Where have the Brisbane Metamorphics gone?!

A few months ago I was reading the 2011 NSW National Parks and Wildlife Service plan of management for the Julian Rocks Nature Reserve just offshore, near Byron Bay. The introduction said the Julian Rocks “are composed of Brisbane Metamorphics which date from the Carboniferous-Devonian period 345-405 million years ago and are the most resistant rock type in the region”. Sounds fine as a bit of background but why can’t I find recent geological work that refers to the Brisbane Metamorphics anywhere else?

Academics from Southern Cross University have used the term in published works as recently as 2007 (Specht and Specht 2007). But I can’t find it on any map or in any geological publication after 1990. Surely the rocks haven’t been eroded that quickly especially since it is “the most resistant rock type in the region”. I can, however, find reference to the Brisbane Metamorphics on the 1: 1000000 scale NSW geological map from 1962. But at such a scale it is hard to figure out exactly where it is. Broadly it appears to be located in some areas near Murwillumbah and some areas near the border with Queensland. The most specific paper I have is by Holcome (1977) which discussed the Brisbane Metamorphics in depth but doesn't say where it goes!

When in doubt try Google? But the result you get when typing in “northern rivers geology” is the website Big Volcano. It can be found here. Here too the geological history summary refers to the Brisbane Metamorphics but mistakenly links to a site that shows a small contact metamorphic area at Mount Coot-tha just to the west of Brisbane. This is a bit confusing because the metamorphic rock here is called the Bunya Phyllite which is a regional metamorphic rock which has undergone a second metamorphic even during the emplacement of the granite that makes up Mount Coot-tha. Interesting in itself, but it does not answer our question why the Brisbane Metamorphics were said to be at Byron Bay!

Well, The answer is simply a case of one of the most difficult aspects of geology, nomenclature. Geoscience Australia provides an excellent service in maintaining a database of all geological units named in Australia (past, present and proposed). It includes an entry on the Brisbane Metamorphics which can be found here. On the webpage you can see three fields that are important for knowing where this unit has gone. “Current: No”, “Status: Obsolete”. The comments field answers the question finally: “Name superseded by Rocksberg Greenstone, Bunya Phyllite, and Neranleigh-Fernvale Formation.”.

What this means is that the one description of Brisbane Metamorphics did not reflect the ages, genesis, and history of these three rock units.  You will get more information about the structural history and rock composition if you deal with the new units individually. Indeed, Holcome (1977) discusses the constituents of the Brisbane Metamorphics at length and notes that the Rocksberg Greenstone, Bunya Phyllite and Neranleigh-Fernvale Group are the constituents of the Brisbane Metamorphics but these are substantially different in terms of formation, metamorphic history and exposures. In northern New South Wales some of these units are present as part of what is called the Beenleigh Block (Holcome (1997).

There are many cases where geological units have been renamed or reclassified after further research has been done. This is no different to any other area of science. The only challenge is keeping up with the change.

References/bibliography:

*Holcome, R.J. 1977. Structure and tectonic history of the Brisbane Metamorphics in the Brisbane Area. Journal of the Geological Society of Australia. V24.
*NSW National Parks and Wildlife Service. January 2011. Julian Rocks Nature Reserve: Plan of Management.
*Specht, R.L. , Specht, A. 2007. Pre-settlement tree density in the eucalypt open-forest on the Brisbane Tuff. Proceedings of the Royal Society of Queensland 113 p9-16