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rattlehead

Michigan Fossil Question

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rattlehead

I find a lot of these Michigan Lightning Stone (septarian/concretions) here locally. Often times there is a visible line, where one part is the clay part and the bottom (i'm guessing) is this material that has tubes and odd shapes. Sometimes fossil is clearly visible like the worm looking one below (2nd pic). Can anyone tell me more about these types of stones? I'm very interested in their formation but have been unable to find much information.

The top pic shows the darker clay on top and the line where it appears to be fossil. The material is still clay like but has a lot going on. It looks similar to the crinoidal limestone death floor stone I find, but without the obvious fossils.

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PFOOLEY

Maybe created by glaciers...similar to a puddingstone.

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fossilized6s

Clearly you have a awesomely preserved Dinosaur heart in pic #3! Lol

No, i have no idea, sorry. But i do love that one!

Your first pic looks like pockets of chert.

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rattlehead

Thanks for looking.

These come in a lot of strange varieties. The are quite soft overall, maybe 3-4 on mohs or roughly just softer than a Petoskey stone. The one in pic 3 is the outside of a large one that was cracked in half. The inside is shown in pic 4 with some of the lightning pattern. Would have loved to have found the egg intact. Looks alien to me!

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painshill

The Michigan septaria known as “lightning stones” probably originated from siderite (iron (II) carbonate) deposited from iron-rich sea-water concreting around organic debris at the bottom of the ocean. Bacterial action plays a part in that. Further accumulation of argillaceous clay-rich mud created approximate ball-shapes which cracked as they experienced shrinkage and hardening through water-loss (chemical desiccation, not evaporation). The shrinkage cracks filled with calcite deposited from carbonate-rich groundwater at a later time. The calcite is normally white, in contrast to the iron-stained matrix, but may also be yellow through to brown from trace amounts of iron.

They’re known by all kinds of other names in other locations and the process above is not the only mechanism by which they can form. It depends on the prevalent geology.

I’m not aware of the Michigan examples being reliably dated, but an upper-Devonian age has been suggested. Note also that their age is not necessarily the same as the deposit they may be found in and also that “age” would be an ambiguous term here since they almost certainly formed over a long period in two distinct phases – the concretionary process, followed by the calcite deposition.

The first picture is not a septarian concretion, nor is it fossiliferous. It’s a chunk of brecciated jasper – likely composed of fragments of rocks/minerals derived from the same kind of argillaceous mud. The upper grey layer is likely argillaceous mudstone. It doesn’t show any obvious signs of metamorphism but if it has seen any metamorphosis it will be trending towards shale.

The second picture looks like iron-rich sandstone with fossils of some kind, but I know not what. I presume that’s the bed matrix for where you are finding the concretions.

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Auspex

How absolutely fascinating! Knowing something about them now, I really like them :)

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Plax

#1 looks like a change in deposition such as a transgressive lag, we get breaks like this here in NC between formations

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