Monday, 16 February 2015

Geology part 2

This post follows on from some notes on last weeks lecture.  The second lecture was about the physical geology of Canberra.  As such it mainly relates to point 5 in my structure suggested in that post.

The area is founded on rocks about 470 million years old.

There are three stages to the evolution of the landscape.  The first was extending the continental crust of Gondwana.  This effectively moved the coast about 1000km from about Broken Hill to well East of it's current location.  This would seem to be the Lachlan Orogeny.
The turbites are sediments washed out to sea and then transformed into rock.  Granite is a volcanic rock formed below the surface: where granite is exposed today indicates some 5km of material has been eroded away..

This seems to have happened through three tectonic cycles.  At the time of the first of these (the Benambran cycle) the location of Canberra was basically on the equator, and there was no life on land: the dry bits of the planet would have looked like Mars does today.

The next stage of tectonics is the Tabberaberran cycle.  This is when shales were laid down.  It included the Devonian age, which my memory tells me is of the big primitive fish in tropical seeas.  Many fossils exist (eg at Wee Jasper).

There then came a 'big squeeze' in the Kaniblan cycle.  This effectively shoved the coast back about 300km and involved 4 episodes of rock folding.  This gave something like the shape of Australia as we know it today.  However it was at 70oS so there was a fair bit of glaciation and erosion which filled up the Sydney basin.

The major features resulting from this are visible today.
This has 3 rifts - shown in grey-blue - and two uplifts  - shown in pink/mauve.  The boundaries of these are the major North-South Faults which are very obvious when travelling around.  For example:

  • climbing out of Queanbeyan towards Bungendore we ascend the Queanbeyan Fault between the Canberra Rift and the Cullarin Uplift
  • the Murrumbidgee River (and its shadow, the Monaro Highway) follow the Murrumbidgee Fault between the Murrimbidgee Batholith (ie Namadgi National Park) and the Canberra rift.

I am particularly interested in the Whiskers Fault as we seem to live right on it.  Further research is needed to find out:

  1. exactly where it runs; and 
  2. the derivation of the name!

One of the major achievements relevant to this area in the early 20th century was the way Marion Burley-Griffin was able to fir the city plan into this crumpled landscape.  Neither she nor husband Walter ever visited the area but did there work on the basis of models and maps which were sent to them.  This included Pittman's geological map from 1910, of which an example was pinned up on the board for our class.
There were several mentions in the class about people coming to the area seeking resources to exploit.  This includes Wild, Vaughan and Throsby-Smith who came lookng for limestone, for making building mortar.  They found it and named the area the Limestone Plains.

That'll do for now!

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