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.

References/Bibliography:

*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.