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.

Why lava is unable to cross the state border!

Nimbin Rhyolite (front), Mt. Warning (right), Binna Burra Rhyolite (distant)

It is interesting to see how being north or south of the Queensland border influences so many things. North of the border you can get cheaper car registration, get away from NSW politics, follow a worst rugby league teams, follow the best soccer teams, see narrower gauge railways (which even frequently have trains on them!) and find that even the rocks have changed.

Actually, the rocks have not changed but for some reason I cannot fathom (like most of the other points raised above) the rocks often have different names but many have the same ones. Rocks of the Mesozoic Clarence-Moreton basin have the same names, rocks of the Palaeozoic basement have the same names, but rocks of the Lamington Volcanics are named differently. You can stand on the Lismore Basalt and take one step into Queensland and you are on the Beechmont Basalt. Suffice to say it can be confusing. So, based on Duggan and Mason (1978) here is a table to show what the rocks units in the Lamington Volcanics are called in either state:

New South Wales  - Queensland

Kyogle Basalt - Albert Basalt

Homeleigh Agglomerate Member - Mount Gillies Rhyolite

Lismore Basalt - Beechmont Basalt

Nimbin Rhyolite - Binna Burra Rhyolite

Blue Knob Basalt - Hobwee Basalt

I note that there is some other rock units that are named differently in NSW and Queensland for example McElroy (1969) shows that such as the Evans Head Coal Measures, Ipswich coal measures and Red Cliff Coal Measures (Parts of the Ipswich Basin) are equivalent to each other, but these are separated by different rocks and so occur in distinctly different geographical locations. But as far as I am aware the Lamington volcanics are the most obvious example where an invisible dotted line representing the state border can name one half of the same rock, formed in the same way, at the same time, at the same outcrop something different.

It is also important to note that stratigraphy is often refined once more is known about rock units. A good example is that some authors such as Cotter 1998 dispute the existence of the Homeleigh Agglomerate Member which is considered part of the Nimbin Rhyolite. Also the Mount Gilllies Rhyolite has been renamed the Mount Gillies Volcanics, Therefore a different unit called the Georgica Rhyolite would be an equivalent of the Mount Gillies Volcanics.

I know I’ve said it elsewhere, but geology is not usually too difficult. The worst part is the nomenclature. I think this is a good example. What do you think?

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.