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.

Geological diversity of the Toonumbar Dam area

Toonumbar Dam is a lovely area that, like so many other places, wish I could visit often. It would be lovely to relax around the dam, maybe stay the night camping or in a cabin. When I last visited, I was rather pathetic... I was looking at the rip-rap on the dam wall and trying to figure out where it was likely to have been quarried! I later found out and visited the quarry to obtain samples and look for structures. But that is a story for another day. As I was saying, the dam is a lovely place and like many beautiful places owes itself to the geological conditions of the area.

The oldest rocks (Mesozoic aged Clarence-Moreton Basin) exposed in the area are actually exposed downstream from the dam itself. Several hundred metres downstream are poor exposures of what appears to be rocks of the Jurassic Walloon Coal Measures, immediately downstream (and all around the dam) is the Kangaroo Creek Sandstone which is obvious to identify up close. The rocks which are apparently of the Walloon Coal Measures are a little harder to distinguish. It is possible that they are members of the MacLean Sandstone (which are considered part of the Walloon Coal Measures) or maybe Woodenbong Beds or even the underlying Bundamba Group but they are certainly younger than the Kangaroo Creek Sandstone.

Inclined bedding in Kangaroo Creek Sandstone
In Iron Pot Creek below the dam. Cross-bedding is also evident

It is worth noting the bedding plains in the sedimentary rocks if you are downstream of the dam. The plains are actually inclined to the west in this area and the further you go down stream the flatter the beds become, then they tilt back the other way (eastward) for a short distance. This is actually a large basin structure called the Toonumbar Anticline (the top of a fold in the rock layers). Another structure, much bigger and of regional significance is located only another couple of kilometres to the east. This is the East Richmond Fault which extends into southern Queensland and down almost to Grafton. I have actually never seen evidence of this fault in the field, but there is geophysical evidence for it and I'm assured it is there. Apparently the fault is much more evident further south between the villages of Mummelgum and Mallanganee.

The large rugged hill and ridge about 5km north of the dam is made from basalt lava, I'm not sure of the exact composition of this rock but it is likely to be part of the Kyogle Basalt which is associated with the Focal Peak Volcano. Interestingly, I think that the basalt is likely not to have been sourced from the actual peak of the volcano but from a distant vent on the side. This is because a few kilometres to the north west just on the north side of the lake is actually one of at least two intrusions of gabbro (the intrusive equivalent of basalt) near Toonumbar, one of these is crossed by Murrays Scrub Road. It is possible that these intrusions were the feeder systems for vents which erupted the Kyogle basalt in this area. This probably demonstrates the nature of volcanism in the area during the Cenozoic period. It seems apparent that the central volcano models of the Focal Peak and even the Tweed Volcanoes appears to be a bit too simplistic.

But, whether you are interested in geology or just enjoy the forests of the Northern Rivers, a trip to Toonumbar Dam is worth while.

Note that the stratigraphy of the Kangaroo Creek Sandstone has been revised since this blog post. See the this post for details.

References/Bibliography:

*O’Brien, P.E., Korsch, R.J., Wells, A.T., Sexton, M.J. Wake-Dyster, K. (1994) Structure and Tectonics of the Clarence-Morton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Bell, A.D.M. (1968). Report on the geology of Toonumbar Dam and Appurtenant Works. Water Conservation and Irrigation Commission.

Lindesay and the volcano

I recently went to Woodenbong via Kyogle. The trip along this section of the Summerland Way is very pretty as you climb into the McPherson Ranges. It also provides many opportunities for good views of imposing Mount Lindesay which is around 1180m high, located right on the state border and is a reminder about mistakes that people make when seeing mountains that are shaped the way they are.

Mount Lindesay from the south

Mount Lindesay is often referred to as a volcanic plug. I've heard this from different people several times. This is not surprising as the shape does imply this, but this is a trick of nature. The upper parts and 'peak' are flows of what is called the Binna Burra Rhyolite (or Mount Gillies Volcanics in Queensland) and some basalt, below this is a layer of obsidian (rhyolitic glass) overlying a layer of rhyolitic ash and agglomerates. The lower parts of the mountain is made from another volcanic rock, basalt (Kyogle Basalt). This basalt however overlies sediments of the Clarence Moreton Basin.

Mount Lindesay gets its shape by the rhyolite that forms the top most layer. The rhyolite is hard, resistant to weathering and therefore remains relatively difficult to erode. It is for this reason that the rhyolite has protected the underlying softer rock at Mount Lindsay and you can see the same process for ridges to the east and south of the mountain too. The actual vents for the rhyolite and underlying basalt lavas is actually a little tricky to definitely locate but we do know that the main volcanic centre for these rocks was at the Focal Peak Volcano located in the vicinity of present day Mount Barney a significant distance to the north. Additionally, there are some real volcanic plugs further to the west which I mention below.

Rhyolite from focal peak was thought by Duggan and Mason (1978) and other authors to extend as far Nimbin to the east. However, recent work by Cotter (1998) has shown that this is not the case but the Binna Burra Rhyolite still extends a long way to the east past places like Wiangaree.

There are however, some clearly identifiable volcanic plugs in the region. A good one is sometimes referred to as the Nightcap Peak and is located half way between Woodenbong and Urbenville just a little to the west of the road. It stands out from the rolling hills, is difficult to miss and is made from the rock granophyre (fine grained granite-like rock). At Urbenville the Northern Obelisk is another example of a plug, a bit one! Additionally, large dykes exist to the south west of Urbenville too.

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

More on the Tweed Volcano

I had the pleasure in obtaining a copy of a University of New England honours research thesis by Howden (2009) a week ago. For one thing, I'm pleased to see that there is still some research being conducted on the Tweed Volcano and Focal Peak Volcano, despite the state of our Country's university science faculties these days. Howden has put a great deal of effort into distinguishing between the mafic rocks of the volcano (basalts) including some detailed petrographic and geochemical analysis. One of the points of interest to me is the attempt to distinguish between the Blue Knob Basalt and Lismore Basalts, sadly, the work undertaken by Cotter (1998) was unavailable (lost to the world until recently) to her. This would have clarified some issues which were difficult to resolve in her thesis.

Previous authors such as Duggan & Mason (1978) noted that there appeared to be very little (if any) distinction between the Blue Knob and Lismore Basalts except for their apparent stratigraphic location. Duggan & Mason (1978) determined that the Blue Knob Basalt appeared to overlay the Nimbin Rhyolite and the Lismore Basalt under it. However, Duggan & Mason and other authors such as Smith & Houston (1995) suggested a possibility that the Blue Knob Basalt could actually be inter-collated with rhyolite flows indicating that it was possible that the basalts were really just occasionally interrupted by flows of the Nimbin Rhyolite.

Howden (2009) has through comprehensive geochemical and petrological study of the Lamington Volcanics demonstrated that the only way to distinguish between the two basalt units was on the basis of phenocryst size with the Blue Knob Basalt showing larger grains of plagioclase feldspar. In the absence of any other geochemical or petrological distinguishing characteristics this shows a very uninspiring difference between them, I would suggest, insufficient to say that they were in fact different.

Because of the absence of significant differentiating features it is likely that the Blue Knob Basalt is really just the Lismore Basalt which continued to erupt at various times with intervening periods of large rhyolitic eruptions of the Tweed Volcano. This means that this can be confirmed if flows of basaltic lava can be identified between rhyolite. In Queensland the equivalent of the Nimin Rhyolite, the Binna Burra Rhyolite shows intercollated flows of Hobwee Basalt (the equivalent of the Lismore Basalt). The plagioclase phenocryst grain size difference probably just reflects slightly different magma residence periods in the magma chamber becoming more obvious at the volcano became older. This is also demonstrated as the Hobwee Basalt in Queensland shows the upper flows have larger phenocrysts.

Slowly we are gaining a clearer picture of our present day landscape and the mechanisms that made it. Sometimes difference between the way we think they have occurred and they way we later find out seems quite minor, yet the implications are significant in understanding how the landscape actually behaves under the ground. The small areas of 'Blue Knob Basalt' were thought to be a last spurt of eruption of the Tweed Shied Volcano (either centred on present day Mount Warning, or other vents on the flanks of the volcano), I think that Howden (2009) has presented us with enough evidence how to say that the way the volcano formed included two different types of lavas (basalt and rhyolite) erupting at essentially the same time.

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.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Howden, S. 2009. An Evaluation of Mafic Extrusives Spatially Assoicated with the South-Western Aspect of the Tweed Shield Volcano, BSc(Hons.) thesis, University of New England, Armidale.
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.

The volcano of the Border Ranges - Focal Peak

I was going to do a blog on the Focal Peak Volcano and the Cenozoic aged volcanic rocks associated with it in the Northern Rivers/New England NSW but to get an understanding of these rocks on the southern side of the dotted line you really have to know a bit, or a lot about the geology across the border. With that in mind I was going to write this blog but then I remembered that the wonderful Queensland branch of the Geological Society of Australia have some excellent information sheets on Mount Barney and Mount Barlow that would do just the trick. So instead of starting from scratch I thought I'd just link directly to the PDF. Here it is.

The authors of this information sheet are Neville Stevens and Warwick Willmott who in my view are/were some of the best science educators in the country and happen to be geologists! I have enjoyed some of their presentations (and many others) at the Theodore Club in Brisbane when I lived there and it is one of the things I do miss about living away from that city. Alas, Neville passed away earlier this year.

While I'm talking about Queensland I should recommend a couple of books which gives an excellent account of the geology of Southern Queensland these are Rocks and Landscapes of the National Parks of Southern Queensland by Warwick Willmott and Rocks and Lanscapes of the Gold Coast Hinterland by the same author. I understand this Gold Coast one has just been revised and expanded. You can get a copy of the Southern Queensland one for less than $25 and the Gold Coast one for less than $15 including postage from the Queensland Division of the Geological Society of Australia. For details on ordering these books click here.

What's the difference between the basalts?

A vesicular (air bubbles) example of Alstonville Basalt

There are three recognized Cenozoic aged "basaltic" geological units in the area between the Queensland border and Evans Head. These were first classified by Duggan and Mason (1978) and are the Blue Knob, Kyogle and Lismore Basalts. These 'basalts' are all part of the Lamington Volcanics.C otter (1998) has also proposed a new unit known as the Alstonville Basalt and included these in the Lamington Volcanics too but the information by Cotter was 'lost' until recently.  All four of these units are described below from oldest to youngest.

Alstonville Basalt
This is a new unit proposed by Cotter (1998), dating by this author gives a date of around 41 million years. This means that the Alstonville Basalt is too old to have formed through the same mechanism as the Tweed Volcano/Mount Warning basalts that are discussed below. No model of formation has been proposed but other research Vickery et al (2007) from the basalts of the New England tablelands area has proposed that a basalt of similar composition and age known as the Maybole Volcanics formed during rifting associated with the opening of the Tasman Sea. So this mechanism may be appropriate for the Alstonville Basalt too.
The Alstonville Basalt is actually similar in composition to the Kyogle Basalt in that it consists mainly of basalt and andesite called hawaiite which means that there is no mineral quartz in the rock but the mineral olivine is commonly found instead.

Kyogle Basalt
In Queensland the Kyogle Basalt is called the Albert Basalt. Wellman and McDougall 1974 give the age of the Albert Basalt at 22.5 million years (and accordingly the Kyogle Basalt would be the same age). The origin of this unit is regarded as the Focal Peak volcano which is situated today around Mount Barney. The Kygole Basalt predominately consists of a basalt called hawaiite with minor basanite and alkaline olivine basalt (basalts which are silica poor with no quartz in the rock but some olivine). Rarely tholeiitic basalt also occurs (basalt with some quartz which has crystallized in a specific geochemical pattern). The minerals that make up the smallest crystals in the rock (the groundmass) generally have a green colour giving the Kyogle Basalt a green tinge which often helps with identification in the field.

As the Australian Plate drifted over a hot spot in the mantle a chain of volcanoes was formed with the oldest situated in Queensland and the youngest (and still active or just dormant) volcanoes situated in Victoria and out in the Southern Ocean. The Kyogle Basalt represents the commencement of hot spot volcanism (i.e. the beginning of the Tweed and Focal Peak volcanoes) in the region.

Lismore Basalt
The Lismore Basalt is called the Beechmont Basalt in Queensland which has been given an age of between 22.6 to 22.9 million years. In some areas Duggan and Mason (1978) have mapped the Lismore Basalt as directly overlying the Kyogle Basalt. However, it is important to note that in the field the distinction between the two units can be difficult at times. The Lismore basalts are mainly tholeiitic in nature (usually contain a little bit of quartz and no olivine). The distribution of the Lismore Basalt is greatest for all the units of the Lamington Volcanics in NSW with the unit exposed over an area of greater than 3 000 square kilometres. It is the major eruptive unit originating from the Tweed Shield Volcano which is centred at present day Mount Warning.

Blue Knob Basalt
There is actually very little difference between the Blue Knob and Lismore Basalts except that the two units are separated by units of rhyolite known as the Nimbin Rhyolite. Some authors such as Duggan and Houston (1978) and Smith and Houston (1995) have even suggested that they represent continuing sporadic eruptions of the Lismore Basalt during the period of eruptions of the Nimbin Rhyolite. The basalts outcrop on top of or inter-collated with the Nimbin Rhyolite and may actually represent a continuity of occasional basalt lava eruptions while the rhyolite lavas were erupted. However, the Blue Knob Basalt represents the final preserved eruptions known of the Tweed Volcano.

In Queensland the Blue Knob Basalt is called the Hobwee Basalt.

Note: Now, if you are a little bamboozled by all the weird names of the basalts and how basalts can appear to be identical and called something else in a different location (especially given state borders) please keep with me because in the near future I will do a post that explains the difference. I'll also have to find some sources online to explain how basalts are different from each other (and how to tell that difference in the field). In the mean time the glossary may provide some assistance.


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.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Smith, J. V., Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences v42.
*Vickery, N. M., Dawson, M.W., Sivell, W.J., Malloch, K.R., Dunlap, W.J. 2007. Cainozoic igneous rocks in the Bingara to Inverell area, northeastern New South Wales. Geological Survey of New South Wales Quarterly Notes v123.
*Wellman, P. & McDougall, I. 1974. Potassium-argon Dates on the Cainozoic Volcanic Rocks of New South Wales. Journal of the Geological Society of Australia v21.

How big was the Mount Warning Volcano?

Mount Warning looking over the Nightcap Ranges

I think I will start by my first blog by asking a rhetorical question.

How big was the Mt Warning/Tweed Volcano? It is certainly not a question that just jumps into ones head unless you love geology!

The traditional view by respected geographers such as Cliff Ollier is that Mt Warning is a remnant of a huge shield volcano that erupted during a time that was known as the Cenozoic (or Tertiary). The extents of the shield was from Evans Head in the south to Mount Tamborine in the North to Mount Lindsay in the West to somewhere out to sea to the east. It must have covered around 7 000 square kilometres in area and been almost 2 000m high. Ferrett (2005) gives its height as 2000metres and a diameter of about 100km. It was big. But I think it is wrong. Well, at least partly wrong.

The funny thing about scientific discovery is that once one is made once something is finally thought to be understood, contradictory information is seen as too hard to deal with. It is a kind of scientific inertia. Especially once the general public think something is true. For example, I've heard again and again that the Great Wall of China is the only man made object to be seen from space (it cannot be seen; whereas cities, irrigation channels, farmland and other objects are seen commonly). This is true too for Mount Warning/Tweed Volcano.

I've been able to find some 'forgotten' (but not long forgotten) research recently that I think turns things on its head. These are:

Masters research from Southern Cross University (when they had a geology department) by Cotter (1998) (the only online reference I can find is here but there is a copy of his thesis in the archives of Southern Cross University, which you can read under supervision only!), A journal article by Duggan and Mason (1978) here and another journal article by Smith et al (1998) here.

Can you put it all together?

I even think that Duggan and Mason (1978) are a little generous with the Lismore Basalt. I think that more of what they called the Lismore Basalt (from Mount Warning/Tweed Volcano) is actually Kyogle Basalt (from Focal Peak/Mount Barney). This makes the extent to the west much less. At a push Smith et al (1998) show that there are no Cenozoic basalts exist in the Evans Head area. But most significantly Cotter shows a even more:

1. basalts between Evans Head and Alstonville are different compositionally from the Lismore Basalt and are probably part of the Chillingham volcanics and therefore they are Mesozoic aged (much, much older than the Tweed Volcano.
2. the land form would have directed lavas away from the south and
3. most of the basalt in the Lismore/Alstonville area is likely to be twice the age at around 40 Million years and definitely would not have been part of the hot spot volcanism that formed the Mount Warning/Tweed Volcano around 23 million years ago.

This all shows that a lot of the recent volcanic geology of the area needs to be reviewed (Is there a 'Alstonville Basalt'). Was the basalt around Alstonville actually similar to basalts in the New England tablelands (such as the Maybole Volcanics) which were associated with the formation of the Tasman ocean? What were the southern extents of the Lismore Basalt after all?


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.
* Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Ferrett, R. Australia's Volcanoes. New Holland Publishers 2005.
*Learned Australasian Volcanology Association, 1998. Lava News, December 1998.
*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