Thursday, 1 January 2015

The name of Paddy's Emu? At last a good answer

Paddy's flat is an area that many consider the middle of nowhere. It is not a well known area but it probably should be. There is a nearby place called Pretty Gully and this name gives a better indication of the Paddy's Flat area. It is some of the headwaters for the mighty Clarence River and includes major tributaries such as the Cataract River and Emu Creek. Researchers have returned to the Paddy's Flat area numerous times for more than a hundred years to try and resolve the tricky geology. But agreement on the geological relationships of the area has been mainly unreachable. However, one of the latest papers in the Australian Journal of Earth Sciences may have resolved many of those issues.

Gideon Rosenbaum and his team from the University of Queensland has been responsible for huge advances in geological knowledge in the Northern Rivers headwaters. The latest paper from Gideon Rosenbaum's team (Hoy et al. 2014) is another for which we should be thankful. The level of research by local universities is sadly very close to non-existent and one of the preeminent research universities has thankfully filled some of the gaps. But, I digress. What is so great about the Hoy et al (2014) paper?

The many ideas about the Stratigraphy of the Emu Creek Block.
from Hoy et al (2014)
There are many great things in Hoy et al (2014) but to me the biggest is something I've struggled with for a few years. It is how and when the area formed. It demonstrated that some of the rocks of the area probably formed in the same geological environment and time as those to the west of Tamworth. Hoy et al (2014) has resolved the three stratigraphic units of the Emu Creek Block. In doing so has demonstrated that the block was formed during the late Carboniferous period. This was when a great unit of subducting crust was sliding from the west under the New England region to the east. According to Hoy et al (2014) the rocks seem to have been deposited in a shallow ocean basin (a fore-arc basin) formed at the front edge of a chain of volcanoes (a volcanic arc). A modern day active fore-arc basin is the area between Sumatra Island and its offshore islands in Indonesia. This means it was the same processes that occurred in the Tamworth area. At the same time it showed just how big the continental collision zones were that created the New England region.

In proposing a new stratigraphy for the Paddy's Flat area, Hoy et al (2014) has now come close ending more than 100 years of head-scratching. There has been more than eight different relationships proposed for the units in the Emu Creek Block starting from the first in 1906. The best one until now was probably the Geological Survey of Queensland (Murray et al 1981).  Hoy et al (2014) proposes that the youngest unit in the block is the Emu Creek Formation which is overlain by the Paddy's Flat Formation which was deposited after a haitus. The Paddy's Flat Formation is then overlain by the Razorback Creek Mudstone. Hoy et al (2014) dated zircons in the rocks using the uranium and lead composition and compared this with the age of the fossils found in the area. The results were inconsistent with the Paddy's Flat fossils. This lead to the conclusion that in-situ fossils are present in the Emu Creek Formation but probably not in the Paddy's Flat Formation. Any fossils that were found within the Paddy's Flat Formation were probably eroded out of the Emu Creek Formation. Coming to this conclusion brings to an end to so much confusion that was present.

Once the Emu Creek Block were formed along with its related rocks from Coffs Harbour to Texas through to the Tamworth area, there was large scale bending of the New England area. So much so that the western facing fore-arc basin at Paddy's Flat was bent around so that it seems to be facing the north-east. This is what Rosenbaum (2012) terms the Coffs Harbour and Texas Oroclines and is the biggest but largely unknown tectonic features of our part of Australia. But more about that in a future post.

References/Bibliography:

*Hoy, D., Rosenbaum, G.,Wormald, R. & Shaanan, U. (2014) Geology and geochronology of the Emu Creek Block (northern New South Wales, Australia) and implications for oroclinal bending in the New England Orogen. Australian Journal of Earth Sciences. Vol8.
*Murray C., McClung G., Whitaker W. & Degeling P. (1981) Geology of late Palaeozoic sequences at Mount Barney, Queensland and Paddys Flat, New South Wales. Queensland. Government Mining Journal V82.

4 comments:

  1. Hi Rod, great blog as usual - keep up the good work.

    I'm sad to say that I think I have spotted an error with your otherwise excellent article. Where you say the block was formed in the late Permian to Triassic, should say late Carboniferous (although they do say that the overlying Razorback Mudstone is of probable early Permian age).

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    1. Thank you for the correction. I have erred with the date. Definitely much too young. All fixed now.

      Cheers!

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  2. Hi Rod, I haven't kept up with your blog for a while, and I'm glad you're still casting light on the real world!

    My interest is plants and plant communities and the varied geology of the Drake-Tabulam-Mallanganee area makes it special indeed.

    Look forward to the post on the orocline with more graphics please!

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    1. Thanks Nick, I'm glad that you still follow the blog and that it still seems to be of interest!

      Images say a thousand words - as the saying goes. This post is quite text heavy with very little backup visually. I'll try and make sure that future posts have some illustrations to help more.

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