Reviewing stratigraphic units defined in the ACT

Marais-van Vuuren, Christo1 , Brown, Catherine1, Jarrett, Amber1

1Geoscience Australia / GSA ACT and External Territories Stratigraphy Subcommission, Canberra, Australia

The usual role for Australian Stratigraphy Commission members in most of Australia is to assist geologists writing up unit definitions or redefinitions, in areas of recent study . The situation in the ACT is a bit different. Although there has been recent mapping work in surrounding areas of NSW, and recent geochronology done on 9 units in the ACT, there has been no new mapping or revision of geological units in the ACT since Bob Abell’s (1991) Geology of the Canberra 1:100 000 sheet area, which only covers the northeastern part of the ACT. Geological mapping over the rest of the ACT (Brindabella, Tantangara and Michelago sheets) is even older. Many of the original unit descriptions and definitions were done by Armin Opik in 1958. Some of these have never been revised.

Prior to self-government in the Australian Capital Territory (ACT), the Bureau of Mineral Resources had an engineering geology and mapping role in the Territory, but that role has not been maintained at either local or federal level. So, given the urban development and other landscape modifications in the ACT, the ACT Stratigraphy Subcommission has decided it would be useful to review the type section locations of units defined in the ACT. The initial aim is to make it easier to find most of them, but we will also need to consider replacement type sections/type areas, in cases where the original is no longer accessible, and possibly some additional reference sections too.

There are 78 current stratigraphic units identified in the ACT, many of them also extending into NSW. At least 22 of those units have been defined/redefined in the ACT. All of the type sections or type areas need locations revised to a modern, known co-ordinate system (GDA 2020). It is already clear that locating some of the type sections will involve some historical research and comparison of old maps with modern ones. We have also identified that some of the original locations are no longer accessible due to road realignments, modification or ‘treatments’ of road cuttings, flooding (creation of artificial lakes) etc. Where type or reference sections have been nominated in drill core, we need to ascertain whether or not the core is still accessible, as well as the location of the hole.

The project is in the early stages, and has started with a review of the information available through the Australian Stratigraphic Units Database  and information in other Geoscience Australia and ANU databases. We expect the review will lead to some local field trips and hope that our work may encourage further review of ACT geology generally, to improve edge-matching with more recent geological work by the Geological Survey of New South Wales, in particular.

Ultimately this work will represent a major update to type section understanding and definitions of stratigraphic units in the ACT, which is decades overdue. If it also encourages some other old definition reviews, that would be a bonus.


Christo graduated from Macquarie University in 2014 with a BSc majoring in geology and geophysics. He then worked for industry as a geophysicist prior to joining GA in 2016 in the stratigraphy section where he works on maintaining the Australian Stratigraphic Units Database. Recently he has joined the Stratigraphy Commission.

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