Eidsvold Earthquake 2015

I woke this morning to the news that a town to the west of Bundaberg had experienced a substantial earthquake. Well, substantial by Australian standards anyway. Geoscience Australia gives the intensity of 5.2 on the Richter scale. The quake occurred at about 2am local time (3am for those of us in the other eastern states coping with daylight saving).

The preliminary report from Geoscience Australia can be found here.

Seismograph from Eidsvold Station

It is in an interesting area because the area of the earthquake is in the northern part of the New England Orogen. This belt of squashed rocks extends from the Bundaberg area in a big arc all the way to Port Macquarie in the South. There are many faults in this area and some are still active, although they are generally small. An earthquake between Gunnedah and Tamworth in 2013 springs to mind.

The scale of the earthquake is quite large for Australia. Indeed the Newcastle earthquake was measured at 5.6. I've not done the maths but the new Eidsvold quake of 5.2 is about half the size of the Newcastle one (The Richter Scale is NOT linear).

Historically, the area is prone to small to medium sized earthquakes with Bundaberg being hit by a size 6.0 in 1918. This is nearly ten times more powerful than the most recent one though the 1918 quake occurred just off the coast.


Oh... and humour starts quickly:


https://twitter.com/iampatwilliams/status/567040767172952065/photo/1

A magma chamber under Cabarita Beach

Again and again, I am amazed at how little we know about what is under our feet. It often takes an unexpected source of information to reveal some incredible knowledge of our region. The lastest information that has recently come to hand has been the preliminary geophysical survey results for the Grafton to Tenterfield survey. There are many results that may indicate some strange goings on, from some inconsistent features in the Mount Warning area (possibly indicating that the Tweed Shield Volcano might actually be a myth! More of this in a future post or two), to strange lineaments and responses showing hidden intrusions. This post is about just such a possible hidden intrusion in the Cabarita area.

Smith (1999), curiously reported that within the Neranleigh-Fernvale Beds at Norries Head, Cabarita (located on the coast midway between Tweed Heads and Mullumbimby) there appeared to be evidence of thermal metamorphism in the rocks there, but no evidence of what caused the heating. Metamorphism is a characteristic of the Neranleigh-Fernvale Beds, but the style of metamorphism is pressure related due to the formation being accreted (squashed) onto the Australian continent during a period of subduction during the Palaeozoic period. Not much heat was generated in this formation and based on the minerals identified in the rocks it is possible to estimate the pressure and temperature when these rocks were squashed. The feature that Smith (1999) identified was biotite crystallisation (a variety of the mica mineral group). This mineral is indicative of heating of rocks to a medium to high grade but the lack of a preferred orientation of this platy shaped mineral shows us that the metamorphism postdates the accretion period. ie. the heating of the rock has occurred some time after the pressure, meaning at least two periods of metamorphism.

As discussed in a previous post, the New South Wales Geological Survey has been collecting geophysical data over the region. One measurement has been the intensity of magnetism (related to the iron content of rocks). Magnetic results can display what is happening under the earths surface, not just on top. It is known to show a characteristic feature where intrusions are known, either a strong negative or strong positive anomaly, depending on the rock type. The picture to the left shows the total magnetic intensity map (courtesy of the 2012 preliminary data package from the geological survey) for the area around Cabarita. I’m sure you can pick out the obvious red and blue anomaly. the pattern is consistent with intrusions, indeed exactly the same feature can be seen in the Mount Warning area (and others that I will discuss in future). As such, I suggest that this anomaly is actually good evidence of an intrusion hidden below the heat affected surface rocks. Smith (1999) thinks that the biotite grade metamorphism occurred during the Mesozoic period (well before the Cenozoic aged Lamington Volcanics) and that there was once a body of molten rock below the ground in this area.

I’m so pleased to be able to see the preliminary dataset, it is obvious that there are many features that can be better understood.


References/bibliography:

*Smith, J.V. 1999. Structure of the Beenleigh Block, northeastern New South Wales. New England Orogen: Regional Geology, Tectonics and Metallogenesis. Papers presented at a conference at the University of New England.
*Geological Survey of New South Wales. 2012. Grafton Tenterfield Airborne Geophysical Survey: Gridded and imagery data. Preliminary package from the Department of Trade and Investment: Resources and Energy.

Geology in the air

A few months ago the New South Wales Geological Survey released their preliminary data package for the Grafton-Tenterfield Airborne Geophysical Survey (link to info here). The survey was conducted as part of a program that has been going on for several years known as New Frontiers New South Wales. Nearly all of the state has now had high quality data obtained for it instead of the old, poor quality data for the whole state. But why has this been done?

Area of latest survey from NSW DPI

The regional survey has been conducted to better understand the geology of the region and as a result this information may guide mineral exploration as well as provide additional data that can help land management especially in relation to soil transport, erosion potential and soil chemistry. This is done through three primary measurements: radiometrics (measurement of natural radiation), magnetics (measurement of how magnetic the earth is), and DEM (measurement of the elevation of the earth). How these can be used to understand the earth through the aerial survey is outlined below:

Radiometric measurements show the relative abundance of three naturally occurring elements in the rock and soil these are Potassium, Thorium and Uranium. The different ratios of these elements can distinguish broadly between different rock types without the need to visit the site and the intensity can be used to do the same. If an area shows up as being rich in radioactive elements it may be underlain by a granite or similar rock as the radioactivity is directly related to the mineralogy of the parent rocks. If an area is eroded the visibility (or comparative lack of) of radioactive minerals may be seen in downstream. The different types of radiation also provides an indication of the chemistry of the soil and rock. This process can only show the radiation on the surface (e.g. in soils, sediment and exposed rock)

Magnetic measurements do a similar thing as far as identifying elements. But instead of radioactive elements it shows the abundance of iron especially in the form mineral magnetite. Again, the geophysical response will be based on the rock mineralogy. Rocks such as basalt will have a strong response (more iron) and so will wet areas. However, unlike radiometrics, magnetics can show deeper structures in the earth such as buried plutons, faults, dykes and others. This means that drilling is not necessarily required to have a good idea of what lies beneath the surface. It is interesting stuff and the maps produced are often very pretty colours! All of the measurements require “calibration” with a digital elevation model (DEM) which shows the shape of the earth.

Recently (2005?) obtained information in the New England and Tamworth areas have shown up major hidden fault systems, areas prospective for gold, deformation of sedimentary basins that may increase prospectivity for oil and gas, new granite plutons, hidden granites, a better understanding of volcanism including confirmation of new volcanic centres and its associated rock (such as the Maybole Volcano). One aspect that appears promising that the survey will aid in determining the viability for is geothermal energy. Geothermal energy may be obtained from related to deep buried granites and magnetics can be used to find likely locations.

It is interesting to note that I have seen reports in the media (and formal submissions to Coal Seam Gas parliamentary inquiries) that some people have been upset by being “buzzed” by aeroplanes from companies undertaking coal-seam gas exploration. Looking at the timing, this may be related to concern about Coal Seam Gas exploration by several companies in the area and I tend to think that it is likely that the aeroplane may have been part of the regional geophysical survey.

It is important to know just how useful this information is to understanding our planet, even if the information can be used in a way that some do not appreciate. Geology usually has two aspects, one is scientific (understanding the world we live) and the other is applied (using such knowledge for other purposes such as mining). The geophysical information obtained by New Frontiers New South Wales is purely scientific but this knowledge can be applied to Mining but it can also be applied for Environmental purposes. Unlike many others, I am cautious about stopping research because of what it may be potentially be able to be applied to.

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.

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.