The NSW/QLD border geological mess and other matters - Talk at Binna Burra

This year marks the 85th birthday of the Binna Burra Wilderness Lodge in Southern Queensland. As part of the 85th celebrations the lodge has invited many people to give talks at the lodge between the 20th and 24th of June. There are many interesting science and nature talks open to the public on these days and two of the talks will have a geological theme. The Lodge is situated on the northern side of the Tweed volcano and the landscape and ecology of the area is intimately connected with the geological history of the area.

I will be giving a talk on the evolution of our understanding of the Tweed Volcano over the years and how politics can affect how we scientifically look at our part of the world. My talk will be on Friday 22nd June at 9.30am. On Sunday 24th, Warwick Willmott will be giving a more geology overview walk and talk including discussions on how the Tweed Volcano and Hawaiian Volcanoes have many similar characteristics.

UPDATE: Due to personal matters I have had to cancel my talk. The Talk by Warwick Willmott on Sunday is unaffected.

The details of my talk are as follows:

Talk title:

Our understanding of the Tweed Volcano: A Learning, Unlearning, Forgetful and Confused Experience.

Abstract:

The landscape of the NSW/QLD Border (Lamington and Tweed areas) being the result of a single volcanic centre, has been recognised formally for less time than the establishment of Binna Burra Lodge (only 70 years). Since this first realisation, many researchers have added to understanding of how the landscape has evolved. However, sometimes even in our modern and scientific world new knowledge can get lost, be ignored, or repeat old myths. This talk will cover some of the evolution of our understanding of the Tweed Shield Volcano and examine some of the persistent ‘popular science’ myths of this landscape.

For details of events being held at Binna Burra Wilderness Lodge you can visit their facebook page:

https://www.facebook.com/pg/binnaburra/events/?ref=page_internal

List of Mount Warning and Tweed Volcano blog posts

To try and keep some form of organisation on this blog I need to make a few more lists. Here is a list of blog posts for blogs where the topic is related to Mount Warning and the Tweed Volcano.

posts on the Mount Warning Central Complex

• The Right Age for Mount Warning
• A Warning About Mount Warning
• How Big Was the Mount Warning Volcano?

Posts on the Tweed Volcanic Rocks

• Crystals or No Crystals?
• Mythical Geology at the Mouth of the Tweed River
• Rocks and Landscapes of the Gold Coast Hinterland
• More on the Tweed Volcano
• Nimbin Rocks!?
• What's the Difference Between the Basalts?
• 'Recent' Rhyolite: The Nimbin Rhyolite at Minyon Falls
• How Big Was the Mount Warning Volcano?

Posts on the Cenozoic Volcanic Rock Preceding the Tweed Volcano

• Old Lakes Between Lava
• A Westward Wilsons River
• A Volcano of the Border Ranges - Focal Peak
• Groundwater in the Alstonville Plateau
• What's the Difference Between the Basalts?

The right age for Mount Warning

In previous posts on the Tweed Volcano, especially those relating to the Mount Warning Central Complex I indicated that there were some strange anomalies to do with the dating of these intrusions. Graham (1990), in his natural history summary of the Tweed region illustrated how confusing the dates recorded for the rocks that made up the Mount Warning Central Complex could be.

Mount Warning Central Complex from the southern rim of eroded shield

Wellman and McDougall (1974) summarised existing and provided new evidence for the date of the Mount Warning Central Complex and the surrounding Lamington Volcanics. Wellman and McDougall (1974) and earlier researchers used a very good technique of dating called potassium-argon dating (K-Ar dating). This is a radiometric dating method based on measurement of the radioactive decay of an isotope of potassium (40K) into argon (40Ar). Note, that the numbers in front of the chemical symbol for each element refers to the number of neutrons in the atoms nucleus. The decay rate of 40K to 40Ar is known accurately because the time it takes for half of the 40K to turn into 40 Ar is about 1.25billion years (the half-life). Therefore, the ratio of the two can be used to determine just how old the rock is.

The accuracy of using the K-Ar dating method is very good, but has some provisos. The most important being that the rock sample must be very 'fresh'. There must be no weathering, alteration or metamorphism of the sample. Because potassium is more reactive than argon and it can be removed or added respectively during weathering and alteration. Additionally, the K-Ar dating 'clock' can be reset during any recrystallisation during metamorphism.

K-Ar dating by Wellman and McDougall (1974) and earlier authors showed that the intrusive complex at Mount Warning was emplaced between ~23.7Ma and 23.0Ma and the surrounding lavas erupted from ~22.3Ma to ~20.5Ma. This doesn't make a lot of sense because an intrusion of magma needs to intrude into something else (otherwise it is not an intrusion!). In the case of shield volcanoes this intrudes the earlier lava that was erupted before. The K-Ar dating shows this is apparently not the case.

What is going on? No one could suggest any reasonable ideas. Cotter (1998) suggested a possibility there may have been a large volume of pre-existing Palaeozoic and/or Mesozoic sedimentary rocks that have now been eroded away. However, Cotter (1998) did date a sample of basalt lava from the Terania Creek area at ~23.9Ma (using K-Ar). This suggested maybe the dating by Wellman and McDougall (1974) and earlier authors might have either missed later lavas or maybe there was something else wrong.

Cohen (2007) spent a lot of time resampling the K-Ar dated volcanic rocks of eastern Australia. This time instead of using K-Ar he used another technique called 40Ar-39Ar dating. This is similar to K-Ar dating in concept. It instead measures the abundance of two isotopes of argon and is much less affected by any effects of weathering and alteration (though not metamorphism). What did he find? He found some of the K-Ar dates were wrong.

Cohen (2007) found the actual date of the lavas was within the range ~24.3 to ~23.6 million years, about 2 million years older than first thought. Though the 40Ar-39Ar date of the Mount Warning Central Complex was quite close at ~23.1Ma it fell within the range of the K-Ar dating (23.7-23.0Ma). This reverses the idea the intrusion of the Mount Warning Central Complex was before the lavas. So, now we know that the final intrusions of the Mount Warning Central Complex does indeed fit the model for shield volcanoes. That is, the intrusions were likely to have been emplaced into already erupted volcanic rock. They were also erupted and emplaced over a period much quicker than first thought. The new dating shows volcanism possibly lasting 1 million years instead of the 3 million previously suggested.

References/bibliography

*Cohen, B.E. 2007. High-resolution 40Ar/39Ar Geochronology of Intraplate Volcanism in Eastern Australia. PhD Thesis, University of Queensland.
*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.
*Graham, B.W. 1990. A Natural History - Tweed Gold Coast Region. Tweed River High School Library.
*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.

Mythical geology at the mouth of the Tweed River

My knowledge of Gaelic mythology is a bit limited but it is interesting to see where geology, Gaelic mythology, Captain Cook and Tweed heads have something in common. I’ve not been to Ireland or Scotland but I’ve experienced a feature that is quite famous in these countries that is also present on the northern rivers.

Fingal Head, clearly showing the basalt columns

Just to the south of the Tweed River mouth lies Fingal Head and Cook Island. Cook Island, is of course named after then-Lieutenant Cook who sailed along the section of coast in 1770. Fingal Head, however, is named after Fingal, a mythological Gaelic hero from Scotland, who never came to this part of Australia! So why is it named so?
To understand the name of Fingal Head you need to know about the story of the Giants Causeway in Ireland and Fingal’s Cave in Scotland. I’m not a good story teller so here is a link (if this link is still not working try this one instead). My summing up of the story is that one of the two warring giants built a causeway to the other side of the Irish Sea so that he could fight the other. The other giant tore it down so that only each side of the causeway remains, one in Northern Ireland the other, Western Scotland. Local tourist information says that Fingal Head is named after the Irish hero. This is actually incorrect, the Irish hero is named Finn MacCool. The name Tweed River should hint that it is actually the Scottish hero that Fingal Head is named after. So, where does the geology come in?

The giants causeway is made from basalt. The volume and thickness of the basalt lava flows means that different parts of the lava flow usually cool at different rates (though, as pointed out by Goehring et al. 2006, the actual mechanism is completely unknown). However, the general idea is that in the case of Fingal Head the lava flow has cooled quite quickly, resulting in contraction of the rock and cooling joints being formed. The incredible thing about nature is that these cooling joints forms columns of rock that are of similar thickness and cross sectional shape, usually hexagons. This formation style is called columnar basalt. Indeed the rock that makes up the causeway has been shown to extend under the sea all the way from Ireland to Scotland. While the scale is not as great as in the British Isles, Cook Island just a short distance off the coast is part of the same lava flow at Fingal Head. This area, therefore has very similar features as the Giants Causeway and in my opinion the name Fingal Head is very appropriate.

The lava at Fingal Head is apparently derived from the Tweed Volcano (classified as Lismore or Beechmont Basalt, depending on what side you are of the Queensland border). Whether it is a lava flow erupted from the original central vent or vents on the northern flank of the volcano is not known. It is worth knowing that columnar volcanic rock is actually fairly common. Indeed, even better columnar formations can be seen elsewhere in the region. If you travel inland from Bellingen up to Ebor and visit the waterfall there (Ebor Falls) you will be able to see some spectacular formations. Columnar jointing is not restricted to basalt lavas either, some rhyolite cliffs around the Tweed Volcano also show this feature too.

References/Bibliography:

*Goehring, L, Morris, S.W. &  Lin, Z. 2006. Experimental investigation of the scaling of columnar joints. Physical Review. V64.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

Rocks and Landscapes of the Gold Coast Hinterland

Since rocks tend not to follow political boundaries but our understanding of them often does it is good to know about what is north of the Northern Rivers/New England Border in southern Queensland. Last year I was going to do a post on the Focal Peak Volcano but then I remembered that the Queensland Division of the Geological Society had produced some excellent publications on the subject and recommended one in particular, so the post was essentially a recommendation for the Book the Rocks and Landscapes of the National Parks of Southern Queensland. But I deliberately omitted from the post comments on another brilliant book that had recently been fully revised so that I could deal with is separately.

The other book is called Rocks and Landscapes of the Gold Coast Hinterland by Warwick Willmott. I enjoy this book very much because it is simple to understand but goes into a good amount of detail. It also shows you exactly where to go to see a feature of interest just like a self guided tour.

However, the detailed knowledge of the northern part of the Tweed Volcano may have skewed research and our understanding of the volcano in general. For instance, although the Tweed Volcano has been assumed to be centred around the site of present day Mount Warning in New South Wales most of our understanding including the detailed research of PhD and MSc level on the volcano actually comes from the University of Queensland. The University of Queensland has been the driving institution for decades in research on these northern flanks by exceptional researchers like Professor Anthony Ewart and Dr Jan Knutson.

As I have discussed in numerous other posts on the Tweed Volcano, the model of what the volcano looked like and how it was formed has recently been questioned by authors such as Cotter (1998). In my mind this raises some questions about elaborating the northern side of the volcano to the remainder in New South Wales. While I have nothing to question the good work on the northern side of the border, including the wonderful books produced by the Australian Geological Society's Queensland Division, it appears that the model of volcanism of the Tweed Volcano has been interpreted to fit into a Queensland model. This has occurred ever since authors like Duggan & Mason (1978) and continued to Stevens et al (1989) and most recently by Howden (2009). I do not question to model of rock formation to the north of the border (it works for what is there) but according to Cotter (1998) south of the border pre-volcanic geological conditions seemed to be different and this had a significant effect on the mode of volcanism in the area. This however, does not mean that the Rocks and Landscapes of the Gold Coast Hinterland is incorrect in any way on its description of Queensland geology, it is just important to note that interpreting the geology south of the border can sometimes be problematic even if a cursory look means that it appears reasonable.

But I have digressed a great deal. Back to the Book! The Rocks and Landscapes of the Gold Coast Hinterland is formatted in a way that makes it a geological tour. If you end up traveling through the Gold Coast area, do get a copy of this book. It is only about $12 including postage and is quite large and detailed for its price. In fact I'm surprised that the cost is so low, but I think that all the time that Warwick Willmott has put into writing it has been for free. As I have said in other posts, Warwick is one of the great science educators in Australia, and the book really helps understand the Gold Coast area a lot.

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.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

A warning about Mount Warning

Here are some common quotes about Mount Warning:

"World Heritage listed Mount Warning (Wollumbin) is the remnant central plug of an ancient volcano." 

"The Mount Warning volcano was a huge shield volcano."

"Considered the central magma plug, Mt Warning and a system of ring dykes, being extremely hard rock, have resisted erosion, and dominate the valley landscape."

"Mt Warning, Wollumbin, the cloud catcher, is the basalt plug of the world's largest and oldest extinct volcano. "

"Now, Mt. Warning is the first place that that the sun hits at sunrise… the highest point in New South Wales….almost the highest in Australia!"

These are quotes typical of tourist and even educational resources. They are quite definite and the comments makes sense, mostly. There are also some points of view that I espoused for a long time... Except aspects of each of the quotes are technically wrong and in some cases completely wrong. Like my post on the "erosion caldera" something that is technically incorrect has become general knowledge. It is a little pedantic of me, but it is one of my hobby horses... so what is technically wrong with the quotes above?

Western face of Mount Warning (composed of syenite).
One of the ring dykes is visible in the foreground
and Mount Uki and the Pacific Ocean in the background

Interestingly, Stevens et al (1989) and earlier authors noted that the rock composition of the intrusions that make up Mount Warning (the Mount Warning Complex) is different from most of the lavas (The Lamington Volcanics) that exist in the region. It is also slightly older than most of the lavas. Geologically speaking the age difference is not huge at only about 2-3 million years, but still significant enough.

It is apparent from Smith & Houston (1995) and other authors that much of the rhyolite lavas that remain of the Lamington Volcanics were not erupted from the central area now the site of Mount Warning but from vents on the flanks. Given the coverage of the mafic components (the Lismore Basalt, for example) it is more difficult to identify any vents.  

An idea has been raised by Cotter (1998) which questions the volume of lava that was erupted from the Tweed Volcano. It is known that the Palaeozoic aged meta-sedimentary rocks of the Beenleigh Block, called the Neranleigh Fernvale Beds and the Mesozoic aged Chillingham Volcanics and Clarence Moreton Basin were not domed upwards by the underlying magma except a little around the Mount Warning Complex itself. However, other areas such as the nearby slightly older Focal Peak Volcano have been lifted by the Cenozoic aged volcanism. But in the case of Mount Warning, Cotter (1998) felt that lithology, the remnants of the rhyolitic lavas, the pre-existing Chillingham and Alstonville Volcanics was the main control on the geomorphology, not as suggested by others the volcanism that formed the shield volcano itself.

The idea suggested by Cotter (1998) has significant implications for the size of the Tweed Volcano. The volcano is considered the biggest by far of its age in eastern Australia. It appears likely that the extent of the shield volcano is not as great as originally thought. The underlying Chillingham Volcanics would have been an existing mountain range and therefore reduced the thickness of the Tweed volcanic pile and the Alstonville Basalts would have reduced the southerly extent. I think that when you add to this the idea that the rhyolite units have erupted away from Mount Warning, but instead from flanks on the volcano, the volume of lavas from the Tweed Volcano may actually be more in keeping with the other intra-plate volcanoes in Eastern Australia. It was also possible that before it was eroded into the present shape (which implies a central shield type volcano) it may have looked more irregular than we imagined...

But don't get me started on the comments about the biggest volcano in the world and the highest point in New South Wales!!! What were these people thinking?!

...but does any one want to talk down something that was presumed to be huge, just to something large? Emotionally, many (including myself) have an emotional attachment to the beauty and wonder of the Tweed Volcano, sometimes it is hard to take a step back and consider it is not quite as fabulous as originally thought, but what we see is still stunning... and it is still very, very big. To put that in perspective I think that even the small volcanoes in the region are stunning. We don't need to exaggerate something for it to inspire us.

Bibliography/References:

*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. 
*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.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

Old lakes between lava

As some people have commented, I have not posted for some time, nearly a month in fact. Sorry for the delay. I have been affected by some unexpected (and some expected) health problems including some surgery (which was the expected part). I am still recovering and will be for a little while so posts will continue to be infrequent too.

In the mean time, this is a short post about some rock called diatomite which occurs in at least two locations on the Alstonville Plateau. The Alstonville Plateau is comprised mainly of basalts previously thought to be Lismore Basalt sourced from the Tweed Volcano/Mount Warning area but now according to Cotter (1998) should be referred to as the Alstonville Basalt from an earlier volcanic event during the Cenozoic. But there are at least two locations where the basalt created areas where lakes were formed by natural dams created by lava flows. These areas are Tintenbar and Wyrallah and there is possibly another one or two.



'Potch' opal from Tintenbar

Diatomite is formed from the preservation of silica from plant and animal remains and looks a lot like chalk. It is white, powdery and often retains impressions and fossils. It was formed in a fresh water environment, in other words a lake. This is referred to as a lacustrine environment.

The Wyrallah deposit was mined up to the 1950s (as diatomite can be used for anything from kitty litter to beer filtration) and is located just up the ridge heading towards the Rous area from Wyrallah. The Tintenbar deposit was also mined but also contained opal in lowest parts of the overlying basalt lava flow. not very good opal, a type called 'potch' but worth looking for at least for interest sake. This deposit was just to the west of Emigrant Creek just south of Tintenbar village. Both deposits are underlain by basalt and overlain by it showing that the lakes must have existed during the period of volcanism.

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.
*Herbert, C. 1968. The Tintenbar and Wyrallah Diatomite Deposits. Departmental Report. New South Wales Geological Survey.

The backward Clarence River

I’ve always wondered about a couple of the major rivers in our region and why the flow the direction they do. In particular I have been interested in the Richmond River and Wilsons River systems and also the Clarence Systems. I recently did a blog post on the Wilsons River and how along a large part of its length it seems to have flowed away from the sea for quite some time, maybe millions of years. In the comments on that post, Mark asked me why a tributary of the Clarence River seems to join that river in a peculiar way and that reminded me to do a post on the topic. So here it is.

From Ollier and Haworth (1995) The effect of uplift on a denritic drainage

If you have a look at a map of the Clarence River system and its tributaries you will observe that the shape of the catchment is unusual. The major tributaries of the Clarence such as the Mann River, Orara River, Timbarra River, Boonoo Boonoo River and Cataract River flow north into the south flowing Clarence River. Usually the dendritic shape of river tributaries means that tributaries join with the main river at an acute angle. The major tributaries above, however, join at an obtuse angle. They almost look like they are flowing backwards.

How did this come to be? Ollier and Haworth (1994) came up with a surprising solution. Essentially, they thought that the angles the tributaries were joining the Clarence would make sense if the Clarence River once flowed the opposite direction. Today this seems like a far fetched idea, I mean, water can’t run up hill, can it? But Ollier and Haworth (1994) thought that prior to the Cenozoic volcanoes that make up the Main Range, Focal Peak and Tweed Volcanoes the land surface would have been much flatter. Indeed the sediments closer to these volcanic centres has been uplifted by hundreds of metres. If there were no mountain ranges along the Queensland border it would be quite conceivable for a river to flow from the New England highlands northward into the Condamine River in Queensland and then into the Murray-Darling River system.

The current trace of the Clarence River is a little bit strange. In many places it crosses between hard Palaeozoic basement rock of the New England into the softer rock of the Clarence Morton Basin and then back again. Rivers usually cut river channels preferentially into softer rock and will rarely flow from gentle valleys in softer rock into steep hard valleys as the Clarence River does along its southward path. Combining this with the knowledge that parts of the Clarence Morton Basin have been shown through various seismic exploration techniques that is has been warped in various directions adds further to the argument.

I for one, am convinced. I think that the Clarence River once flowed north before the Macpherson Range came into existence and the River would probably have joined with an earlier Condamine River. Need to check for yourself? Have a look at the rivers of the region on a map, follow the route of the Clarence River from the Pacific ocean and observe the rivers that join in. Surprisingly, you might find it makes sense!

Postscript: Still can't picture the Clarence River flowing inland? This post discusses the location of the Great Australian Divide and where it would have been when the Clarence River flowed into the Murray-Darling/Condamine System.

References/Bibliography:

*Ollier, C., Haworth, R.J. (1994) Geomorphology of the Clarence-Moreton Basin. In Wells, A.T. & O’Brian, P.E. Geology and Petroleum Potential of the Clarence-Morton Basin. Australian Geological Survey Organisation, Bulletin 241.

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.

A westward Wilsons River

The Wilsons River flows from east to west between Booyong and Lismore which is unusual for coastal rivers in the region. You’d expect a river to find the path of least resistance and head to the sea quite quickly, in the case of the Wilsons River the path of least resistance appears to have been away from that range of hills or mountains in the Alstonville area and away from the sea.

In his masters research, Cotter (1998) discovered that the landform of the tweed volcano was more complex than the simple shield volcano model proposed by earlier researchers. The shield volcano model essentially shows a radial drainage pattern from the centre of the shield a bit like the spokes on a bicycle wheel. While this does hold up well for the remaining skeleton of the Tweed Volcano of particular interest is the area to the south where a previously unidentified Cenozoic volcanic unit was discovered and shows that pre-existing structures explain the river drainage. Cotter (1998) suggested the name of Alstonville Basalt for the new Cenozoic (up to 41 Million Years old) unit as it appeared to pre-date the tweed volcano (23 Million Years). Additionally, it has been identified that the even older Mesozoic Chillingham Volcanics (but here consisting of basalts rather than the rhyolites that are seen further north) occur on what was once considered the southern flank of the volcano.

Brodie and Green (2002) observed that the dip of the Alstonville Basalt is to the north west which to me seems to indicate a volcanic centre further to the south east (in the opposite direction for lavas from the Tweed Volcano) assuming that not too much deformation has occurred since the rocks were erupted. Taken together this implies that during the Mesozoic hills existed to the south of the present day Alstonville Plateau and that during the early Cenozoic volcanic hills were emplaced and created a barrier for southerly or easterly discharge.

Cotter (1998) suggests that the Wilsons River has actually roughly followed its current path since the Late Mesozoic. The diatomite deposits located at Tintenbar and Wyrallah are of lacustrine origin and may be the result of lakes forming on the newly erupted Alstonville Basalt as the Wilsons River was intermittently impounded by the existing hills of the Chillingham Volcanics. It has been following a westerly course certainly before the Tweed Volcano (c. 23 Million Years) for the Chillingham Volcanics and Alstonville Basalt has stopped the Wilsons from flowing south or east. This continuity of flow direction implies that any lavas from the Tweed volcano would have cut through the Lamington Volcanics of the Tweed volcano unless the lava was of significant enough volume to change river direction. This volume of lava appears unlikely given the distance of this area from the centre of the volcano now represented by Mount Warning.

Putting all the background together shows that the section of the Wilsons River from Booyong to Lismore may have been flowing away from the sea for more than 40 Million Years, yet, additionally it is worth noting that the majority of the length of the Wilsons River, Richmond River and even Clarence River is north-south parallel to the coast. This implies some form of pre-existing structural control, probably associated with the deposition of the Clarence Morton Basin or even older Palaeozoic basement rocks, in turn; suggesting that the northern rivers have been following similar flow paths for a long, long time. However, this is a discussion for another post.

References/Bibliography:

*Brodie, R.S. & Green, R. 2002. A Hydrogeological Assessment of the Fractured Basalt Aquifers on the Alstonville Plateau, NSW. Australian Bureau of Rural Sciences, Australia
*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.
*Ferrett, R. Australia's Volcanoes. New Holland Publishers 2005.

Nimbin Rocks!?

Sadly, I don't visit Nimbin even though I live quite close. This is mainly because you do risk your health through passive (hemp) smoking, getting beaten up if you stumble across someones 'crop' while trying to find geological features in the bush, or just threatened with a knife for money so they can get their family birthday presents (or something important like that). Another thing too is the distrust that many people have in the area for geologists thinking in their ignorance of what I am trying to find may lead to a gas well in their front yard and assuming that I have to be working for a coal seam gas company.



One of the Nimbin Rocks, Cathederal Rock

 But one feature stands out near Nimbin and that is the Nimbin Rocks which tower above the surrounding valleys. 'Google' "Nimbin Rocks" and you will find lots of short snippets on these grand rock formations. Unfortunately, I've found that these descriptions are technically wrong. For instance wikipedia (and many, many travel websites) use terms to describe the Rocks as being derived from a dyke and also as being extrusive. Well, technically, a dyke is an intrusive body only and extrusive rocks are better known as lavas. So what is correct?

The Nimbin Rocks are comprised mainly of the quartz rich volcanic rock called rhyolite overlying a section of agglomerates (reworked volcanic rock) and volcanic glass known as perlite. Below the perlite lies basalts of the Kyogle Basalt. And here may lie the clue. The rocks appear to be layered because they are deposited on top of each other. First the Kyogle Basalt, then the perlite and agglomerates and then the rhyolite lavas (with some bands of perlite within it). The rhyolitic lavas are referred to as the Georgica Rhyolite Member according to Duggan and Mason (1974), or historically and more recently as Nimbin Rhyolite according to McElroy (1962) and Cotter (1998) and others.

If the Nimbin Rocks were related to a dyke they would have formed through pushing through the surrounding rocks such as those of the Kyogle Basalt or the Clarence Moreton Basin sediments, metamorphosing them and displaying different diagnostic textures than those I know about. However, it is still quite possible that the rocks may have been vents since the nearest identified vents seem to be about 8km away to the north east in the Nightcap Ranges and rhyolite lava flows tend to not move great distances, indeed rarely greater than 5km. However, the vents located further into the Nightcap Ranges are characterised by thick erosion resistant units of rhyolite which we don't see so much near Nimbin other than the Nimbin Rocks themselves. But conversely, the shape of the rock monoliths does imply a dyke.

So, what is the answer? Well the Nimbin Rocks are either one or more volcanic vents or they are the remnants of thick lava flows possibly from vents in the nightcap ranges located on the flanks of the Tweed Volcano. Which is almost not an answer at all. But one thing is obvious, it is interesting just how little we know about the landscape in which we live, work and see.

Blog Note: I like to provide photos for these sort of posts but recently where I store photos (skydrive and/or GoogleDocs) has changed its method for providing URLs to allow embedding of these files and Blogger doesn't like the new URLs. So, these next blogs might be a bit more bland looking until I figure out a better way to store and embed photos.

Since writing the above I have come across a report by Relph (1958) which says the following:

“Quartz-feldspar porphyry [granite, the intrusive equivalent of rhyolite] has intruded the sediments at Lillian Rock and the eastern portion of the Nimbin Rocks area. In the latter occurrence the porphyry forms two prominent pinnacles, with columnar jointing evident, and outcrops to the east, and in the bed of Goolmangar Creek. In neither case have the surrounding sediments been affected to any marked degree, but it is thought that it is intrusive and of dyke or plug form rather than of extrusive nature. Under the microscope this rock revealed no sign of flow structure.”

Although Relph considered two of the Nimbin Rocks intrusive he did not find any diagnostic evidence of them being either intrusive or extrusive.

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.
*McElroy, C.T. 1969. The Clarence-Moreton Basin in New South Wales. In Packham, G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16
*Relph, R.E. 1958: Geology of the Nimbin area. Technical Report. Department of Mines NSW, 3.
*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.

Ground water in the Alstonville Plateau

A palaeosol in the Alstonville Basalt

Ground water is a valuable source of water for stock watering, domestic uses, irrigation and town water supply in the area of the Alstonville Plateau. For example both Ballina Shire Council and Rous Water operate ground water bores as sources of water for municipal use. The reason for the popular use of the ground water from this source is its yield and also freshness. The quality of the water in the aquifers is excellent and the quantity good. In fact the popularity of ground water from the Alstonville Plateau is such that it threatens to be over used with many aquifers being badly drawn down and for this reason the NSW state government has put in place a water sharing plan that prohibits new water extraction licenses from some areas of the plateau and all ground water bores in the area require a license.

So what is the Alstonville Plateau ground water source anyway. Where does the water come from? Well, in short, the Alstonville Plateau ground water source is a series of aquifers that occur in the Cenozoic basalt that defines the area of the Alstonville Plateau. The plateau extends from beyond? Lismore in the west almost to the coast at Lennox Head, past the little village of Newrybar in the north (almost to Bangalow) and south almost to the Richmond River at Broadwater. According to Brodie and Green (2003) there are several aquifers with the upper most being an unconfined source of water within the upper weathered and/or fractured zones of the basalt. Below this is at least one confined aquifer which flows through permeable layers such as paleosols (old soil horizons) or through fractures in the basalt. An example of a paleosol from the Alstonville Basalt is shown above (not acting as an aquifer in this case).

The unconfined aquifer is usually able to be intercepted within several metres of the surface but this depth can vary wildly depending on the depth of soil weathering zones and local topography. This shallow source is usually easy to find but yields are usually low and are often subject to drying out during periods of drought due to the local surface water influence on these aquifers. In general when it rains the streams tend to recharge the aquifers and when the weather dries out the aquifers tend to return base flow to the streams (until the aquifers run out of water).

The deeper aquifers are confined between layers of basalt. The layers that the water is found in is either made from substantially fractured rock or paleosols that were developed on lava flows and were subsequently covered up by new lava flows (i.e. are directly related to the eruptive conditions during the formation of the basalt). Interestingly, the dip direction of the aquifers is generally from east to west which is somewhat inconsistent with the idea that these rocks were sourced from the Tweed Volcano which is the established theory since Duggan and Mason published their paper on the volcanic rocks of the area in 1978.

The interesting thing about the importance of this ground water source is that despite the area being mapped as Lismore Basalt  most other areas of the Lismore basalt away from the Alstonville Plateau are not in as high demand for ground water as the Alstonville Plateau. Why is this? It is possible that there are peculiar features of the plateau such as extensive paleosols but it is possible that it is related to the plateau being derived from an older basalt unit that was identified by Cotter (1998) but has not been followed up in detail by any other authors since. See my older posts on this subject here and here.

References/Bibliography:


*Brodie, R.S. & Green, R. 2002. A Hydrogeological Assessment of the Fractured Basalt Aquifers on the Alstonville Plateau, NSW. Australian Bureau of Rural Sciences, Australia
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*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.

What is meant by some of these names (1)

I have a habit of blasting people with technical jargon sometimes and I keep forgetting that I'm a bit of a geology geek and sometimes I'm hard to understand. So I thought it might be wise to have a quick comment on some of the names that I use. There are many different types of geological names. The main types (in my opinion) are:

1. geological ages;
2. mineral names;
3. rock names; and
4. rock unit names

But just to complicate things each of these can be broken up with further names for instance:

Geological ages from the International Stratigraphic Commission.

1. geological ages: the age Cenozoic era (65.5 million years to the present) includes smaller age periods called the Quaternary (present to 2.6 million years ago), Neogene (2.6 million years to 23 million years) and Paleogene (23 million years to 65.5 million years) periods. These too can be subdivided.

2. mineral names: minerals like quartz and feldspar will be familiar to most since they are the two most common minerals on earth but these can be broken down further based on slightly different chemical properties. Feldspar can also be called plagioclase (if it is richer in the elements sodium and calcium - [chemical formula NaAlSi₃O₈ to CaAl₂Si₂O₈]) or orthoclase (if it is richer in the element potassium [chemical formula KAlSi₃O₈]). Needless to say, these mineral names too can be subdivided.

3. rock names: you've probably heard of basalt but what about hawaiite, mugarite, tholeiite and benmorite? Well, these are just fancy names for different basalts based on slightly different mineral compositions. E.g. tholeiite has quartz (due to higher silica) and hawaiite has olivine (due to low silica). Thank goodness, these basalts are rarely subdivided any further.

4. rock unit names: One I will refer to regularly on this blog is the Lamington Volcanics. This is a unit that refers to all the rock sourced directly from the Tweed Volcano (Mount Warning area) and the Focal Peak Volcano (Mount Barney area). Itself it contains sub-units such as the Lismore Basalt which is mainly comprised of basalt (mainly of the tholeiite type) that was erupted during the Cenozoic era (Neogene to Paleogene periods). Yes, some of these units can further be subdivided.

When you get right into geology it becomes evident that it can be quite tricky. But most of the trickiness comes from learning all the names not from understanding what actually happens with rocks! I will continue to occasionally post on nomenclature in the future. In the mean time you may find some help in the glossary.

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.