Geologists are Cool and Sexy

I saw this link on the Amphibol blog, a blog in german but the clip is in English.

NOTE: The clip has been removed from Youtube, I'll try and find another source.



I couldn't agree more. It is funny how cartoons can speak the truth so well!

Happy Christmas all. Remember why we have the day.

Coraki has its faults

Coraki is a nice little town on the Richmond River just near its confluence with the Wilsons River. The town is located on the flood plain and therefore many parts of it can be inundated in the case of major floods. The flood plain provides a relatively fertile plain that grows excellent pastures and much sugar cane, especially the further down stream on the Richmond you go. But Coraki has its hidden faults.

Being an active flood plain the area surrounding Coraki is dominated by recent alluvial deposits generally of Holocene age but with lots of slightly older Pleistocene alluvial and estuarine sedimentary deposits. Areas that are under permanent shallow unconfined ground water influence tends to retain pyrite which is produced by bacteria in an anaerobic (oxygen poor) environment (i.e. under stagnant water). When this pyrite is exposed to the atmosphere or more oxygenated water by the action of drainage for agricultural, construction or flood mitigation purposes the pyrite oxidises. Pyrite is Iron Sulphide (Fe₂S) which with water (H₂O) forms H₂SO₄ which is more well known as sulphuric acid. This acid can then be discharged causing degradation to aquatic life or degradation of land creating unproductive acid scalds.

Not all of the town is in the flood plain, in fact about half is located on some low hills that are comprised of Kangaroo Creek Sandstone. The Kangaroo Creek Sandstone is part of the Clarence Moreton Basin and its exposure here may be partly due to a fault called the Coraki Fault. In the area of Coraki and also at Tullymorgan and maybe even places like Clifden near Grafton the faulting of the Coraki Fault has created some unusual features within the Mesozoic Clarence Morton Basin and the underlying Palaeozoic basement rocks. These features cannot be seen on the Earths surface but can only be identified by geophysical techniques, in particular seismic surveys.

So, what are the features that can’t be seen? Well, there is the Coraki fault itself which is a dextral strike-slip fault meaning that the eastern side of the fault has moved northwards relative to the western side. But there is also a weird structure which is referred to as a “flower structure”. This occurs when another fault is present perpendicular to the main fault. This creates a central wedge shaped block which near Coraki has been squeezed by the faults upward and created here, slightly more elevation in the Kangaroo Creek Sandstone and possibly other units of the Clarence Morton Basin. This is probably hard to visualise, so maybe a diagram will help when I can get one to embed.

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.

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

References/Bibliography:

*O’Brien, P.E., Korsch, R.J., Wells, A.T., Sexton, M.J. Wake-Dyster, K. Structure and Tectonics of the Clarence-Morton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241. 

From deep within the earth lies Baryulgil

Deep within the earth below the seas (so deep in fact we begin to enter the Earths upper mantle) we find material that is solid but so hot that it is viscous. This material is very low in quartz and when we see this rock on the surface it is unusual. The only way for such rock to come to the surface is through great wedges being thrust on to the edges of continents as the great oceanic plates move on the mantle. The upper units of rock from oceanic plates is greywacke from turbidites from collapsing continental shelves or the pelagic sediment accumulated over vast periods of time. But also you will find volcanic rocks erupted under the water at mid-ocean ridges and below these great thicknesses of basalt cooled into columns and even further below these great plutons of the mafic rock called gabbro which is the source of the basalt on the surface. Yet even deeper we start transitioning into the mantle and here we find rock that contains very little silica (ultramafic rocks) but is rich instead in iron and magnesium. These are called peridotites and dunites when found in rock form. From top to bottom the section is called an ophiolite sequence and these occur infrequently on the earths surface.

Given that the highlands of the New England region are derived from accretionary material scrapped off the sea floor during collision with the Australian Plate we have a good chance to find some. And we are in luck. I know of three significant areas in this region where ophiolite is preserved the two biggest are located north of Tamworth along the peel fault and at Port Macquarie. A smaller area can be found north-west of Grafton at the little village of Baryulgil, located midway between Tabulam and Copmanhurst.

Sepentinite from a location south of Baryulgil, the host rock for the asbestos

The ophiolite at Baryulgil is unusual because only a portion of the ophiolite is preserved, this being the peridotite and dunite altered to a rock called serpentinite and a small area of gabbro. It is also worthy of note because of the damage such a rock has caused the local people. The serpentinite at Baryulgil is known as the Gordonbrook Serpentinite and includes such serpentine minerals as chrysotile – better known as a mineral of the asbestos group. Mining of this industrial mineral by Australian Asbestos and later by James Hardie occurred at Baryulgil for quite some time and it is this that has caused many problems.

Stepping slightly into the area of politics and aboriginal relations (and then quickly away again) the Baryulgil asbestos mine was often held as a wonderful example of how an indigenous population could be assimilated into the good things of western culture. Alas, as we know too well today that model of assimilation was flawed, in part in the case of Baryulgil because of the harm to its workers from such a carcinogenic material. Reportedly the mine and its processing plant had an appalling reputation for dust which is the main mechanism that causes the entry into the body and the subsequent long term damage including a massive increase in the risk of cancer. As an aside, it is worth noting that even the Nazi party in Germany before the Second World War (and greater than 40 years before the closure of the Baryulgil mine) introduced regulations to ensure that dust was minimised when working with asbestos because of the probable heath effects.

The Gordonbrook Serpentinite is a body approximately 25km long elongated unit right on the edge of the New England Fold Belt accretionary terrain. Geophysical surveys including gravity and magnetics indicate that the unit probably much larger than the area exposed as it appears to underlie the Clarence Morton basin just to the east of Baryulgil. The unit shows a gravity anomaly given its composition from heavy minerals and the magnetic signature shows up because of the richness of iron when compared to the more recent Jurassic aged sediments (Laytons Range Conglomerate and Gatton Sandstone) of the Clarence Moreton Basin and the accretionary complex meta-sediments to the west.

The gabbro unit of the ophiolite sequence is present as a small remnant unit on the north western most part of the serpentinite body on the northern side of the Clarence River. Interestingly the Clarence River pretty much runs straight though the middle of the serpentinite as it meanders from the mesozoic clarence moreton basin sediments into and out of the older accretionary terrain. This meandering has implications for indicating the history of the river development of the Clarence. But more about the Clarence River in another future post.

The minerals present in the serpentinite are mainly comprised of serpentine (a type called antigorite) but there is asbestos (chrysotile) occurring naturally in vein systems. Altered serpentinite also locally forms magnesite which is a white chalk like mineral formed through the affects of carbon dioxide rich ground water. The nature of the serpentinite and ground water alteration and reposition of secondary minerals is such that metals such as arsenic, and particularly nickel and cobalt are also quite rich in small patches. But these minerals are hard to come by unless intersected by cuttings or mine workings.

If you pass through that way to explore the more remote corners of our region take note of the roads. The councils that managed the area have previously maintained and unpgraded the roads with locally sourced rock. This means that the road base is often made from serpentinite. This has caused made road management problematic because the current Clarence Valley Council to minimise the risk of exposure to asbestos when staff or contractors are maintaining the roads!

Another feature of the Baryulgil Serpentinite is that it helps to demonstrate a theory about a major period of deformation in Eastern Australia. This formed tectonic features called the Coffs Harbour Orocline and the Texas Orocline, but there is too much to discuss about this now so I will have to dedicate a post about this in the future.

References/bibliography:

*Cornwell, J 2004 Hitlers Scientists: Science, War and the Devil's Pact. Penguin Books
*Henley, H.F. , Brown, R.E. , Brownlow, J.W. , Barnes, R.G. , Stroud, W.J. 2001 Grafton-Maclean 1:250 000 Metallogenic Map SH/56-6 and SH/56-7: Metallogenic Study and Mineral Deposit Data Sheets Geological Survey of New South Wales.
*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.

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.

Looking out from the lookout at Point Lookout

The view of the National Park from Point Lookout

One of my favourite places is Point Lookout at the New England National Park between Ebor and Dorrigo. Point Lookout is spectacular for it scenery and feel. On most days you can see the pacific ocean while looking over rugged hills and valleys and I particularly like going there during winter where icicles hang from trees and the waterfalls below the peak are frozen. Point Lookout is nearly 1560metres high which I understand makes it the highest point in northern New South Wales. Like the beauty of the Mount Warning area and Tweed and Brunswick River region, Point lookout owes its attractiveness to the erosion of a large shield volcano.

Point lookout is located on the rim of an escarpment which formed through the erosion of the Cenozoic aged (in this case 19-18 million years) Ebor Volcano and the much older Devonian to Carboniferous (up to ~416Ma) accretionary complex rocks that make up the balance of the New England tablelands. Today, only the north western portions of the lavas (called the Ebor Volcanics) and the central weathered volcanic plug from the Ebor Volcano remain. Research by Ollier (1982) suggested that the central volcanic plug of Ebor Volcano was centred on what is called the crescent which is actually a fairly insignificant looking feature when compared with the rugged valleys today.

It is interesting to note that even though the nearby 23 Million year old Mount Warning (located near and over the Queensland border) is regarded as one of the biggest shield volcanoes in the southern hemisphere, having a height of around 2000 metres before it was eroded, the Ebor volcano was probably a similar size or bigger at its greatest too. It is a bit of a mystery why so little is left of Ebor Volcano when so much remains of the Tweed Volcano/Mount Warning.

The Crescent complex once thought to be Permian (290Ma-250Ma) as recently at the 1970's and was considered part of the intrusives that constitute the New England Batholith. In fact most of the most 'current' geological maps of the area were drawn at this time and so they are incorrect. But since investigations on the radial drainage patterns and geological features by Ollier in the late 1980s followed by dating by Gleadow and Ollier (1987) (which is difficult due to how weathered the Crescent is) and more recent work by Ashley et al (1995) now it is known to be the centre of the Ebor Volcano and aged around 19 Million Years. Ashley et al (1995) also discovered that a nearby basalt called the Doughboy Basalt was around 46 Million years old which is clearly not related to the Ebor volcano but is consistent with other locations where an older Cenozoic basalt is present before the hot spot volcanism that formed the Ebor, Mount Warning and other volcanoes existed.

When I was last at Point Lookout there were several bush walks from long and difficult to short and easy. The most difficult ones take you into the valleys where the rock has been eroded into the older accretionary complex. But even on the short one you can see some interesting 'recent' volcanic rocks. On a section of the walk around the top of the cliffs where security fences are necessary (lest you plummet away!) there is cuttings through the rock. In this rock look closely and you'll see some big crystals within a fine groundmass. This rock is a type of basalt called tholeiite (which means that it has crystalised with a certain geochemical signature) and the crystals are feldspars which is a common rock forming mineral. The feldspars here quite obvious and seem to catch the light at two angles, this feature is called twinning and is characteristic of the calcium rich variety of feldspar called plagioclase. Along the bigger walks below the point dacite can be found as well as basaltic and dacitic breccias at the stunningly beautiful during winter, weeping rock and numerous palaeosols.

The remnant of the shield volcano shows the characteristic radial drainage pattern for volcanic shields but the eroded central areas of the volcano (including the caldera if there was one) drains fairly directly to the east via the Nambucca River. The radially draining creeks and rivers are well known for their waterfalls such as Dangar Falls and Ebor Falls.

The road from Dorrigo to Armidale is not a busy route, it is often missed by many people but I always recommend people visit the New England tablelands because of its beauty and uniqueness in Australia. Point lookout is just off the Waterfall Way which name probably gives you an indication of many of the other attractions. In my opinion, the depths of winter are the best times to visit to get the mood and subtle beauty of the area. I should get back there myself... it has been too long since I was last there.

You may be interested in a self-guided geological tour. Bob and Nancy from Armidale have a wonderful site which includes an excellent (and expanding) range of geological tours including ones of the Northern Rivers Area. Their tour guide on Point Lookout can be accessed from their webpage (very much worth the look) or  directly linked from here.   

References/bibliography:

*Ashley, P.M., Duncan, R.A. & Freebrey, C.A. 1995 Ebor Volcano and the Crescent Complex, northeastern New South Wales: age and geological development. Australian Journal of Earth Sciences V42.
*Gleadow, A.J.W. & Ollier C.D. 1987 The age of gabbro at the Crescent, New South Wales. Australian Journal of Earth Sciences V34.
*Ollier, C.D. 1982 Geomorphology and tectonics of the Dorrigo Plateau, NSW. Australian Journal of Earth Sciences V29.

To the geologist a road cutting is a tourist attraction!

I dont know where this came from as it was sent to me in an email but it is so good I had to reproduce it... well good as far as geological humour goes!

You know if you are a geologist if:

• You can pronounce the word "molybdenite" correctly on the first try.
• You think the primary function of road cuttings is for a tourist attraction.
• You associate the word "hard" with a value on Moh's scale instead of "work".
• The pile of rocks in your garage is taller than you are.
• you have a strong opinion as to whether pieces of concrete are properly called "rocks".
• The local university's geology department requests permission to hold a field trip in your back yard.
• There's amethyst in your aquarium.
• Your wife has asked you to move flats of rocks out of the tub so she could take a bath.
• You spellchecker has a vocabulary that includes the words "polymorph" and "pseudomorph".
• You think Rocky, Jewel and Beryl are good names for your children.
• You were the only member of the group who spent their time looking at cathedral walls through your hand lens during your last trip to Europe.
• Work wont give you time off to attend the national gem and mineral show and you go anyway.
• You begin fussing because the light strips you installed on your bookshelf is not full spectrum.
• You've purchased an individual, unfaceted rock, regardless of the price.
• You've ever spent more than $50 on a book about rocks.
• You shouted "Obsidian!" while watching "The Shawshank Redemption".
• You find yourself compelled to examine individual rocks in driveway gravel.
• The Geological Survey identifies your rock collection as a major contributing factor to isostasy in your state.
• You know the location of every rock shop within 2 hours drive of your home and when they haven't seen you for a week the shop owners send you get well cards.
• You have retired but are still thinking you need another room on your house for your collection.
• You get annoyed when people think you are talking about petroleum when you are discussing matters about petrology.
• Your idea of a "quiet romantic evening at home" involves blue mineral tack and thumbnail boxes.
• You plan to use a pick and shovel while you are on holiday.
• You can point out were Tsumeb is on a world globe.
• You associate the word "saw" with diamonds instead of "wood".
• You consider a microscope useless unless it has polarising lenses and an accessory plate.
• You begin wondering what a complete set of the Mineralogical Record is worth and when you find out you actually consider paying for it.
• you've installed more than one mineralogical database program on your computer.
• You throw out clothes instead of rocks to keep the weight of your baggage down before checking into your flight.
• You receive a letter from the local council informing you that you will need a landfill permit if you place any more rocks on your property.
• Your internet home page as pictures of your rocks.
• There's a copy of Dana's Manual of Mineralogy next to your toilet.
• You still think that pet rocks are a pretty neat idea.
• You get excited when you discover a hardware store that stocks 16 pound sledge hammers.
• You debate for months on the internet concerning the relative advantages and drawbacks of vibratory versus drum tumblers.
• Your employer has requested you don't bring any more rocks into the office.
• You really want to keep the rock on your wife's wedding ring.
• You know that the word "aa" does not refer to the alcoholic support group.

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.

A rock forming mineral: Olivine

Everyone has heard of the very common mineral called quartz, most people have heard of the very common mineral called feldspar, but surprisingly few people have heard of the very common mineral called olivine. I speculate that this is for two reasons. one being that quartz is resistant to weathering and is very easy to find, feldspar often occurs in big crystals and is also somewhat resistant to weathering, whereas olivine quickly breaks down into clay and occurs in mafic (quartz poor) rocks. the second being that it is often only obvious as large crystals in some basaltic rock.

But firstly olivine is made from similar components as most of the other common minerals. In particular it is comprised of silica with either/or some magnesium (Mg2SiO4), known as forsterite or iron (Fe2SiO4), known as fayalite. Its chemical formula is often given as ((Mg,Fe)2SiO4) because the magnesium or iron can substitute for each other and are usually present together. Because of the nature of the chemical bonds between the magnesium, iron and the silica group the mineral weathers quite rapidly (geologically speaking). Forsterite (mg rich) tends to be an olive green colour and because of the iron content fayalite is more browny-green.

Bowens Reaction Series from Encyclopedia of Earth

Olivine is crystalised in volcanic rocks at high temperatures. This means that as a mafic (basalt like) magma chamber cools the first mineral to form into crystals is olivine (see figure opposite). This indirectly means that if you see olivine crystals in the field it is usually because the rock was a lava that was erupted relatively rapidly to the surface from deep in the earths crust or upper mantle. But, sometimes you can come across rocks that are almost entirely made from olivine. These rocks are called dunite. It is formed at the boundary between the crust and the mantle and has crystalised there. It is thought that it has been bought to the surface through the action of plate tectonics where sometimes large chunks of oceanic crust can be scraped onto a continental plate as the process of subduction takes place. This is called an ophiolite sequence.

A metamorphic source of olivine is through the contact metamorphism of dolomite limestones.

Particulars:

Chemical Formula: (Mg,Fe)2SiO4
Fracture:Conchoidal
Hardness (Moh): 6.5-7
Specific Gravity:
Colour: Olive Green (Forsterite) to Browny-Green (Fayalite)
Luster: Vitreous (glassy)
Crystallography: Orthorhombic
Gem: Peridot
Common accompanying Minerals: Not found with free quartz crystals. reguarly found with feldspar, pyroxene, augite

More information on olivine can be found one the Mineralogy Database.

Just a quick note on dunite and ophiolite sequences, this rock type is named after Dun Mountain in the northern part of the South Island of New Zealand. Dun Mountain is almost exclusively made from dunite and is part of a geological feature known as an ophiolite sequence which stretches along and off the Alpine Fault in New Zealand. Another ophiolite sequence is present in New Caledonia. Closer to home, the Peel Fault which runs along the western side of the New England Tablelands past Tamworth eventually to somewhere near Port Macquarie, also resembles an ophiolite sequence. I will discuss the Port Macquarie part of the Peel Fault at some time in the near future.

References/Bibliography:

*Klein, K. Hurlbut, K. Manual of Mineralogy (After Dana, J.D.). Wiley 21st Ed.
*Encyclopedia of Earth: www.eoearth.org

Radioactive paradise (slightly)

The areas of the Tweed Valley, Nightcap National Park and Byron Bay are often seen as fresh clean and natural. Well, I can argue that especially Byron Bay may be a little unnatural but certainly there is a feeling of 'freshness' with the rainforests and the beaches. Given this, few people would think that you'd get a bigger dose of radiation from living in these areas than you would in Brisbane or Sydney (even living near the Lucas Heights Reactor).

Few people realise that radiation occurs naturally in the environments in which we live. Yes, most of you would know that the Sun is a thermonuclear power station bombarding Earth with gamma radiation on a daily basis. But it is also a natural part of the earth and actions either natural or man made can result in these areas being elevated in radiation. In the cases below the sources are formed through different ways but all provide an increase in radiation sometimes thousands of times higher or more than what would be considered background.

Let us look at the little village of Uki first. This little place is located in the Tweed River valley and is known for its rainforest surroundings and rugged, scenic landscapes. Geologically some of the area around Uki is situated on mesozoic aged rhyolite of the Chillingham Volcanics and this rock type provides an added level of radiation due to the minerals that exist naturally in it. But even more interesting is that a mineral exploration company discovered a very tiny sized but significant anomaly in the radiation levels just south of the village. The source was not clear but sampling showed that a five square metre anomaly existed in the already slightly elevated rhyolite terrain background radiation. Analysis showed a nearly 0.05% concentration in uranium which is quite high. This is many thousands of times higher than the normal level expected. The reason for this anomaly remains unknown.

Byron Bay is located on the southern side of expansive active and historic beach systems. Much of the Byron Bay area (and much of the north coast itself) was subjected to heavy mineral mining up until the 1980's but this has ceased now. The heavy minerals sought after were mainly titanium rich ilmanite and rutile and there are other heavy minerals too such as zircon and monazite. These minerals were naturally enriched through the processes of wave and tidal action which created zones amongst the dunes that were targeted for mining. But many of the left over heavy mineral sands were not needed once the rutile and ilmanite were removed. So the left over mineral sand was discarded in some cases used as fill for future building sites. Little did people realise monazite rich left over sand would cause issues which may be unsafe for building homes on. This is because monazite is a radioactive mineral and when the residually enriched sands were dumped this increased the concentration of thorium and uranium and the associated radiation. In fact this situation didn't just occur at Byron Bay but all along the north coast.

More broadly, but less significantly many areas where rhyolite or granite is the underlying rock also have higher than normal background radiation. This too is because of radioactive minerals being enriched naturally when these sorts of magmas are being formed. So this would apply to areas in or close to the national parks of the nightcap ranges and many areas inland in the headwaters of the northern rivers such as the Clarence or Bellinger Rivers and large expanses of the New England tablelands.

References/bibliography:

 *Pechiney Resources (1970). Report on air and ground prospection, Clarence-Moreton Basin, EL 278, Nimbin - Murwillumbah area. Unpubl. Exploration Progress Report.

Coal seam gas gets a seismic thump!

Ok, time for me for foray into an area that is politically sensitive. But I hopefully do so factually.  I'm starting now because of recent developments by the Lismore City Council to first approve a Review of Environmental Factors (Which is strange since the Mineral Resources Division of the NSW Government approves these (or was it approval to use road reserves? I can't seem to figure out what authority the council has from the news reports)) and then to rescind this approval once they found that the work being undertaken could be used to target the area for further coal seam gas exploration (well... that is how I read it). Here are two newspaper reports that discuss the matter: Northern Star and Northern Rivers Echo.

So, I guess the centre of the matter is 'what is seismic exploration anyway?' Seismic exploration comes under the category of geophysics and in this case refers to the use of sound waves to try and understand what is below the ground. It is a non invasive method with the major environmental impacts believed to be limited to noise pollution and a small area of squashed grass. Practically one of the common processes used (and I understand this may have been the method outlined in the REF) is a truck with a pad under which will drop and hit 'thump' the ground. Sensors in the truck (or an accompanying vehicle) then receive data back from the ground as the vibrations made are reflected off layers of rock under the ground. The truck will then drive off a hundred or so metres further along and then do it again and so on.

The Department of Mineral Resources have previously conducted some of this work in the area since the 1970s. But I understand that Metgasco are currently undertaking more detailed work which may provide them with information that can lead to (or rule out) possible places to target exploration drilling. The Roads and Traffic Authority (RTA) frequently use this technique when planning routes for roads, the last time I heard this was between Byron Bay and Grafton about 6 months ago.

The decision by Lismore City Council is therefore one I find a little hard to understand unless it is simply a matter of the Councillors not understanding the technique itself or just deferring to the current political environment. Personally, I'd certainly love to get some more geophysical information on the Clarence Moreton Basin (for the sake of scientific knowledge itself) since I've recently had some discussions various people that prove that we know very little about faults, stratigraphy, intrusions, groundwater, deformation, metamorphic events, etc that have occurred in the basin since the rocks were laid down. Seismic exploration would go along way to answering some of these questions. But I guess if the politicians and general public don't want the information to be used specifically in coal seam gas exploration then it is important that we do not learn about these features in our region. That might be a price many in our region are prepared to undertake.

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.

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