Help me ID this rock sample

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Help me ID this rock sample

Postby Heimhenge » Mon Sep 11, 2017 4:54 pm UTC

I submitted this question with photos to the ASU School of Earth & Space Exploration which has an "Ask a Geologist" service, but it's gone unanswered twice now. Figured there must be some geologists on this forum so thought I'd try here.

I had some excavation done recently on my property in New River, AZ and uncovered a rather strange looking rock. My 5-acre property includes a 200 ft hill that's a westward extension of Gavilan Peak geology. A surveyor told me the main "rock" in my hill is a metavolcanic basalt with quartz intrusions covered by a varying depth of caliche. This rock was found near the surface after a shallow excavation to regrade my driveway. Photos are attached.

The specimen is about 8 cm in its longest dimension and has a mass of 80 grams. Difficult to get its density since there a fairly large "hollow" that runs through the rock but it feels pretty light for its size. I could measure it's volume/density via water displacement if needed but didn't know how important that info would be. Estimate around 2.5 g/cm3 ... about like sandstone.

I don't have an "official" Mohs hardness test set, but I can scratch the rock with a piece of copper so I'd guess it's around 3.0.

I showed it to some amateur geologist friends in my astronomy club, one also a member of a geology club, and got the following educated guesses:

fossilized wood
fossilized coral (since this area used to be an inland sea)
failed geode (no crystals are seen in the interior void)
just a weird example of effluvial erosion boring a hole through a normal rock

But nobody was really sure. Any insights would be most appreciated. Here's the photos:

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Re: Help me ID this rock sample

Postby Soupspoon » Tue Sep 12, 2017 7:59 am UTC

Not my field1, but the pictures are making me think "fossilised burrow". But not in the usual sense of the "negative space" being obvious as distinct rock 'fingers' to show where worms/etc have passed and later sediment has infilled, but rather the void walls having been made denser during construction (there seem to be 'folds' as if original clay forced out) and survived subsequent eroision better than the undisturbed surrounding sediment and whatever infill would have seeped in to sustain the shape.

And then eroded out from its home rock in a way that also eroded out the infill. (Like said otherwise, weird erosion. Even assuming the interior were far more erodable.)

Likely I'm far wrong, though. Don't expect your share of our combined Nobel Prize For Paleontology by return of post.

1 And the old "if you don't know what it is, and how it got there, it's a Glacial Erratic2" probably doesn't work in this case.
2 Like "ritual artefact" in archæology. Almost tempted to tell you it's this. ;)

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Re: Help me ID this rock sample

Postby Heimhenge » Fri Sep 15, 2017 6:28 pm UTC

Your "fossilized burrow" idea makes a lot of sense. In this desert ecosystem (Arizona) there's quite a few critters that burrow into the ground ... mammals, reptiles, insects. I can see how an abandoned burrow could get filled with softer "silt", fossilized, and later be cleared out. In fact, there was still a lot a crap in the void before I blew it out with compressed air.

Also, on closer inspection, this sample may be on the borderline between true "rock" and merely compressed/hardened clay. I have a feeling a could break it quite easily.

Good suggestion Soupspoon, thanks.

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