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The Great Artesian Basin: more than the eye can sea


 
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Hydrogeologist, John Polglase, digs into the many layers of the Great Artesian Basin.     
The sedimentary rock formations and strata that make up the Great Artesian Basin, and which are home to the groundwater, ore bodies, coal and petroleum products mined by humans, are essentially flawed. Not only were many different sedimentary materials laid down over millions of years by different erosion and weathering events, but also, they have subsequently been affected by temperature, pressure, fluids and tectonic plate activity. As a result they are not as cohesive, continuous or as horizontal as commonly depicted. (ie the sponge cake diagrams of sedimentary layers). These diagrams of horizontal 'subterranean layers' are only drawn to show simple concepts, and are not a reflection of reality - and certainly not of any local 3D reality beneath our feet.

 
The terms 'clay', 'silt' and 'sand' (from which we get 'mudrock', 'siltstone' and 'sandstone') actually define the rough diameter of the particles or grains transported and sorted by gravity, wind and water.    
Various minerals may comprise the grains.
'Clay' particles or colloids are <0.002mm in diameter;
'Silt' particles are between 0.002 - 0.02mm; 
'Sand' particles, 0.2 - 2.0mm in diameter.
The different size and mixes of particles not only makes for rocks of variable strength and weakness, porosity and permeability, it also makes for planes of weakness between them.
Even then, not all rocks of any one particle size are alike. For example, a sandstone can be 'immature' - less coherent and weaker than a 'mature' sandstone. And although buried, dewatered and cemented into a rock, the minerals that make up the grains of the sandstone may continue to experience mechanical (eg. tectonic plate activity) and chemical (eg. hydrothermal fluids) attack.

Left: bed of rock joints

 
Indeed, there is a whole cycle of 'weathering' that has been going on for hundreds of millions of years upon and within the Earth’s crust. For instance, deeply buried mudrocks and sandstones may be recrystallised and converted into granite by heat and pressure. Since this hot rock will have a lower density than the cold, surrounding country rock, the granite pluton will rise up through that material to ultimately be exposed on an eroded surface, and surficial mechanical and chemical weathering will again resume, eventually turning the granite back into sandstone and mudrocks.
Mudrocks are also 'flawed' in that they have vertical and horizontal cracks in them - even though there has been no rock movement. These cracks are called joints (see above left). Joints are formed as the mud is buried, dewatered and turned into mudrock. Mudrock jointing can be seen in road-cuttings, mine and quarry walls, sea headlands and rock platforms. The loading and unloading (ie erosion) of sedimentary material above the buried mudrock, causes load and relaxation cracks to form in the mudrock. In addition, if a brittle mudrock is gently folded by basin tension (ie compression), further cracks and fissures may form. Ultimately, tectonic tension or extension (ie stretching) may cause catastrophic breakages or fault swarms. Over time, all these mechanical weaknesses can open or close, and when open, are 'highways' for fluids and gases under pressure.

 
The Great Artesian Basin is a loosely-connected series of sub-basins, troughs and embayments: separated by different basement highs, and formed and filled at different times and in different tectonic contexts over >100 million years.     


Beneath the surface there may be several sedimentary basins on top of each other, with each filled and completed in different gelogical periods.
For example, the Jurassic-Cretaceous Surat Basin, which is a major member of the Great Artesian Basin, overlies the Permian Gunnedah Basin and the 'living' Murray Darling Basin sits atop the Surat Basin.

 
The GAB is not a homogenous and uniform structure. With respect to GAB groundwater, bores drilled into the GAB can go down to depths of 1½ km, and pass through seven or more underground streams on the way.
Each stream has a different history and hydrogeochemical expression.
While relatively shallow groundwater is typically relatively fast moving and high in bicarbonates, the further and deeper it travels, the slower it gets and the carbonates precipitate out, leaving a sulphate dominated water. Deeper and further still, the groundwater becomes dominated by sodium chloride and is virtually stagnant, moving less than 100cm a year.
Even deeper and older still, the water is so salty it may be saltier than seawater (>35,000 ppm of dissolved solids/salts) and is a brine. In addition, at such depths, the hot, salty, hydrothermal fluid may be the reactive agent for the slow formation of ore bodies, deep within the sedimentary basin pile.
Typically, all subsequent deep mining in a sedimentary basin involves the management of salty groundwater.

 

 
The extent of the Great Artesian Basin is not generally appreciated, stretching far beyond the interior of Australia.    
It extends via the continent shelf under the Arafura Sea to New Guinea as the Carpentaria and Papuan Basins. The continental shelf under The Great Australian Bight has a series of sub-basins that are part of the GAB (eg the Recherche, Eyre and Ceduna sub-basins).
It is vast both in extent and time (>100million years) making its geology and groundwater incredibly diverse. The Great Artesian Basin is thus in reality far removed from many of the popular and simplistic characterisations we hear and read of today.
It is literally is more than the eye can see!

 
John Polglase was interviewed for A Question of Balance by Ruby Vincent. Images and diagrams provided by John Polglase, Summary text from Victor Barry, October 2011

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