however, that the GSA believed the fill level on the eastern face needed to be reduced to a height of 40m to ensure that “almost the complete section [of the diatreme] currently exposed above the water level would remain intact and available for public and scientific examination”.

The GSA suggested the resulting incline down to the quarry floor could serve as a mountain biking trail. “This would provide a great public amenity [and] benefit both the diatreme cross-section and the need to fill the quarry in with the road spoil,” Percival explained. He added that preserving the diatreme could lead to economically beneficial “geotourism” opportunities.

RMS position

When Quarry approached the RMS for comment, a spokesperson said, “RMS is aware of the diatreme and will be taking all appropriate steps to ensure sections of the eastern wall of the quarry will remain visible during the partial filling of the quarry. It is too early to provide exact detail of the fill level as planning approval is yet to be received.

“The location of the conveyor system has been selected to minimise impact on remnant vegetation,” the spokesperson added.

The spoil management project website indicated that the RMS aimed to gain approval from the NSW Department of Planning and Environment by the end of 2015, and the RMS spokesperson confirmed to Quarry that planning approval was “expected to be received shortly”.

More reading
EIS released for quarry spoil management project
Spoil deal to transform quarry in six years
Local, state, federal govts team up for quarry rehab
Adventure experiences touted for disused quarry











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Monday, 26 August, 2019 1:43pm
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