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.