Sunday, 8 July 2012

The 'older' Rhyolite in the North East

In some of my earlier posts I mentioned that there are many areas in the mountains around the Tweed Valley that are comprised of rhyolite. I mentioned that this rhyolite was formed during eruptions associated with the Tweed Volcano during the Cenozoic era. This rhyolite is called the Nimbin Rhyolite or the Binna Burra Rhyolite (depending what side of the state border you are on). However, there is actually another large distribution of rhyolite not associated with the Tweed Volcano, erupting much earlier, during part of the Mesozoic known as the Triassic. These older mainly rhyolitic rocks are called the Chillingham Volcanics with the type location unsurprisingly located at Chillingham, a village west of Murwillimbah. Those of you who have seen my earlier posts will recognise that I have briefly mentioned the Chillingham Volcanics before, but in this post I intend to go into it further.

Layers of pyroclastics and volcaniclastic of the Chillingham Volcanics
(Murwillimbah - Kyogle Road)



The Chillingham Volcanics have been studied in a fair amount of detail by Roach (1997) in his thesis. This included all of the Triassic volcanic rocks from Brisbane to Uki. So, obviously there is a relationship with the rocks of the southern Queensland, Indeed Roach (1997) indicates that the Brisbane Tuff is a deposit of volcanic rock of rhyolitic composition. The Brisbane Tuff is most well known by the Kangaroo Point Cliffs opposite the Brisbane River in Brisbane City and was erupted during the same general period of time as the Chillingham Volcanics.

The Brisbane Tuff provides a miniature version of the Chillingham Volcanics and is well known because the volcanic centre can be identified in the northern suburbs of Brisbane and the tuff was laid down in the valleys that existed in the Palaeozoic aged basement. The situation which lead to the formation of the Brisbane Tuff also developed further west and south where a larger valley now known as the Ipswich Basin was forming. The eruptions occurred in and around the basin as the crust in this area was subsiding during thermal fluctuations and as the basin filled up with volcanic rocks subsidence continued leading to a very thick unit of mostly rhyolite and reworked volcanic rocks (actually a sedimentary rock known as a volcaniclastic rock). So the Chillingham Volcanics are actually the lower most stratigraphic unit in the Ipswich Basin.

The Chillingham volcanics are mainly comprised of rhyolite in the form of lavas, pyroclastic, ash and tuff deposits as well as the above mentioned volcaniclastics. Many volcanic vents are recognised from structural characteristics of the rocks, however, only one area really shows an obvious modern geomorphological character. This area is around Uki and Clarie Hall Dam where eruptions formed a large mass due to the slow moving nature of the lava. Interestingly the northern most parts of the Chillingham Volcanics in Queensland shows us that there was not just rhyolite but also some andesite and even basalt, but in the area between Chillingham and Uki it is pretty much all rhyolite.

Outcrops of the Chillingham volcanics occur over a long distance with the eastern most side of the Ipswich Basin exposed in New South Wales meaning that a band of the Chillingham Volcanics is visible within the eroded valleys of the Tweed Volcano. The band is actually interupted by the Mount Warning Complex which appears to have intruded right along the line of the pre-existing Chillingham Volcanics. Also the volcanics are covered by the Lamington Volcanics of the Tweed Volcano too, both along the Queensland Border and between Clarie Hall Dam and Evans Head. Indeed the Chillingham Volcanics appears to change composition through this area with authors such as Smith et al 1997 and Cotter 1998 identifying andestite and basalt at Evans Head and an area near Wardell.

The Chillingham Volcanics overlie palaeozoic aged rocks of the Beenleigh Block, mainly rocks of the Neranleigh-Fernvale Group. The overlying rocks are more components of the Ipswich Basin such as the Ipswich Coal Measures and its equivalent (such as the Evans Head Coal Measures).

Although I have said that the Chillingham Volcanics contain the older rhyolitic rock in this area, there are actually still older rhyolites in the region... But I'll talk about those rocks in a future post.

References/Bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Roach, A. 1998. Late Triassic Volcanism of the Ipswich Basin, Masters Thesis, Macquarie University.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45

2 comments:

  1. Road cuttings must be such a boon for geologists. Great post from Book of Job.

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    1. You are quick Mark, you posted a comment about 2 minutes after I posted the blog! Road cuttings can be tourist attractions to geologists. Once abandoned quarries were good too but because many are considered a safety risk most are filled back in these days.

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