On Friday 22nd January Chris and I set off from Perth, accompanied by John and Julie, to sample what geology in the bush is like. It was almost like old times for John and Julie were fellow (sorry Julie!) members of my geology class at Aberdeen University, many years ago.
We were to be away for four days and three nights, and, this being Australia, the first day was devoted to getting the miles done. As we took to the dirt roads we began to see kangaroos hopping across and emus running in front of us, too daft to get off the road. We were headed for a place called Sandstone and accommodation in a donga. Dongas are semi-mobile accommodation, designed for use in the bush. Really they are shipping containers with bathroom plumbing and an air conditioner. Ours was the noisiest air conditioner in Western Australia.
Actually the place was quite good as there was a bar, grass and a small swimming pool. The proprietrix took in damaged kangaroos from the Conservation Department and looked after them till they were able to be sent on to zoos etc. They were no longer wild so could not be released into the bush.
We had our evening meal at the Sandstone National Hotel, which was full of prospectors, geologists and drinkers. (Many of them were all three.)
Our main purpose on Saturday was to look at John’s astrobleme. But we looked at several things along the way. The first was London Bridge, a natural arch in weathered basalt.
There seems to be quite a lot of these natural bridges – I saw several as we drove along but this one is close to a town and has a road leading to it. They form when erosion forms a ridge. The uppermost layer is often lateritic and is therefore harder than the weathered rock underneath, so erosion breaks through the subsoil first.
From there we headed to the Barrambie layered intrusion. This is comparable in size to the Bushveld in South Africa, but has had a much more active geological history. Despite decades of search no one has found a viable platinum mine but vanadium is mined as well as iron ore.
John took us to an ilmenite – magnetite cumulate.
And to prove that magnetite was present we used a magnet, dropped onto the soil covered with dirt and picked up.
It is not easy to clean the magnet after this – magnets and magnetite have a strong affinity!
Then we were off to see the Yarrabubba astrobleme. Time has not been kind to this meteor impact. Any crater there was has long been eroded away – all we have left is what happens deep below the impact. It was first noticed several decades ago when a Survey geologist noted some shocked quartzs in thin sections of rocks collected in the area.
Nothing was done with this until John was working on other impact structures and came across the reference. The felsic Barlinga granophyre and the nearby potassic Yarrabubba granite are rather unusual for this part of WA. Geophysical evidence shows that area have been demagnetised.
Field work showed that the Barlangi granophyre had been injected horizontally and forcibly into the country rocks.
Xenoliths of country rock can be seen.
And shatter cones were seen.
Pseudotachylite veining can also be seen.
The Barlinga granophyre was probably formed instantaneously when the meteorite impacted and injected itself higgledy-piggledy into its surroundings, making things most uncomfortable for all the minerals down there. In particular quartz had to equilibrate quickly.
After this we set off for Cue, another small town. And on this portion of the trip we found ourselves to be very lucky. After driving for over a hundred kilometres on dirt roads with no other traffic, we turned onto the Great North Highway just north of Cue – and almost immediately had a puncture. It turned out that our vehicle did not have a wheel brace and we could not change the tyre until another vehicle came to our aid. If the puncture had happened while we were on the dirt roads we would have been in some difficulty.
We left “The Queen of the Murchison” B&B in Cue for Walga rock and found it covered in puddles. There had been rain overnight and, later, we found that some of the rivers were actually flowing!
Round the edges of these granites is a good place to look for water.
We then went on to Jokers Tunnel – an unsuccessful gold mining venture. A tunnel, about a hundred metres long, was driven through a ridge, hoping to find some gold. Nothing was found . And the tunnel still survives. It was driven using hammers and chisels and probably without explosives.
There are countless reasons for this sort of thing not happening in the UK, but it was fun and no bats (or wetas) were harmed.
We overnighted in Mingenew and set off for the Coalseam Conservation Park, where the first coal in WA was found. It is Permian and is pretty poor stuff and was never mined seriously. Coals of similar age are mined near Collie to the south of Perth.
Parts of the succession are fossiliferous.
We then went to Cervantes, on the coast, to look at Lake Thetis which has stromatolites – or thrombolites if you listen to the experts.
And lastly, for a final treat we visited The Pinnacles which are spectacular structures of controversial origin. See HERE.
It does look rather like a temple for penis worshippers, but even so it was a nice place to end our tour of the bush.
I am filled with admiration for the geologists of Western Australia who have done so much with little outcrop and even less fresh rock. And they are working in extremely harsh environments of heat, drought and flies. Surviving in the bush is a full time occupation – doing geology as well is remarkable.