GeologyRocksWhat Geological Process Caused these Rock Formations?
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Hi and welcome to GR They
Thu, 06/21/2012 - 12:06Hi and welcome to GR
They look like stromatolites to me. These are amongst the oldest known fossils and are colonial structures built by blue-green algae. They thrived in warm aquatic environments and built reefs much as coral does today. Domed columns are typical, and the fossilized structures can be quite extensive. There are modern examples at a number of points in Australia, which is where I'm assuming you're based.
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thanks
Thu, 06/21/2012 - 13:05Hi Gus,
Thanks for the reply. I like your theory however as much as I would like it to be true it does not fit into the geology of the area (Illawarra, 100km South of Sydney NSW Australia) or have many similarities to other stromatolites formations.
Apologies that the photos are not higher res.
The rock appears to be sandstone with the outer 'shell' being made of something denser & harder, possibly molten caused from heat or pressure.
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I don't think they are
Thu, 06/21/2012 - 20:15I don't think they are stromatolites. I think they are sedimentary concreations. The "swirls" are patterns of different amounts of cementation, with lots of cementation occurring in the centre of each concreation.
It reminds me of somewhere I've been to, but I can't remember where...
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My first thoughts were
Fri, 06/22/2012 - 06:17My first thoughts were stromatolites as well. So since that is ruled out (although there is no reason why they should not be 'inland' now) a couple of other suggestions - could it be exfolioation (onion skin weathering) or box iron. It is similar to a box iron quary (disused) near me.
John
“Civilisation exists by geological consent, subject to change without notice.” - Will Durant
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I was under the impression
Fri, 06/22/2012 - 08:59I was under the impression that Liesegang Rings were staining or colour change rather than a '3D' type of feature. What do you say Jon?
John
“Civilisation exists by geological consent, subject to change without notice.” - Will Durant
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I concur with the idea that
Wed, 06/27/2012 - 20:13I concur with the idea that these are diagenetic in origin, as opposed to any 'molten' or metamorphic processes. They look like concretions (perhaps carbonate) that have formed via chemical processes - most likely linked to paleogroundwater flow.
Seems hard to indentify the exact mechnism though, especially as the contacts between each feature are so sharp. It may be that the precipitation of the carbonate is centred on a nucleus - some studies on concretions in sandstones have found that carbonate will preferentially precipitate around fragments of decaying organic matter, for instance.
Fantastic examples if so!
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What Geological Process Caused these Rock Formations?
Submitted by RickJo on Thu, 06/21/2012 - 09:26.Dear group,
I am a new member but a keen amateur geologist.
I have been attempting to put together some info on the local geology of my area, basic info can be found here: http://immersionimagery.com/geology-of-the-illawarra-southern-highlands.html
I came across these formations and am trying to put a date and the basic process that caused the formations.
Would I be right in speculating that this was formed through metamorphic processes? Are the swirls caused by mini convection's? I noted that the outside of the different 'swirl convections' harder material has been deposited and take longer to erode.
Also are the 'ball' type of rocks kind of 'bubbles'?.. This however may not make sense as you would speculate that lighter materials would be inclined to rise however these 'bubble balls' take longer to erode?
What process causes this kind of formation as I can see that a great deal of pressure must have been at play but not enough for complete metamorphic changes in the sedimentary silica rock.
I understand that putting a date on this would be difficult for geologist outside my area however would I be correct in assuming that it was part of the Lachlan Fold Belt?
Below are some photos.