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.

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. 

5 comments:

  1. Thanks for creating this fascinating blog. I am wondering whether I can find out what the ground is like underneath my Coraki house. A friend at the southern end of town has built on a sandstone rise, but I really don't know how deep the mud is here at the far northern end, near the bridge. Is there somewhere on line I can find out this information? Many thanks. J

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    1. Hi Kehla,

      Sorry for taking so long to reply to your question.

      The short answer is no, I'm not sure where you can find a definitive answer to this question. But it maybe worth doing a search for "Groundwater Works" which is a free state government mapping program for ground water bores. Find the bores that are closest to where you are and hopefully they will have descriptions of what the material the bores are installed into are.

      My broad understanding is that there is often more than 30 metres of alluvium including clay in the Richmond River valley. At Coraki this overlies sandstone called the Kangaroo Creek Sandstone or Grafton Formation. The hill you mention is made from Kangaroo Creek Sandstone I think.

      Hope this helps? Sorry I can't give you a more accurate answer.

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  2. hi my name is david i was wondering why coraki is so dusty inside houses must be bad for your lungs i can see particles floating in the air.i was told the council quarry causes this?thanks for your time DAVID.

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    1. Hi David,

      The short answer is possibly. The long answer is probably not much dust is from the quarry.

      Quarries are sources of dust and this can be an issue - but you'll have to check with the local council about their monitoring. They will have limits on the amount of dust allowed from the site and are usually very strict about controlling it. Most dust is from wind blowing across the landscape - whether there is a quarry or not (ANY exposed dirt can be a source of dust). This is a diffuse source and simply blows dust indoors unless it is raining. Alternatively, the sugar cane farms in the area often burn the cane and this can place soot into the atmosphere causing dust issues. Vehicle traffic can also add to the issue (though probably not much in Coraki), even a neighbour mowing the lawn on dryer days can be a big dust source for a neighbour!

      However, with regards to health - dust from any source can be an issue to people with respiratory problems. Though it is nearly impossible to completely remove dust from any environment.

      Hope this answers your question somewhat.

      Cheers,

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  3. hi not sure if my message went through why is coraki so dusty indoors i can see particles floating is council quarry to blame cant be very healthy for lungs.thank you david

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