Monday, 1 April 2013

A Volcanic Sedimentary Rock

My Wife and I have been in South Brisbane for a few weeks while my daughter has received treatment in a hospital there so I have not compiled any posts on the geology of the Northern Rivers during this time. However, I thought it might be worthwhile to tell you about an interesting rock I found in Brisbane that is of a type that can occasionally be found in the Northern Rivers especially in the New England Tablelands.

One morning while walking to the hospital, down the driveway of the apartment I was staying at I caught a glimpse of a rock fragment that was different to what I had previously seen in this area. The driveway was cut into weathered old Paleozoic aged rock called the Bunya Phyllite. But the rock fragment that I saw of interest because it was quite different from the phyllite as it had a large quartz cobble in it. Later when walking back to the apartment I had a closer glimpse. It appeared that this rock had fallen down the slope and there were other rocks inconsistent with the phyllite. I picked the piece up that first got my attention and washed it clean. It was a conglomerate, with large rounded clasts of quartzite and basalt and an angular clast of the aforementioned phyllite. The clasts were cemented together with a grey material with small angular crystal fragments. All of this was a surprise until I remembered that I was close to Kangaroo Point which is a cliff line made from a volcanic rock called the Brisbane Tuff part of a Triassic aged volcanic terrain.

It was apparent that what I had was conglomerate formed in the throws of the volcanic eruptions that created the Brisbane Tuff. The Roach (1997) and earlier authors interpreted the Brisbane Tuff as series of pyroclastic flows, surges and air falls that were deposited in pre-existing valleys formed during the Triassic. The valleys probably have had rocky streams evident from the rounded nature of the clasts in the conglomerate. After or during an eruption of the volcano combined with a lots of rain or the failure of a natural dam or lake a mud flow probably ran down the valley mixing all the rock, debris, mud and what ever got in its way stopping after the energy had been spent. The conglomerate would then have been covered and preserved by material from subsequent eruptions.

The sort of volcanic related mud flow described above is called a lahar. They are actually quite common in modern volcanic terrains but are often quickly eroded away so tend to be a little less common than would be expected in older volcanic terrains. Lahars are part of a larger group of volcanic-sedimentary rocks called volcaniclastic rocks. Volcaniclastic rocks are found in the Northern Rivers areas, particularly in the areas of the escarpment and tablelands where the Permian (pre-Brisbane Tuff) Wandsworth Vocanic Group is present (Barnes et at 1991), (The Wandsworth Volcanic Group includes such diverse units as the Annalee Pyroclasics near Armidale to the Drake Volcanics near Drake). The group is very extensive and deserves to be considered in several future posts. It is also worth noting that the Brisbane Tuff was deposited at the same time in a similar way as the Chillingham Volcanics which filled the bottom of the Ipswich Basin and now outcrops in the Tweed and lower Richmond River Valleys.


*Barnes, R., Brown, R.E., Brownlow, J.W. & Stroud, W.J. 1991. Late Permian Volcanics in the New England - The Wandsworth Volcanic Group. Quarterly Notes of the New South Wales Geological Survey.
NSW geosurvey quarterly notes, 84.
*Roach, A. 1997. Late Triassic Volcanism of the Ipswich Basin. Macquarie University, PhD Thesis.

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