Ahh Ahh

Readers of this blog will probably notice I have an intense interest in volcanology. Volcanology has a wide variety of aspects some of which I’m comfortable, some less so. These aspects can be the chemistry of molten materials, the physics of earthquakes or the dynamic processes of pyroclastic flows. Volcanology and igneous rocks more generally seem to have their own weird language that can stop you and make you turn to a dictionary.

One of my favourite words in the ‘language’ of geology is the name of a large scale structure of lava flows. It is called aa. So, turning to a dictionary (this time the Omnificent English Dictionary in Limerick Form) you get the following possible definitions:

No consonants! Does this seem ominous? 
It's with rough-surfaced lava synonymous.
Yet the thought it conveys
With two capital A's 
Is, of course, Alcoholics Anonymous. 
By Chris J. Strolin


I'm ascending a gentle volcano; 
The climb's not the cause of my strain. 
No, This lava is stressed, 
Pretty jagged at best. 
Cut my feet on sharp aa — the pain, Oh! 
By Aliza


On Hawaii the lava's aflame 
As observers, in awe, cry its name. 
When that molten rock's oozing 
Down paths of its choosing, 
It's "A'a!" that tourists exclaim. 
By David

Probably one of the more interesting dictionary definitions I’ve seen. I Hope that helps with understanding? If these are a little bit obscure you can always visit my Glossary.

Rocks named after a creek named after an Australian marsupial

Note that the stratigraphy of this formation has been revised since this blog post. See the this recent post for details.

One of the most widely outcropping rock units of the mesozoic aged Clarence Moreton Basin is the Kangaroo Creek Sandstone named after its type locality at Kangaroo Creek in the Nymboida area. It is also one of the most recognisable stratigraphic units in the basin.

McElroy (1963) showed that the Kangaroo Creek Sandstone consisted mainly of white to cream coloured quartz sand. The texture of the sandstone is saccharoidal, that is, it has a glistening sugar like appearance of the quartz sand grains. This sand glistens more than usual because while buried, fluids in the rock caused extra silica (quartz) to crystallise on the existing sand grains creating new tiny crystal faces that reflect light in a vivid way. The nature of the rock in this formation tends to weather less readily than other units and as a result tends to form prominent topographic features such as hills, cliffs, ridges and the like.

Crossbedding and typical saccharoidal texture in Kangaroo Creek Sandstone

The Kangaroo Creek Sandstone was deposited in a fluvial (river) setting and as a result cross bedding structures are very common in outcrops. Sorting of grains in the unit is very well developed, that is, the grain size is very similar at any particular outcrop. Additionally, the thickness of the beds is very consistent which together indicates that the tectonic setting was relatively unchanged through the period of deposition. Following burial of the sandstone fluids present in the rock caused extra dissolved silica to precipitate out onto the existing sand grains filling in voids and creating the characteristic texture.

The Kangaroo Creek Sandstone is considered by some authors (Wells and O'Brien 1998) to grade into the Woodenbong Beds in the north west of the NSW portion of the basin. However, it is noted that others (Willis 1998) consider the Woodenbong Beds the equivalent to the McLean Sandstone Member of the Walloon Coal Measures (but more about this in future post). The Kangaroo Creek Sandstone underlies the Grafton Formation but the contact with this formation is gradational. According to (Wells and O'Brien 1998) it also sometimes shows a conformable boundary with the underlying Walloon Coal Measures, however, in most areas the boundary is shown by an unconformity. It is easy to tell the difference however, because compositionally any sandstones in the Walloon Coal Measures are composed of feldspar and lithic grains rather than the quartz of the Kangaroo Creek Sandstone.

Outcrop of Kangaroo Creek Sandstone on the Clarence River near Grafton

It is interesting to note that the recrystalisation of quartz in the Kangaroo Creek Sandstone means that this unit is now essentially dry with respect to Ground Water. There is very few spaces left for the water to travel through. for example O'Brien et al (1998) shows that most other sandstones in other basins such as the Great Artesian Basin, is where most ground water is obtained. In fact, in the whole of the Clarence Moreton Basin the only unit to have useful ground water bores is the Grafton Formation which is recharged from rainfall. The Kangaroo Creek Sandstone does have some bores that produce a very little water in the upper most portion of the unit (probably rainwater recharging fractures in these locations (Kwantes 2011), like the overlying Grafton Formation) but it appears that no other bores obtain water from the Kangaroo Creek Sandstone because the formation actually behaves like an aquiclude or aquitard. Water is not obtained from aquifers below the Kangaroo Creek Sandstone because the water quality is generally poor.

It is interesting to note that according to some gas exploration results it is apparent that areas of the Kangaroo Creek Sandstone (assuming this is not mistakenly identified McLean Sandstone) that are directly overlying the Walloon Coal Measures contain substantial areas of conventional natural gas. This is gas that has migrated from the underlying Walloon Coal Measures and been trapped in either pore spaces or fracture zones. I understand that several companies in the area such as Metgasco and Red Sky Energy intend to exploit these reserves.

Pollen spores in drill holes give an age of middle to late Jurassic for the Kangaroo Creek Sandstone (Wells and O'Brien 1998).

References/Bibliography:

*Kwantes, E. 2011. Future Water Strategy: Groundwater Options - Position Paper. Report for Rous Water by Parsons Brinkerhoff.
*McElroy, C.T. 1963 The geology of the Clarence-Moreton Basin. New South Wales Geological Survey, Memoir 9, 172 pp.
*Moran, C., Vink, S. 2010 Assessment of impacts of the proposed coal seam gas operations on surface and groundwater systems in the Murray-Darling Basin. The University of Queensland.
*New South Wales Government. 2010. State of the Catchment Report: Groundwater. Northern Rivers Region. Department of Environment, Climate Change and Water.
*Wells, A.T. , O'Brien, P.E. 1994 Lithostratigraphic framework of the Clarence-Moreton 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.
*Willis, I.L. 1994 Stratigraphic Implications of Regional Reconnaissance Observations in the Southern Clarence-Morton Basin, New South Wales 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.

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.

Feline Rock Classification

Igneous cats from the Northern Virginia Community College blog.

Maybe you don't need to visit the glossary after all?

Top of the Basin: The Grafton Formation

The Clarence Moreton Basin covers a large proportion of the catchment areas of the present day Clarence and Richmond Rivers in northern New South Wales and extends a significant distance more into south east Queensland. The portion of the basin which is most well known is the Queensland section but slowly we are learning more about the southern areas. The basin consists of many individual stratigraphic units which were deposited in slightly different environments at different times. The youngest unit is called the Grafton Formation and is thought to have been deposited during the Mesozoic era called the Cretaceous period which could be as young as 65Ma but it may be as old as late Jurassic.

The extent of the Grafton formation is small by Clarence Morton Basin standards because the majority of the unit appears to have been removed by erosion. Exposures can be found as far as 30km south of Grafton to about 10km north of Casino. The full remaining thickness of the formation has been estimated at up to 442m but is probably less with the best estimate of 267m obtained from a drill hole at Grafton.

Grafton Formation lithic sandstone near Casino

The formation is comprised of interbedded lithic to quartz arenites (sandstones), clayey siltstone, claystone and minor coal, sometimes 2metre thick conglomerate layers are present too. The lithic fragments frequently include the volcanic rock andesite implying active volcanism upstream at the same time as the sediments were being deposited. The bedding can be thin to thick and commonly a ferruginous (iron rich) lateritic weathering profile is present creating a very red coloured soil. This is particularly evident in the hills just to the north of Grafton such as Junction Hill. The sandstones are fairly characteristic in that they are usually tough and green-grey in colour.

One author (Wells and O'Brien 1994) suggests that the Grafton formation (and the Kangaroo Creek Sandstone) may also be equivalent to the Woodenbing beds (located between Urbenville/Woodenbong and Kyogle) and even though they are lithologically (rock composition) different this is still possible. An alternative by Willis 1994 is that it is the equivalent of the McLean Sandstone Member of the Walloon Coal Measures. But this will be discussed in detail in a future post.

The formation overlies the Kangaroo Creek Sandstone and is gradational meaning that the Kangaroo Creek Sandstone grades into the Grafton formation. Thankfully, recognising the difference is not hard on the basis of lithology (rock type) because the Kangaroo Creek Sandstone is very consistent in appearance (saccharoidal texture and abundant cross bedding) and consistent rock composition (quartz sandstone). The top Grafton formation has been eroded and is overlain by the more recent Cenozoic volcanics.

The Grafton formation was deposited in a mainly fluvial (riverine) environment with the more common siltstones and mudstones in the south probably being deposited in a lacustrine (lake) environment. This led to an idea that the source of the rivers and lakes that laid down the sediments in Grafton Formation was from the north but recent revisions of the probable mountain chains that existed at the time means that this many not necessarily be the case. Wells and O'Brien (1994) give the maximum age of the Grafton Formation as late Jurassic.

Interestingly, Grafton Formation is the only rock unit in the Clarence-Moreton Basin that has any significant or active ground water sources. The basin has proven to be a very poor source for water because of the lack of volume. In fact the only volume of water obtained from the Grafton Formation is really only unconfined aquifers recharged from surface water and overlying alluvium.


Note: Since writing this post it has been suggested in a new paper that the Grafton Formation appears to be made up of two members. The new paper by Doig & Stanmore (2012) significantly increases our knowledge of the Grafton Formation. I will endeavour to do a new blog post with the updated details.




References/bibliography:

*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 16.
*New South Wales Government. 2010. State of the Catchment Report: Groundwater. Northern Rivers Region. Department of Environment, Climate Change and Water.
*Wells, A.T. , O'Brien, P.E. 1994 Lithostratigraphic framework of the Clarence-Moreton 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.
*Willis, I.L. 1994 Stratigraphic Implications of Regional Reconnaissance Observations in the Southern Clarence-Morton Basin, New South Wales 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.

What is the Mount Warning erosion caldera?

It is very popular to refer to the Mount Warning area as the Mount Warning erosion caldera or Tweed Shield erosion caldera. Many sources indicate that it is the biggest erosion caldera in the world. For example Bigvolcano or good ol' wikipedia use the term. There are some very informative books by top class geologists such as Rocks and Landscapes of South East Queensland by Warwick Willmott also use the term. It is certainly an imposing volcanic influences landscape, but what is an erosion caldera anyway.

Ok. Let us start somewhere definite. A geological dictionary definition. Lets just look at caldera: A large circular crater left after the collapse or explosion of a volcanic cone. Now, lets look at erosion: The wearing away of rocks or other materials by the action of water or ice or wind.

So, Adding the term erosion to the front of the word caldera implies this: a landscape formed through the actions of wind or water or ice but also simultaneously formed through the collapse or explosion of a volcanic cone. Hopefully, you agree that this is possibly misleading. We have two different formation concepts equally applicable at the same time. What gives? How can the same feature form at the same site twice? once by erosion and once from the collapse of a magma chamber.

Mount Warning in the centre of the volcano remnant

Mount Warning was once the centre of a large volcanic cone called the Tweed Volcano. The centre of the Tweed Volcanic cone was a crater which may have collapsed or exploded as some stage to create a larger caldera. But this process is not definitely known because if this caldera actually existed it has since been eroded away to reveal the valley systems that we see today.

I think is is becoming obvious that there has been some sort of mistake in the development of the name erosion caldera. So, why use this term? A short answer is that most geologists tend not use this term, unless informally to illustrate the grand nature of some valleys that are formed in the remnants of large volcanos. That is not to say that some geologists don't mistakenly use the term anyway, I mean even geologists are human!

My suggestion, is not to use the term erosion caldera at all since it often results in confusion on the mechanism for the formation of thing it is actually used to describe.

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.

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.

Where have the Brisbane Metamorphics gone?!

A few months ago I was reading the 2011 NSW National Parks and Wildlife Service plan of management for the Julian Rocks Nature Reserve just offshore, near Byron Bay. The introduction said the Julian Rocks “are composed of Brisbane Metamorphics which date from the Carboniferous-Devonian period 345-405 million years ago and are the most resistant rock type in the region”. Sounds fine as a bit of background but why can’t I find recent geological work that refers to the Brisbane Metamorphics anywhere else?

Academics from Southern Cross University have used the term in published works as recently as 2007 (Specht and Specht 2007). But I can’t find it on any map or in any geological publication after 1990. Surely the rocks haven’t been eroded that quickly especially since it is “the most resistant rock type in the region”. I can, however, find reference to the Brisbane Metamorphics on the 1: 1000000 scale NSW geological map from 1962. But at such a scale it is hard to figure out exactly where it is. Broadly it appears to be located in some areas near Murwillumbah and some areas near the border with Queensland. The most specific paper I have is by Holcome (1977) which discussed the Brisbane Metamorphics in depth but doesn't say where it goes!

When in doubt try Google? But the result you get when typing in “northern rivers geology” is the website Big Volcano. It can be found here. Here too the geological history summary refers to the Brisbane Metamorphics but mistakenly links to a site that shows a small contact metamorphic area at Mount Coot-tha just to the west of Brisbane. This is a bit confusing because the metamorphic rock here is called the Bunya Phyllite which is a regional metamorphic rock which has undergone a second metamorphic even during the emplacement of the granite that makes up Mount Coot-tha. Interesting in itself, but it does not answer our question why the Brisbane Metamorphics were said to be at Byron Bay!

Well, The answer is simply a case of one of the most difficult aspects of geology, nomenclature. Geoscience Australia provides an excellent service in maintaining a database of all geological units named in Australia (past, present and proposed). It includes an entry on the Brisbane Metamorphics which can be found here. On the webpage you can see three fields that are important for knowing where this unit has gone. “Current: No”, “Status: Obsolete”. The comments field answers the question finally: “Name superseded by Rocksberg Greenstone, Bunya Phyllite, and Neranleigh-Fernvale Formation.”.

What this means is that the one description of Brisbane Metamorphics did not reflect the ages, genesis, and history of these three rock units.  You will get more information about the structural history and rock composition if you deal with the new units individually. Indeed, Holcome (1977) discusses the constituents of the Brisbane Metamorphics at length and notes that the Rocksberg Greenstone, Bunya Phyllite and Neranleigh-Fernvale Group are the constituents of the Brisbane Metamorphics but these are substantially different in terms of formation, metamorphic history and exposures. In northern New South Wales some of these units are present as part of what is called the Beenleigh Block (Holcome (1997).

There are many cases where geological units have been renamed or reclassified after further research has been done. This is no different to any other area of science. The only challenge is keeping up with the change.

References/bibliography:

*Holcome, R.J. 1977. Structure and tectonic history of the Brisbane Metamorphics in the Brisbane Area. Journal of the Geological Society of Australia. V24.
*NSW National Parks and Wildlife Service. January 2011. Julian Rocks Nature Reserve: Plan of Management.
*Specht, R.L. , Specht, A. 2007. Pre-settlement tree density in the eucalypt open-forest on the Brisbane Tuff. Proceedings of the Royal Society of Queensland 113 p9-16

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.