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Iowan

Poop Aka Coprolite? Diorite?

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Iowan

Hey guys, found this in a gravel bed in central Iowa and am having trouble identifying it. I had originally thought it might be a native American artifact but the members at Arrowheadology disproved that and sent me here. Any suggestions would be great. Thanks!

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post-17283-0-05192000-1420347572_thumb.jpg

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UtahFossilHunter

It's not looking like a coprolite but I could be wrong. Where did you find this in Iowa? Do you know the approximate age?

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Iowan

Yes I found it in Iowa. Unfortunately I don't know the age. The reason I thought it may be fossilized poo was, aside from suggestions, this auction of a rock suggested to be such looks very similar:

http://s.ecrater.com/stores/16895/4f11e92d49d92_16895n.jpg

I'm new to this forum so if that doesn't hyperlink I'm sorry. New pic is of the one being auctioned that looks similarpost-17283-0-14476600-1420351015_thumb.jpg

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jpc

This looks a lot more like some sort of Paleozoic rock full of fossils that has been tumbled down one of the mighty rivers in Iowa. Certainly not a coprolite. I found the one you linked to. I had to search on the site to see its label. Yes, it is very similar to your rock, but it is not a coprolite either.

not a coprolite

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Doctor Mud

Another vote for it not being a coprolite.

Looks like a rock with some sort of inclusions. A coprolite should look like poop - look up some images of actual coprolites. I would pity any poor animal that passed anything like this!

Lets also say that maybe - just maybe a fossil coprolite got tumbled in a river and didn't look poop like anymore - those inclusions should be evidence of the animals meal. To me they look crystalline and suggest that this is either fossiliferous rock or these are crystal inclusions in volcanic rock..

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Rockwood

Looks like it could be a very metamorphosed rock.

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Iowan

Another suggestion that had yielded a similar match was diorite porphyry. I think this is probably what mine is too. Green example is from the Smithsonian national museum of natural history and the grey one is from some korearth.net Korean geology site.

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post-17283-0-67321000-1420384127_thumb.jpg

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fossilized6s

This looks a lot more like some sort of Paleozoic rock full of fossils that has been tumbled down one of the mighty rivers in Iowa. Certainly not a coprolite. I found the one you linked to. I had to search on the site to see its label. Yes, it is very similar to your rock, but it is not a coprolite either.

not a coprolite

I agree. No coprolite here.

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Iowan

I agree now that it isn't coprolite, would you guys say it's diorite porphyry?

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jpc

what? you expect us to be geologists, too? Just kidding, and saying that the question is too geological for me. Or too metamorphic for me. Anyone else better at these things than me?

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Doctor Mud

I agree now that it isn't coprolite, would you guys say it's diorite porphyry?

I would say so. Your new picture is much clearer and this looks like a plutonic igneous rock. A plutonic rock crystallises in the interior of the earth. You tend to get larger crystals as the cooling is slower.

A porphyry is composed of large crystals or phenocrysts in a fine grained groundmass or "matrix". If your large crystals are plagioclase feldspar then this is a diorite porphyry. The crystals look like plagioclase feldspar to me but there are other tests you can use on the hand specimen, look for cleavage and hardness of the crystals.

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jpc

very good, dr mud

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Rockwood

I would say so. Your new picture is much clearer and this looks like a plutonic igneous rock. A plutonic rock crystallises in the interior of the earth. You tend to get larger crystals as the cooling is slower.

A porphyry is composed of large crystals or phenocrysts in a fine grained groundmass or "matrix". If your large crystals are plagioclase feldspar then this is a diorite porphyry. The crystals look like plagioclase feldspar to me but there are other tests you can use on the hand specimen, look for cleavage and hardness of the crystals.

Sorry if this seems rude, but if I'm not mistaken the new pictures are references not of the original post.

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painshill

It is unquestionably an igneous porphyry but I would suggest it didn't originate in Iowa. The state is not noted for exposures of plutonic rocks and Iowa porphyry samples I have seen came from deep drilling, were distinctly green in colour and with much smaller flesh-coloured phenocrysts.

I would say that it's a glacial cobble from elsewhere, which will make it more difficult to guess the mineralogy just by looking.

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Iowan

Yes Rockwood you are correct, only the pictures in my original post were of my rock. The other ones were examples I had come across that looked similar. I will post some close-ups of my rock.

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Edited by Iowan

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Doctor Mud

Sorry if this seems rude, but if I'm not mistaken the new pictures are references not of the original post.

Whoops! Yes I was confused there. And though the second picture was yours. Thanks for pointing that out Rockwood.

Based on your new pictures this is most certainly a porphyry though.........right conclusion sorry....wrong reasons :blush:

The phenocrysts in your porphyry certainly look like plagioclase.

Sorry my rock ID is a little rusty and it occurred to me that an ultimate identification of this piece (in the absence of their being a local source - as Painshill pointed out) would be by thin section. This is when geologists take a rock sample and mount it and grind it down until it transmits light. You can then look at the mineralogy of the fine-grained stuff.

Why is this important? Well we can most likely say this is a diorite or gabbro porphyry, but telling the difference between the two depends on the chemistry of the plagioclase (calcium vs sodium rich) and the proportions of other things in the groundmass (fine-grained stuff) like pyroxene, amphibole, and olivine. Sometimes you can see these crystals using a hand-lens.

This diagram summarizes the classification of volcanic rocks. (note extrusive means erupted onto the surface, intrusive means intruded into the earths crust and not cooled on the surface.)

Sorry for the confusion folks.

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Edited by Doctor Mud

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Gen. et sp. indet.

Iowan, 100% porphyry.

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painshill

Iowan, 100% porphyry.

What formation in Iowa (apart from glacially deposited erratics originating elsewhere) has porphyry which looks anything like that?

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donnyjoe

I think he was addressing the OP, not suggesting that Iowa was the point of origin.

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Herb

If its glacial in origin it could be from Canada. Looks like porphyry to me also, even though I only got a "B" in rock identification in college. :D

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FossilFisher

Ran by this post and it looks to me that I've found what looks almost exactly like this. Just south of Ledges State Park on the Des Moines River there is a flooded gravel pit that contains a localized Marmaton Group and Upper Cherokee limestone. I never got to clear of an identification when I tried. My thought is that it's delta refuse from the giant river that emptied out here in the Pensylvanian period filled with plant material (Annularia, Chordaites, Lepidodendron) locked in sand. But honestly thats just as far as I got.

post-10531-0-83842300-1421022456_thumb.jpg

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Herb

FossilFisher your rocks have plant remains, the other has mineral inclusions.

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janislav

Those phencrysts look pretty pink for plagioclase. I'm more inclined towards potassium feldspar and therefore a rhyolite porphyry. The dark groundmass is problematical, but looks glassy gray, like some of the rhyolite that crops up beneath the Baraboo Quartzite in central WI. I lost $5 on that black rhyolite back in the 60s. The origin is almost certainly glacial. A precise location would help, Iowan.

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