Mike Smith, Director, National Rock Garden
Published in the National Rock Garden Newsletter No. 28, December 2024
NRG Newsletter No 25 reported on our aspirations to acquire several specimens of radiolarian red-ribbon chert from below the wall of Chaffey Dam, in the Nundle area of central NSW. During November, three boulders of approximately 4 tonnes each, were loaded onto a crane truck under the supervision of staff from the Chaffey Dam management team.
This rock is characterised by its striking reddish-brown hue and intricate banding, composed almost entirely of tiny (~0.5 mm) silica microfossils. Its silica-rich content originates from the skeletal remains of countless marine organisms known as radiolarians. When these plankton die, their organic walls decompose, and their skeletons sink down to the seafloor. If they sink below 3–5 kilometres all calcium carbonate dissolves and biosilica (silicon dioxide from biological remains) begins to accumulate. Accompanying the silica microfossils are fine clays—derived from fine wind-blown dust and from ash clouds produced by distant volcanoes. In the case of the NRG’s specimens, this has occurred far away from any mainland river source of sediment in the deep ocean.
Silica isn’t compatible with clay and limestone, and during compaction, the silica near the bedding base will dissolve away, enhancing the sedimentary beds in the cherts and preserving the radiolarian microfossils in the middle of the beds. Red and black colours within the rock reflect the deposition/precipitation of iron and manganese from the sea water. These oxide minerals also settle on the ocean floor. The rock’s composition tells a story of the rhythmic alternation between quartz-rich fossil beds and clay-rich and metal-rich interbeds, showcasing a unique geological history.
This sedimentary material was originally deposited as soft, flat-lying sediment during the Ordovician Period. Subsequently, over millions of years, these formations were compacted and contorted, and emplaced on land—now forming an elevated outcrop of highly durable rock.
While this specimen is very important from a strictly scientific perspective, it is also very important for engineering works. The hardness of the chert made it suitable for the construction of Chaffey Dam, a major geotechnical project completed in 1979. In order to provide sufficient water for the city of Tamworth and surrounds, and to meet modern safety requirements, the dam was raised in 2016 by 8 m to its current height of 62 m. It is now 500 m long and holds 100,500 ML.
The chert’s physical properties also play a crucial role in the material culture of Australian Aboriginal peoples, and the NRG Steering Committee will seek advice from the Traditional Custodians in order to include relevant Indigenous cultural information in the website description which will be accessible via the QR code on the rock plaque.
The three boulders have been placed in temporary storage, pending the future completion of additional rock platforms and pathways at the NRG site in the Arboretum. The Steering Committee thanks the Sydney Minerals Exploration Discussion Group (SMEDG) which provided the funds for the two stages of transportation of the three specimens, as well as their preparation for display.
References
New South Wales Government, 2024. Chaffey Dam. https://www.nsw.gov.au/visiting-and-exploring-nsw/locations-and-attractions/chaffey-dam. Last accessed 27 December 2024.
Kachovich, S. 2013. Significance of radiolarian biostratigraphy of the southern New England Orogen, New South Wales. Honours Thesis, Faculty of Science, Medicine & Health, University of Wollongong Research Online https://ro.uow.edu.au/thsci/48.
Kachovich, S. and Smith, M., 2023. Specimens from the great ocean depths, soon to travel to the National Rock Garden, Canberra. NRG Newsletter, November 2023 https://www.nationalrockgarden.com.au/featured-article/specimens-from-the-great-ocean-depths/
