At Nashua where chert should not be

In an earlier post, Booyong not in the Clarence Moreton Basin? I indicated that I had been informed that there was evidence of Palaeozoic sedimentary rocks near Booyong, and that if this is the case there is significant implications on the morphology of the Mesozoic aged Clarence Moreton Basin in the area between Byron Bay and Lismore. It therefore has implications for the exploration for coal seam methane too.

Well, I can confirm that after much searching, I came across an ephemeral creek north of Booyong near the Casino to Murwillimbah Railway line at Nashua that clearly contained much chert. Additionally, a railway cutting appeared to have significant folding, but was very weathered so identification of the parent rock and indeed even whether it was folding was not clear.

Chert at a location between Nashua and Booyong

Chert is a rock that is essentially absent from the Clarence Moreton Basin, however, it is common in the underlying rock of the Beenleigh Block, known as the Naranleigh-Fernvale Group. This group of rocks formed in a deep marine environment. It consists of turbidite sediments that occur when parts of the undersea continental slope erode and some slates and chert. The Chert is formed on the ocean floor where chemical and biological material settles over a long period of time. All of this rock was then transported and accreted onto the edge of the Australian Continent before the Devonian or Carboniferous period which was around 333-343 million years ago during the formation of the New England Orogen. This process of accretion has caused folding and low grade regional metamorphism to be common place in this geological unit. By comparison, the sediments of the Clarence-Moreton Basin are dominately continental derived and are un-metamorphosed. They were mainly deposited in river systems, lakes or at best shallow marine environments. Chert does not form very well in these environments.

But, what is so exciting about this outcrop? The Neranleigh Fernvale Group occur along the edge of the hills above the coastal plain south of Ballina and are exposed just about everywhere from the Gold Coast to Byron Bay. If you follow the line of outcrop from near Ballina you will find it is overlain between Byron Bay by the Clarence Moreton Basin which extends all the way to the Tabulum in the west. But because the outcrop occurs at Nashua this shows us that the area to the east is probably a different basin from the main part of the Clarence Moreton to the west. This has significant implications for understanding the structure of the earth here and also affects where resources such as gas can be explored for. For those that were worried about coal seam gas, you won't find any at Nashua.