Sunday, January 11, 2009

Identifying the mystery rock

Folks occasionally stop by the Survey, asking us to identify a rock they've found somewhere. Last week we got an email from the South Dakota Geological Survey with a photo of a rock one of their constituents had collected in Arizona and asked us for help in figuring out what it is.

The description from SDGS is "The rock boulder is about one foot in diameter and probably weighs 15 to 20 pounds. It was collected near the Salt River by Phoenix in 2001. It was collected by this gentleman while drilling an 8-foot diameter caisson for a building foundation; he says it came up from approximately 30 to 40 feet depth. He said he was not drilling in fill material. It is obviously a well-rounded boulder, and so I assume he was drilling in alluvial material. The rock is very hard (> or = 7), non-magnetic, non-calcareous, with a pale reddish streak. The tannish residue in the "burrows" is non-calcareous, perhaps a clay?"

AZGS geologist Steve Richard weighed in with a few alternatives:

It may be a slightly metamorphosed calc silicate rock from the Mescal Formation of the Apache Group. This unit started life in the Middle Proterozoic as a dolostone with some chert, but was subsequently intruded by diabase sills and underwent a karstic weathering and diagenesis event, such that most of the dolomite is now calcite (thus the unit was originally called a limestone) in many places, the rock is variably silicified and iron stained in places, and has enjoyed varying degrees of contact metamorphism in lots of places. The pitting in the sample in the photos doesn't look biogenic to me, it looks like diagenetic (or contact metamorphic) silicification, with the pits forming in layers/globs that were not completely silicified--the carbonate-rich (or incompletely silicified) material in these weathers more readily than the silicified rock. My first guess is Mescal because it seems to form strange textures like this more often than most units.

Same thing could be from the Abrigo Formation (Cambrian), but usually there's more visible sedimentary lamination in the Abrigo.

Laramide or Middle Tertiary volcanic rocks (with some superimposed alteration) could produce similar pitted weathering due to variable replacement, perhaps superimposed on devitrification texture in a felsic lava. Are you sure the protolith was sedimentary? Look for clastic textures in the pits--that's probably where primary texture would be preserved.

Charles Ferguson, a geologic mapper with us here in Tucson, offered additional perspectives:

When I first saw this rock, I did not know it was from Arizona and I thought immediately it was from the Belt Supergroup of Montana, not an impossible feat of transport for South Dakota. I've seen a lot of the Belt that looks like this, especially the argillaceous units from the Wallace or Aldridge formations.

When I read that it was from Salt River, in Arizona, and read Steve's evaluation, that makes a lot of sense. The Mesoproterozoic Apache Group is age equivalent to the Belt and they have similar facies and tectonic settings.

I agree with Steve that those are probably not burrows. You see sedimentary/diagenetic textures like that a lot in the Belt and the Apache. Mescal is a good call, but I've also seen Pioneer Shale that has that wormy weathering as well. Those old argillites often become silicified or contact metamorphosed and make super hard and durable river gravel cobbles.

We invite other opinions and interpretations. Post a comment on what you think this is.


  1. Anonymous5:08 PM

    Chert with the carbonates weathered out seems a good bet. Also I've seen (dirty) quartzites with a similar weathering pattern.

  2. Looks very strange.. especially the orientation of the burrows. If this would not be I would suggest to take a biogenic origin into account e.g. similar to what can be noticed in alpine lakes?

  3. Anonymous4:58 PM

    metamorphosed amygdaloidal volcanic? Thin section it and you will know all.

  4. Anonymous5:03 PM

    maybe a metamorphic amygdaloidal volcavnic or what appear to be vessicles may have been stretched by flow. thin section it and you will know all.

  5. Anonymous3:47 AM

    If not amygdaloidal volcanic, it may be graphic granite. But the streak is reddish.If the fresh surface does not yield that streak then definitely it is. Fresh surface may give better clues. First impression is a banded iron stone but banding is not complete.

  6. Anonymous4:35 AM

    My rock isn't from arizona but it posing a challenge to idenfy at the moment...take a look at this forum...

  7. Last week we got an email from the South Dakota Geological Survey with a photo of a rock one of their constituents had collected in Arizona and asked us for help in figuring out what it is.

  8. I found one just like this last week but not quite this big. slightly smaller than my hand. first obne for me

  9. Anonymous10:09 AM

    Wow, it is awesome just to hear you folks talk about this. To me its just a lovely stone, like any mountain trail or landscaping rocks in Utah. Science gives it some meaning, it's great!

  10. Ok this is a easy one.
    .so happens i was looking for someone i can contact on this exact subject..that i discovered here in phoenix. Ok the rock.. Ready for this... If you turn the picture to the right you will see a big number 5 .on the side dead center... Cant miss it...this is a spanish symbol...this is more than rock.. What u have is a human head. That was poured molten material on... On the side of the rock with the number 5 visable is a depiction of the executioner. And prisoner...cartoonish depiction.on this one.. It has on this rock/head.. In detail of the crime and punishment.. All written in spanish.. They kept great detailed records of the the number 5 in my best conclusion so far is a spanish department..conquistadors that is what mines they regulatedlike a division now there is number 3 6 7 and the biggest one so far is 5 I need to go to spain and hang out for couple months and i will have that figured out for sure.. I find that this is the way to preserve and kind of receipt. By incasing them in material. im finding that these human parts where being sent back to spain.. For king and queen to play with.. Everything under the sun was sent back to them.. From arizona and every place they repositioned to.. I have maybe 10 to 12 heads.. Tongues. One eyeball.. Hands. feet. Half and hole foot thighs. Knee toe. Penises yes with balls.. Half lower torso of female.. Now this rock u have is a little different than my heads . you executioner depiction looks like he is spanish ..he's not indian like this executioner that did my collection.. My executioner looks like Alice Cooper in full make up.. I will send u some pictures of my collection.. My email Maybe you want to give your girlfriend foot or toe for Christmass email me .. And dont open the head up its not good.. All i got to say is when in water it smells like... Like sweet wet dogs hair smell .. Lol half of it floated..

  11. I guess i take the title on being the Arizona archaeological Czar and ambassador

  12. Thomas Cooper12:03 AM


  13. Thomas Cooper12:21 AM

    Come to think of it I probably released the smallpox or what ever the disease was at that time black plague or something.