Saturday, 10 March 2012

Old lakes between lava

As some people have commented, I have not posted for some time, nearly a month in fact. Sorry for the delay. I have been affected by some unexpected (and some expected) health problems including some surgery (which was the expected part). I am still recovering and will be for a little while so posts will continue to be infrequent too.

In the mean time, this is a short post about some rock called diatomite which occurs in at least two locations on the Alstonville Plateau. The Alstonville Plateau is comprised mainly of basalts previously thought to be Lismore Basalt sourced from the Tweed Volcano/Mount Warning area but now according to Cotter (1998) should be referred to as the Alstonville Basalt from an earlier volcanic event during the Cenozoic. But there are at least two locations where the basalt created areas where lakes were formed by natural dams created by lava flows. These areas are Tintenbar and Wyrallah and there is possibly another one or two.

'Potch' opal from Tintenbar
Diatomite is formed from the preservation of silica from plant and animal remains and looks a lot like chalk. It is white, powdery and often retains impressions and fossils. It was formed in a fresh water environment, in other words a lake. This is referred to as a lacustrine environment.

The Wyrallah deposit was mined up to the 1950s (as diatomite can be used for anything from kitty litter to beer filtration) and is located just up the ridge heading towards the Rous area from Wyrallah. The Tintenbar deposit was also mined but also contained opal in lowest parts of the overlying basalt lava flow. not very good opal, a type called 'potch' but worth looking for at least for interest sake. This deposit was just to the west of Emigrant Creek just south of Tintenbar village. Both deposits are underlain by basalt and overlain by it showing that the lakes must have existed during the period of volcanism.


*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Herbert, C. 1968. The Tintenbar and Wyrallah Diatomite Deposits. Departmental Report. New South Wales Geological Survey.


  1. Hi Rodney,

    That is bad news about your health.

    There has also been opal finds around Bangalow and there is a crystal/opal horizon around the 400m mark in the Night Cap but this is more volcanic. "Jacob the Jeweller" in Ferros Arcade Byron is a good man to talk to.

    I hope that you are on the mend and start feeling better soon.


  2. Hi Rodney

    Very interesting is the topic of old lakes between the lava presented by you. Congratulations for your work. Fascinating.

    But this has led me to ask - do you know of chert deposits (if that is what they are? - they appear as such to me but I only have school boy geological knowledge and experience [HSC at Kyogle High School!]) in such lakes in the Umingar locality, located between Kyogle and Woodenbong, on the Summer Land Way?

    Would be very appreciative of your response to my question



  3. Hi Brian,

    much of Umingar state forest is Walloon Coal Measures which is comprised of mainly terrestrial (riverine type) sediments including sandstones and siltstones and some coal. There is possibly some rhyolite lavas or pyroclasitic deposits that may resemble a chert. And it is remotely possible that there is some chert but this is usually a marine derived sediment.

    Cherts are common part of the older New England Orogen which occurs to the west (begins somewhere near Tooloom I think) and also the area north of Byron bay up into south east queensland.

    I would be interested to see this rock you mention to see what it is. I would be interesting to find out if it is chert what it is doing there!

  4. Hi Again Brian,

    I thought some more about your question overnight. I wonder if what you have is a metamorphosed mudstone or some kind of lava deposit in a lacustrine environment or some opal or maybe it is even a chert after all!

    Sorry this doesn't help much. I'd be interested in seeing what it is if you have a picture or similar. Feel free to email me - I've added my email address to the about me page.

  5. Hi Rod. Sorry to hear that you have been unwell. Jim

  6. Hi Rod, I will declare right up front that I am no expert in the field of geology.I am writing because I came across this blog post while researching a paddling guide for Maguires Creek in the Teven Valley. I have noticed that there are quarries on either side and I understand that at least one of them is looking for basalt and meta-argilites. Can you please let me know if you think it would be Asltonville Basalt in this location? Also, is it possible that the argilites were deposited as a result of lakes like those described in your post? Thanks, Scott

    1. Hi Scott,

      Thank you for your inquiry and my apologies for the late reply (you may have noticed I've been out of action for some time.)

      Yes, The basalt in the Teven valley area is most likely Alstonville Basalt.

      But no, the metasediments are much older and were deposited in the sea, probably on the margin of a continental shelf. These were subsequently squashed onto the Australian continent. The basalt flowed over these rocks many millions of years later. The remnant rocks that showed lakes were formed are now only layers in the basalt.

      Hope that answers your question. Let me know if I can help more.

  7. Hi I live in Tintenbar and just wondering were I could have a little dig for that opal .