Friday, February 1, 2013

Nullarbor Plain


New evidence for palaeoenvironmental conditions across the Nullarbor Plain.

New research, from a team led by Dr Simon Holford, shows the now arid Nullarbor Plain was full of fast flowing rivers during the Cretaceous period.

But what is the Nullarbor?

The Nullarbor (from the Latin meaning no tree), is the world’s single largest piece of limestone, and is about 200,000 square km in area. The limestone was deposited in a shallow sea, as evidenced by the presence of foraminifera, echinoids, bryozoans, and red algae fossils. The climate of the Nullarbor today is typical of a desert, with temperature reaching up to 48.5 °C during the day, and reaching freezing conditions at night.

What's the new evidence?

Using evidence taken from sediment cores, the research team has shown that during the late Cretaceous (85-70ma), the Nullarbor was crisscrossed by an ancient river system full of small but fast flowing rivers. The rivers stretched as far as the Eyre and Yorke peninsulas, and the Flinders and Mt Lofty ranges, in SA. The rivers drained into the Ceduna Delta, an alluvial plain 700km wide that lies off of the west coast of South Australia. The delta, located in what is now known as the Great Australian Bight, is approximately the same size as Great Britain, and researchers have always believed that the delta was created by an ancient river system over 2000km long starting in Queensland. However the research conducted shows that at least the younger part of the delta system appears to have been formed from processes relating to the river system stretching across the present day Nullarbor.

So how did they gather the evidence needed?

Due to much of the delta being underneath 500m of water, gathering evidence has been pretty tricky; because of this there has been little research on the geology. The research team had to use rock samples gathered by a petroleum company in 2003. The rocks were crushed, and zircons were extracted for radiometric dating. The team used two methods of dating: fission track dating and uranium-lead dating. In total they dated 1000 zircon grains, and the youngest age they discovered was 83 million years. They then looked at the onshore geology and dated that in order to match zircon ages with like-for-like geology in order to find a source for the material deposited in the delta.

So what's does this all mean?

Due to the size of the Ceduna delta, geologists have long thought that a continental sized system of rivers would have been needed to produce it (such as rivers bringing source material from Queensland). The original idea of the research was to prove this hypothesis, but instead much of the source sediment was local, from the Nullarbor Plain, and there is so far no evidence to prove a large ancient river system stretching from Queensland. It is important however to note that the material for this study came from just one well which was 4km deep and 10cm wide; evidence from more wells will be needed to further determine the source location for the material deposited in the Ceduna delta.

I will leave you with this quote:

"If there is any road not previously traveled, then that is the one I must take." Edward John Eyre, 1841 - The first European man to travel the Nullarbor Plain.


Links:

http://www.australiangeographic.com.au/journal/ancient-rivers-once-drenched-the-nullarbor.htm

http://jgs.lyellcollection.org/content/170/1/3.abstract

http://www.nullarbornet.com.au/

http://austhrutime.com/nullarbor_plain.htm

http://books.google.com.au/books?id=RSIbLENyT2wC&pg=PA165&lpg=PA165&dq=nullarbor+geology&source=bl&ots=NaLcdX6sFu&sig=Wnppi-Ks8dAatkIqvbaLNY6wtEc&hl=en&sa=X&ei=MasJUa7EBqmTiQfysoD4DQ&ved=0CEUQ6AEwAg#v=onepage&q=nullarbor%20geology&f=false

http://www.bom.gov.au/water/awid/id-839.shtml

http://adb.anu.edu.au/biography/eyre-edward-john-2032

Image: A view of the Great Australian Bight as seen from onshore. Somewhere underneath the waves is the Ceduna delta system. Image from http://bikeeasterneurope.blogspot.com.au/



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