Nullarbor once flush with fast-flowing rivers

By Joanna Egan | January 25, 2013

New research has shed light on the landscape around the Nullarbor some 80 million years ago.

NOW ARID, THE NULLARBOR region was once full of fast-flowing rivers, new research reveals.

After analysing sediments extracted from a well in the Great Australian Bight, scientists from the University of Adelaide have a clearer picture than ever before of what the southern Australian landscape looked like some 80 million years ago.

Geology of the Ceduna Delta

Their findings, which were published in the Journal of the Geological Society this month, reveal that between 85 and 70 million years ago (during the late Cretaceous period), an ancient river system made up of many small, fast-flowing rivers, covered the present-day Nullarbor Plain, extending as far as the Eyre and Yorke peninsulas, and the Flinders and Mt Lofty ranges, in SA.

These rivers deposited sediments into the Ceduna Delta, a 700km-wide alluvial plain off the west coast of SA. A delta develops when a river deposits sediments at its mouth as it flows into an ocean, sea or lake. The Ceduna Delta was formed in this way during the late Cretaceous period. Situated in what is now the Great Australian Bight, it is about the size of England.

In the past, researchers believed the delta was created by a 2000km-long river system stretching from eastern Queensland to Ceduna, in SA.

“Our findings were surprising,” lead researcher Dr Simon Holford told Australian Geographic. “Most previous geologists to study the delta had envisaged it being the product of a massive, broadly southwest-flowing river system, larger than the present-day Murray-Darling system, that transported eroded material over thousands of kilometres from the eastern coast of Australia,” he says.

“However, our findings indicate that, for the shallowest, youngest part of the delta at least, much of the sediment appears to have been sourced from regions much closer to the delta.”

Sediment from well reveals late Cretaceous landscape

Despite the enormity of the Ceduna Delta, researchers have collected remarkably little data on its geology. This is partly due to the fact that it is today located in very deep water – mostly greater than 500m, with the deepest parts of the delta under almost 5km of water. To overcome this hurdle, Simon and his team collected samples of sedimentary rocks from a well at the delta’s centre, which was drilled by Australian petroleum company Woodside in 2003.

The researchers crushed the rocks to extract grains of a mineral called zircon, which they dated using two radiometric dating methods (fission track dating and uranium-lead dating).

“Our research is the first to determine the ages of zircon grains contained in the sedimentary rocks deposited in the delta,” says Simon. “In total we dated around 1000 grains – some as young as 83 million years.”

The team then examined onshore geological records in order to identify regions where similarly aged zircon deposits were located. “This essentially allowed us to determine which parts of Australia were being eroded to supply the sediments that were deposited in the delta, and thereby gain some idea of the nature of the river system that fed the delta,” says Simon.

“Our analysis indicates that the sediments were mostly derived from erosion of rocks within about 1000km radius of the delta, thus implying a somewhat smaller, more proximal drainage network than what was previously thought,” he explains.

Smaller ancient river network than first thought

Dr Chris Fergusson from the University of Wollongong says this study is the first to challenge assumptions about the Ceduna Delta’s formation, and that its results are surprising. “Because of the size of the Ceduna Delta, it has been argued that a continental-scale river system was required to provide the sediment, and that the sediment was carried from distant sources such as the emerging eastern Australian highlands or a river system in central Queensland,” he says.

“This paper provides the first test of this hypothesis, and the results were unexpected,” Chris adds. “It is a shock to learn that such a huge sediment pile was of largely local derivation, and that it was established without the need for monstrous river systems carting huge sediment loads from one end of the continent to the other.”

Although the research sheds light on how the delta formed and paints a clearer picture of what the landscape may have looked like during the late Cretaceous period, it is only the starting point.

”Despite the breakthroughs provided by our findings, we need to bear in mind that our dataset, new and significant as it is, is somewhat spatially restricted,” explains Simon. “It is based on one well, about 4km deep but only about 10cm wide.”

The older parts of the delta, which were deposited between about 85 and 100 million years ago, are yet to be reached by drilling. “It is possible that these sediments were indeed deposited by a larger river system,” says Simon. “Our study is a first step, but we need much more data.”

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