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I had the pleasure of visiting a well known collector in SW Wisconsin a few weeks ago. His home is almost as nice as any museum I have ever visited. He showed us some beautifully crystalized stromatolites that were collected in SE Minnesota. Hints were given as to where to find such specimens. So naturally, I had to find myself an example. I am just not sure if these are just geological and not biological. I hate to question a very knowledgeable man, but obviously I am. Opinions are welcomed.

 

Mike

 

 2019-11-054.thumb.jpg.2f148cc5e887dc04dc592ed3bd75aefd.jpg

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It's hard to know what was in those cavities... Something must have filled them and then dissolved out - but what? The vague parttern kind of reminds me of brecciated stromatolites (dissolved away) but can't say for sure.

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 @Wrangellian, I went ahead and looked up "brecciated stroms" and found this from fossil mall. Interestingly, Buffalo County is just across the Mississippi from where I was hunting:

 

Brecciated Blue Agate Ordovician Stromatolite

Brecciated Stromatolite (Blue Agate)

Geological Time: Lower Ordovician

Size (25.4 mm = 1 inch): Stromatolite is 115 by 110 mm, & up to 35 mm thick

Fossil Site: Oneota Formation, Buffalo County, Wisconsin

Code: DS2018

Price: Sold


Brecciated StromatoliteThis is yet another form of stromatolite that is at once beautiful and has interesting scientific characteristics. It derives its name, Brecciated Basal, from the fact that it originates at the base (Basal) of a large overlaying stromatolite structure. Brecciated (angularly broken) signified that, in fact, the original stromatolitic structures were shattered into irregular shapes, probably in a surf, and were then buried under the stromatolite colonies that subsequently grew above. Mineral replacement has been hard at work with a bluish gray silica predominating, making for an attractive creation through the interaction of biology and chemistry. The mineral replacement was likely formed by cold water precipitation, resulting in the crypto crystalline (blue agate) structure that yet retains the definitive imprints of growth of ancient microbial mats.

A final point of scientific interest is the age, Lower Ordovician, more than a billion years after stromatolite reached its peak on Earth, and during a time when stromatolite niches were disappearing because of competing metazoan and benthic life forms. The final major decline of stromatolites on Earth was a mere 50 million years after this stromatolite was formed.

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There you go  ;)

They can be such a mess, it's hard to tell what's stromatolite vs. chalcedony/agate... but I think all the layering in your specimen is agate. The cavities could have been broken strom pieces, but they're gone. That's just my semi-self-educated guess!

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11 hours ago, Wrangellian said:

The cavities could have been broken strom pieces.

 

@Wrangellian, you could be correct, but the nature of the rock in my area leads to frequent porous voids  in the limestone from the "acid" groundwater leaching through the pervious rock.

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