Friday 22 February 2013

Into the Parrots Nest

At least 3 lava flows are evident from the different 'steps'
I had the opportunity a few weeks back to visit a quarry near the locality called Canaiba situated mid way between Casino and Lismore. The quarry is an operating variable quality rock quarry probably excavating Miocene aged basalt lavas from the geological unit known as the Lismore Basalt or possibly the earlier Eocene aged Alstonville Basalt. It was a site I'd wanted to visit for quite a while because the quarry is located at the lower side of a long ridge with an old abandoned quarry located at the top of the ridge on the way to a locality called Parrots Nest. In my mind having two quarries could give an interesting perspective on the any variations in lava flows.  But even before I visited the old quarry, while I was driving along the road to visit the operating one I noticed an interesting feature in the shape of a spur from the main ridge. Visible were several 'steps' in the spur. These steps create what is referred to, unsurprisingly, a stepped topography.

The steps are caused by the erosion of different lava flows. The flows are up to 20 metres of so thick which according to Duggan and Mason (1978) is a bit uncharacteristic for the Lismore Basalt (thin 2-3 thick flows). Looking back along the ridge it is pretty evident that the flows are of consistent thickness through the whole area.  They are probably from the Lismore Basalt that are related to the formation of the Tweed Volcano which was centered around present day Mount Warning. I wonder if there were closer vents that could be the source of the lavas but there is little evidence of any in the immediate vicinity. Indeed authors such as Cotter (1998) feel that the pre-existing topography was such that the area through Blakebrook Quarry (another site north of the quarry I was visiting) through to places like Parrots Nest may have been a valley. The swift flowing basaltic lavas flowed down these valleys filling them and creating thick sequences of rock.

The red layer overlain by another basalt lava flow
indicates the presence of a fossil soil horizon
The operating quarry cuts several of the lava flows that make up the ridge, the boundaries of the lava flows were very easy to make out because of the weathered zones especially the presence of palaeosols, that is, fossil soil horizons. The palaeosol gives an idea of the nature of the eruptions of lava too. Obviously enough time needs to have passed for the formation of a soil profile to occur on the earlier lava flow before the next lava flows over the top of it. Depending on the climatic conditions this could be many decades between flows or even thousands of years.

Anyway, a good trip even if it was just for the palaeosol or the stepped topography alone. But I'd like to do another blog on some of the macro scale igneous textures that are present in the lava including dykes, vesicles, voids and veins and I've still not got to the top abandoned quarry but when time allows I'll get there. I took some samples at the operating quarry to examine under the microscope to see if there were any microscopic textures that are of interest too, but once again, time does not seem to be on my side... though I will get to these tasks sometime!


*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.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.

Saturday 16 February 2013

Where do you go?

I'm aware of two good sources for field trips in NSW that are easily available one is Bob and Nancy's Geological Tours website, the other is a book published by the NSW Geological Survey called the Field Geology of New South Wales. There are also wonderful guides published by the Queensland Division of the Geological Society of Australia for areas just across the Queensland border.

Field Geology of New South Wales was released in its current form in 2000, the third edition of the book that was first published early in the 1900's. The Authors are David Branagan, and Gordon Packham.  This book covers the whole state but does include some local excursions such as Lismore - Dunoon - Nimbin, Grafton - Drake - Tabulum - Casino, Armidale -Point Lookout-Ebor, Armidale - Black Mountain - Toms Gully, Kempsey - Crescent Head - Smokey Cape, Port Macquarie, Taree - Mount George - Comboyne - Camden Haven. As well as brief descriptions of the geology on and near the Pacific and New England Highways. . So, there is definitely something for just about every area. It is also worth noting that one of the authors Gordon Packham is well known for his knowledge of the sedimentary geology of north east New South Wales.

For those interested the Field Geology of New South Wales can be obtained for about $40 from NSW Resources and Energy here.


Branagan,, D.F. & Packham, G.H. 2000. Field Geology of New South Wales. Department of Mineral Resources.

Sunday 10 February 2013

Geologists at the University of New England

The first geology class at UNE 1939 (from Voisey 1964)
L to R: Mary Hindmarsh, Catherine Miller, Rae Anthony,
Frank Wickwood, Sylvia Willoughby & Joan Bates.
Rae Anthony and Joan Bates became the first geology graduates in 1943

I obtained a copy of an interesting publication about the first 25 years of the geology department at the University of New England (UNE), (Voisey 1964) from a fellow member of the Geological Society of Australia. This book was published back in 1964 by Alan Voisey who was then head of the department. Voisey (1964) mentioned many names that are now synonymous with UNE geology, for instance, Bruce Chappell, Barrie McKelvey, Raymond Binns, J Wilkinson and others. In addition, I was surprised to find a wonderful picture of the first 'Geology I' class from 1939. It may surprise many others too that in 1939 the first geology class has only one man in it and five women. in 1940 the class was neatly balanced with four women and four men.

While I'm on the subject of the history of the UNE geology department I came across this interview with one of UNEs most well known geologists, Dr Richard Stanton. It is an interesting read as it demonstrates how little we knew of much of our geological history until relatively recently. In paricular the interview demonstrates how even the concepts that we now take for granted as geologists were only developed in the last generation or so. This includes such fundamental concepts such as plate tectonics and the ideas that arrived from it. The interview can be found on the Australian Academy of Sciences page here. Dr Stanton obtained his undergraduate geology degree from UNE and is still an Emeritus Professor there.

Recently I've also been in contact with a couple of undergraduate geology students and some of my fellow students from UNE which reminded me of the awesome geology alumni that UNE has and what seems a bright future with dedicated students with a love of the subject. I'm so pleased that the department still has excellent staff including Dr Nancy Vickery who with her collegues has recently demonstrated that the 'Basalts' of the Inverell and Glen Innes area are quite different or Dr. John Paterson who has made some incredible palaeontological discoveries in South Australia and Associate Professor Paul Ashley with an inexhaustible knowledge of  geochemistry and metallic mineral deposits. There is a couple of other geology staff now at UNE who I've not met but given the history of the department I'm sure they will continue be a great source of research into our region and examples to their students.

While I'm still on the topic it is worth mentioning that some geologists from UNE have gone to become important drivers of political change such as Professor Peter Flood, or some controversial ones such as Professor Ian Plimer.


Voisey, A.H. 1964. Twenty Years of Geology in New England - The first twenty-five years of geology in he New England University College and in the University of New England. L.A. Cotton School of Geology, University of New England, Armidale, NSW.

Friday 1 February 2013

Really, really thin slices of rock

One of my most keen interests in the geology of our area is about the difference between the 'basalts'. The Lismore Basalt, the Kyogle Basalt and the Alstonville Basalt. Most of the time it is impossible to tell the difference if you are holding a piece in your hand. The size of the crystals that make up the rock are usually too small to see but occasionally you can see the characteristic twinning of a larger feldspar crystal (called a phenocryst) or even some olivine. But 95% of the time you'll just say "this is a black rock - probably a basalt".

Since there is very little in the way of field techniques to determine the difference between the regions 'basalts' other techniques need to be used. The best technique is geochemistry, looking at the actual chemical composition of the rock itself. Alas, this only works for the freshest, most unweathered rocks and of course it costs money for each sample. The second technique is called petrography (no usually nothing to do with petroleum), whereby a piece of rock is ground down to a thickness of around 30 microns, light shone through it and its properties determined by a specialised type of microscope (one with polarizing light filters). This can often give you a qualitative assessment of the rock geochemistry.

Kyogle Basalt in Plane Polarized Light
I'm not very good at making these myself but it doesn't stop me from trying. Sometimes I can make some good ones. With some assistance and advice from a technical officer at the University of Ballarat and a fellow club member who is a Senior Visiting Fellow at the University of Wollongong, I've recently had some more success.

The photos are of a piece of Kygole Basalt that I've made into thin sections. The sample was obtained from a quarry in the Afterlee area on the way to Toonumbar. The first is what the sample looks like under plane polarized light. The larger crystals are olivine which in come cases have been slightly reabsorbed creating an embayed texture on their edges. They are also slightly altered to a clay mineral of some kind around the edges and in some of the cracks. The dark black minerals are iron or titanium oxides and if you look closely you'll see little  prism like minerals which is feldspar.

Kyogle Basalt in Cross Polarized Light
The next photo is the same field of view with a second polarizer inserted at 90 degrees to the first to reveal wonderful interference patterns. The interference patterns are characteristic from mineral to mineral. The olivine clearly showing what are called high order interference colours. In this you can also see little accumulations of bright colours that are different from the olivine crystals. These are pyroxenes, probably the variety called augite. The prismatic crystals of feldspar are more evident in their straw colour and the grey 'background' is nepheline.

Overall the chemistry of the constituent minerals this rock is called a alkaline basalt. This is because it contains abundant 'alkali' metals (sodium (Na) and potassium (K)). Alkaline rocks are common in the Kyogle Basalt of the Focal Peak Volcano. Remember though, that the term Alkaline does not refer to a rock type but a suite of rock types. If this rock sample had remained in the magma chamber for a longer period of time it would factionate or evolve to rocks called nephelenites or similar. This is a different process to what goes on with the other major suite of rocks called tholeiites which tend to evolve towards dacites and rhyolites (such as the Nimbin Rhyolite that that we see on top of the Lismore Basalt - which is mainly tholeiitic in composition). But more about that another day.

Hopefully this weekend I get the chance to make some more thin sections. Which reminds me - this is the way I do it, if you have a polarizing microscope and are keen to learn the art yourself give it a go.