STEP Inc

Talk on Hornsby Quarry

STEP has concerns that the project to fill Hornsby Quarry with spoil from the North Connex tunnel and create public recreation spaces will ignore the unique geological feature, the volcanic diatreme. In this talk Dr Ian Percival, who prepared the original geoheritage report on the Hornsby diatreme in 1979, will outline the geology of the feature, explain its origin, and will argue why it should be protected for future generations of geologists and the public. Ian is Senior Principal Research Scientist (Paleontologist) at the Geological Survey of NSW.
  • Talk on Hornsby Quarry
  • 2015-11-24T20:01:00+10:05
  • 2015-11-24T22:01:00+10:05
  • STEP has concerns that the project to fill Hornsby Quarry with spoil from the North Connex tunnel and create public recreation spaces will ignore the unique geological feature, the volcanic diatreme. In this talk Dr Ian Percival, who prepared the original geoheritage report on the Hornsby diatreme in 1979, will outline the geology of the feature, explain its origin, and will argue why it should be protected for future generations of geologists and the public. Ian is Senior Principal Research Scientist (Paleontologist) at the Geological Survey of NSW.
  • When 24 Nov, 2015 from 08:01 PM to 10:01 PM (Australia/Sydney / UTC1008)
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Why the Diatreme is Special

The Sydney Basin is largely composed of sedimentary rocks of Permian and Triassic age, deposited approximately 300 to 240 million years ago, with some Early Permian volcanic rocks mainly in the Kiama region. All these rocks are well-exposed and widely known.

Outside the geological community, however, very few people are aware that there was a relatively brief interval in the Jurassic Period when volcanic eruptions again rocked the Sydney Basin, with nearly 100 separate sites recognised where maar volcanoes erupted as a result of explosive reaction between lava and groundwater. Immediately following the steam-generated eruption, a diatreme is generally formed as a collapse feature infilled by shattered country rock intermixed with volcanic ejecta. Maar volcanoes and resultant diatremes are fairly rare geological features.

Hornsby Quarry was excavated within the largest diatreme known in the Sydney Basin and provides the last remaining artificial section through a diatreme in NSW. It is regarded as the best example of this phenomenon in NSW, if not in Australia, and is a highly significant part of the state’s geological heritage.

Unfortunately, all this was largely ignored in the recent environmental impact statement prepared by consultants for the Roads and Maritime Services for the proposed infilling of Hornsby Quarry using spoil generated by the NorthConnex tunnel project.

The Geological Heritage Subcommittee of the Geological Society of Australia (NSW Division) prepared a detailed submission pointing out the inadequacies of the proposed remediation of the quarry site as regards safeguarding the diatreme exposure.

While some of these points were partly addressed in the subsequent response by Roads and Maritime Services to submissions received during the statutory public exhibition period, there still appears to be little or no recognition of this unique scientific feature in our region.