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.

A brief geological tour of Evans Head



Half Tide Rocks: Made from Chillingham Volcanics (dark rock)

Evans head is a popular vacation spot. It has some lovely beaches which are interrupted by a proud and attractive headland. During the summer it is impossible to find accommodation in the area and the town is crowded with families enjoying sunshine, boating, fishing, swimming and relaxing. I don't go there for holidays but I'm close enough to enjoy a day or two by the beach as sometimes.
The interesting feature of Evans Head is the obvious rocky headland standing quite proud at the mouth of the Evans River and along a coast line with huge sandy beaches. The reason for this feature is the more erosion resistant rocks that occur here. Click here to link to a basic geological map of the area. The hardest and oldest rock outcropping just south of Evans Head town at the Half Tide Rocks is the Triassic aged Chillingham Volcanics being the earliest part of the Ipswich basin. These rocks can be seen as the darker coloured rock at the two headlands in the photograph above. Here the Chillingham Volcanics are comprised of basalt and andesite (elsewhere in NSW such as at Chillingham the Chillingham Volcanics are mainly rhyolitic in composition) and here at Evans show an uncommon rock called hayloclastite. The formation of hayloclastite in this area was the result of eruption of basalt into a coastal sea. Something you might see in modern day Hawaii or Iceland where lava flows directly into the sea. Unfortunately it is very hard to recognize because of weathering of the rock in this area.

Overlying the Chillingham are the Evans Head Coal Measures. These are located on the southern bank of the Evans River and extend around to the south of the Half Tide Rocks. Despite their name coal is a little hard to find and is only present in occasional thin bands. The Evans Head Coal Measures are therefore mainly comprised of sandstone (a type called arenite with the sand grains mainly composed of quartz sand and occasional small fragments of rock), siltstone, mudstone and some coal. The arenite frequently shows a feature called cross-bedding which is common in rocks that have formed in medium velocity rivers. The Evans Head Coal Measures are equivalent to the Ipswich Coal Measures in southern Queensland and the Redcliff Coal Measures which occurs south of McLean and is exposed on the coast near Brooms Head.

Ripley Road Sandstone with a small conglomerate layer

Ripley Road Sandstone is the youngest exposed rock unit at the headland. This is actually part of the Clarence Moreton Basin which overlies the Ipswich Basin. If you go to the lookout you can see boulders of a pale grey colour. This is the Ripley Road Sandstone. It is mainly comprised of quartz sand lightly cemented together with a grey clay (known as a clay matrix) but occasionally some bands of conglomerate are present such as in the picture opposite.

On the geological map you will notice that the areas around the headland are comprised of different types of sediments these are all very recent which geologically places them at Quaternary (or more specifically Pleistocene to Holocene aged). This pretty much means that these sediments are actively changing and being deposited. Mainly sands in the beach and dune systems and silts and clays around the river estuary. Many of the Holocene aged sediments contain potential acid sulfate soils, which are common in the region but present several environmental management issues when disturbed. In the beach sands there are also commonly found heavy minerals which have from time to time being mined. But more about these heavy minerals some other time.

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 V16.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45
*Stephenson, A.E. , Burch, G.J. 2004. Preliminary Evaluation of the Petroleum Potential of Australia's Central East Margin. Geoscience Australia. Record 2004/06.

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.

'Recent' rhyolite: The Nimbin Rhyolite at Minyon Falls

The rhyolite forms a rugged range around the valley

If you are familiar with the northern rivers you would be aware of grand waterfalls in Nightcap National Park. The grandest (in my opinion) are the Minyon Falls which drop Repentance Creek around 100metres into the gorge below. I remember when you used to be able to stand at the very top and jump over the streams to cross but the National Parks and Wildlife Service of N.S.W. have stopped access (for obvious safety reasons) except at a constructed viewing platform.

Minyon Falls are spectacular. Geologically they represent thick units of rhyolite known as the Nimbin Rhyolite erupted during the later phases of the tweed volcano during the period known as the Cenozoic which was centred on the nearby Mount Warning. Underlying the rhyolite is basalt and andesite of the Lismore Basalt which appears to be from the earlier main phase of eruption from the volcano. At Minyon Falls the Nimbin Rhyolite is greater in thickness than the height of the falls themselves. It mainly shows massive units of rhyolite lava inter-collated with units of volcanic glass (obsidian) darker, but still of similar composition to the rhyolite.

Rhyolite is the volcanic equivalent of granite (which forms underground). It is fine grained due to quick cooling due to its volcanic nature which stops crystals from becoming very large. Rhyolite is silica rich which means that minerals like quartz and feldspar are abundant and other minerals such as olivine that is commonly be present in some of the basalts nearby are absent. The high silica content makes the lava thick and viscus and therefore gas bubbles are commonly trapped in the lava and banding of the lava flows becomes more frequently observed. The composition of rhyolite often leads to violent eruptions which are represented by ash and volcanic glass which can form thick layers themselves (some of these glass layers are present at Minyon Falls too).

If you are fit enough for a big walk at the base of the Minyon Falls are unusual structures which show how viscus the lava can be. Brittle-ductile structures are evident to the trained eye in this area. Smith (1996) identified these as essentially these are structures which show that when the lava was flowing the lava had become almost solid with many small faults mixed in with folding and flow banding of the lava.

Minyon Falls with the rhyolite cliff visible

Fresh rhyolite lava is a hard, erosion resistant rock and for this reason is why we have rugged ranges surrounding the central core of the Tweed Volcano at Mount Warning. The highest portions of the volcano including the rhyolite have been eroded away from the area now occupied by the Tweed Valley. Most of the volcanic rock in the valley has been eroded right down to the much older Paleozoic aged rocks of the Neranleigh Fernvale Group. The creeks that start in the ranges such as Repentance Creek have slowly cut back the face of the rhyolite cliffs as the velocity and power of the waterfalls slowly breaks grains from the rocks and creates cracks that break off in large rock falls.

Are you in Northern Rivers? It might be worth climbing Mount Warning to see the shape of the Tweed Valley and the remnants of the shield volcano in the cliffs seen all around the edge of the valley. Or maybe a trip into the Nightcap Ranges to Minyon Falls. Have a look at rocky creek beds to see exposed rock and many structures.

Note: There are two large areas of rhyolite in the Northern Rivers. These are the Nimbin Rhyolite of Cenozoic age discussed in this post but there is also rhyolites within the Chillingham Volcanics which are much older and are probably the basal units of the Mesozoic Ipswich Basin.

References/Bibliography:

*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. 1996.Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia. Journal of Volcanology and Geothermal Research V72
*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.

How big was the Mount Warning Volcano?

Mount Warning looking over the Nightcap Ranges

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

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

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

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

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

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

Can you put it all together?

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

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

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


References/Bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
* Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Ferrett, R. Australia's Volcanoes. New Holland Publishers 2005.
*Learned Australasian Volcanology Association, 1998. Lava News, December 1998.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45