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Identity HelpMichigan's Keweenaw Peninsula

27th Apr 2024 21:41 UTCCraig Waddell

I found this rock at McLain State Park on the north shore of Michigan’s Keweenaw Peninsula. It’s a 2 on the Mohs hardness scale.

A lot of fossiliferous limestone washes up on the beach, but this is the first piece that I’ve found that looks quite like this in the mushroom-root-looking part.

In addition to front and back, I’ve attached two pictures taken with a digital microscope. Is this oolitic limestone? Or something else?

Questions Answered
Can it scratch glass? : No
Can you scratch it with your fingernail? : Yes
Is it light/heavy for the size? : Normal

27th Apr 2024 23:44 UTCDon Windeler

At a glance, this looks to me like one of the stromatolites (fossil algae mats) that are found a bit farther north along the west side of the Keweenaw Peninsula, closer to Copper Harbor. I have a piece or two of these kicking around from my time as an undergrad, but would need to look for them in long-buried flats to show a comparison.

If that's indeed the case, this example would be broken out of something else. My example(s) looked like the top of your photo, with a rounded whitish lump in a red matrix.

I could be totally wrong, but it's not too much of a stretch that stuff could have moved SW down the coast to McLain State Park.

28th Apr 2024 00:39 UTCCraig Waddell

Thank you, Don. Here's one of the pictures of the bottom section that I took with a digital microscope. I tried to load it earlier, but I must have done something wrong.

28th Apr 2024 00:40 UTCCraig Waddell

Second picture of lower part taken with a digital microscope.

1st May 2024 18:49 UTCGregg Little 🌟

In the first photo, my overall impression of the sample's two textures is that you have a boundary (bedding plane?) between a very fossiliferous unit (lower part) and a fossil "poor" unit, often dominated by micrite (lime mud) deposition (low energy regime or quiescent period). This quieter period of deposition might have abundant micro-fossils in the micrite which can be seen on a wetted surface under magnification. Note, compared to the lower part, the lack of large pore spaces (inter-fragmental) is due to the fine grained nature of the micrite and the lack of larger fossil fragments.

The lower very porous, coarse fragmental portion appears to be coralline (primitive?) with possible shelly material (brachiopods?). There is probably lots of published work on the fossils of this region to nail down the ID if you wish.

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