Sunday 25 December 2011

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.

Thursday 22 December 2011

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.

*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. 

Wednesday 14 December 2011

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.


*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.

Friday 9 December 2011

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.


*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.

Saturday 3 December 2011

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.   


*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.