Monday, 10 June 2013

How wonderfully marbleous!

There are some rock types that are very common around the country and around the world that just don’t seem to rate much of a mention in the Northern Rivers. One very common rock is limestone formed from corals in a shallow sea, just like the Great Barrier Reef. Limestone is made almost entirely of the mineral calcite. Some parts of the world have vast terrains dominated by limestone called karst landscapes and it is quite distinctive. Limestone terrains sometimes form amazing subterranean cave systems as the stone is dissolved by rainwater infiltration into the formation. These karst terrains include north-west Mexico and other parts of North America, a giant band through northern England and a wide area of South Australia along the Great Australian Bight. However, it is a landscape absent from the Northern Rivers.

Outcrop of limestone north west of Tabulam
Having said that vast areas of limestone don’t exist in the region it is worth noting that they do exist in small areas here and there within the older rocks of the New England Orogen. The reason for this is interesting. The New England Orogeny was a period of mountain building during periods of plate collision which included a period of subduction of an oceanic plate under the Australian continental landmass during the Silurian period. The material on the surface of the oceanic plate was often accreted, that is scraped off and squashed onto the Australian continent. Seamounts are old islands in the middle of the sea. Such as, those around modern day Hawaii or Fiji. The seamounts were accreted onto the continental mass where they created little pockets of limestone in midst of the jumbled, squashed mass of deep seafloor sediments.

This means that if you find limestone in the New England area you are actually finding the preserved remnants of a little tropical island reef or lagoon. An especially nice thought, when you find some limestone on a cold frosty New England winter morning. One relatively accessible place to see some limestone is an old quarry on the Pretty Gully Road just north-west of the town of Tabulam which sits on the Bruxner Highway crossing of the Clarence River. The stratigraphic unit that the limestone of the area is part is the Emu Creek Formation which also includes areas of interesting fossils (more about that in yet another post). However, the quarry is interesting for more reasons than just as an occurrence of limestone.

Following the period of subduction and accretion a period occurred where intrusions of molten magma pushed their way into the accretionary sedimentary rocks. It occurred a couple of times including during the Late Permian to Early Triassic and created one part of what is referred to as the New England Batholith. The batholith is an array of granitic rocks that stretches through the whole New England Tablelands. The intrusions of the Late
fresh face of limestone - note the sparkles from the calcite crystals
Permian to Early Triassic included the emplacement of the Bruxner Monzogranite, a type of granite pluton (more about this specific rock in a future post). This pluton heated up and metamorphosed the rocks around it and one of which was that body of limestone near Tabulam. Contact metamorphism of limestone creates the rock called marble and this has happened at Tabulam. Although, the quality of marble is questionable because of the amount of impurities.

Other things happened to the limestone during metamorphism too. The transfer of fluids into and out of the cooling magma created chemical reactions which concentrated elements such as iron. This process develops what is called a skarn, a body of altered limestone with sometimes economic amounts of minerals. The minerals in a skarn can be diverse and very, very valuable but the minerals are based on the chemistry of the granite pluton. In the case of the chemistry of the Bruxner Monzogranite, there was not much of value except lots of iron which formed abundant amounts of the minerals magnetite and haematite. This has been considered for mining in the past but the small size and low grade means it is not a viable iron mine.

There are other small limestone deposits all around the New England and all of them are interesting for one reason or another. Some north of Inverell have lovely caves, others near Tamworth are mined for lime on a large scale. While others, just have interesting little features that illustrate what happened during the formation of our region.


*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. v38.

*Lishmund, S.R., Dawood, A.D. & Langley, W.V. 1986. The Limestone Deposits of New South Wales. 2nd Ed. Geological Survey of New South Wales

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