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:
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.
