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Identity HelpStromatolite?

1st Dec 2018 06:39 UTCCraig Howard

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Hi everyone,


I am hoping for your opinion on this piece of chert I found years ago near Moora, Western Australia.


At the time I just thought it was just an interesting rock. However, having developed an interest in stromatolites since then, I've recognised stromatolitic characteristics in this rock, e.g. fine and gently convex laminations, distinct columns with relatively smooth margins, and - towards the top - a mixture of parallel and divergent branches with small projections.


These attributes are especially visible if you increase the image contrast - see third image below. What do you all think? Is this a stromatolite? And if so, any thoughts on possible species?


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1st Dec 2018 15:56 UTCAlfredo Petrov Manager

I agree about stromatolite, but no idea about species.

1st Dec 2018 18:04 UTCUwe Kolitsch Manager

Yes, a stromatolite.

1st Dec 2018 20:20 UTCRalph S Bottrill 🌟 Manager

There is some argument about they exact nature of this rock type, but possibly not a true stromatolites. It’s pretty common as beds in much of the less metamorphosed Mesoproterozoic to Neoproterozoic rocks in Australia, usually as dolostones or cherts but probably originally algal limestones, dolomitised and sometimes silicified during diagenesis to low grade metamorphism. Stromatolites are also algal limestones but are characterised by spaced, sporadic active columnar growth during deposition, with limey sediment between the columns. The algal limestones or dolostones tend to have a more continuous wavy to lumpy surface, but they are commonly broken up like this example, partly tectonically but probably partly due to (organic) gas escape structures. The columnar structure in this chert appears to be due to such vertical structures, now replaced by white dolomite or quartz, not sediment, though I cannot see enough detail in the photo to be completely sure. That’s not to say there are no true stromatolites in the rock, but it’s not obvious.

2nd Dec 2018 00:23 UTCCraig Howard

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Hi everyone,


Thank you for your feedback, and Ralph in particular for your detailed response. Here are some close-ups of details that seemed stromatolitic to me; do these provide any more clues for determining this rock's nature?


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2nd Dec 2018 02:38 UTCGregg Little 🌟

I would tend to back up what Ralph Bottrill said as it doesn't have the typical and definitive stromatolite structure; distinct columnar growth with intervening sedimentation. Modern day examples are seen in your Shark Bay, of course. From your close-up photos I would still lean towards calling it a "laminated algal mat". Unfortunately the silicification process has not preserved much of the very fine detail, possibly due to previous recrystallization (calcite or dolomite?), probably making species identification unlikely.


One further observation on the tiny white dots that define the algal layers. These may be the included gas cavities from the algal decay, see "birdseye limestone" in the literature.


Algal development was wide spread in the Mesoproterozoic to Neoproterozoic from simple mats (layer textures like yours) to more structured stromatolite development as well as binders of fragmented material in higher energy depositional environments. Distinct algal structures like pisolites and oolites also form in these higher energy environments, possibly in association with bacterial activity.

2nd Dec 2018 07:39 UTCCraig Howard

Hi Gregg (and Ralph et al.), thank you for your gracious time and consideration.


I think it is safe to conclude that this rock is a piece of laminated algal mat that has been dolomitised and then silicified.


Thank you all for your wisdom.

2nd Dec 2018 07:46 UTCRalph S Bottrill 🌟 Manager

Hi Craig

Thats a bit clearer, but again it seems like fracture-related carbonate and/or silica alteration cutting algal-mat laminations in carbonate sediments as Gregg has said very well. The pictures could be upside down if you consider the fluid expulsion would have buckled the soft algal sediments. A great specimen in the story it tells!
 
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