Geology ID?

[Gee.S]

Found an interesting little doo-dad in the Cave Creek area.

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[mcvansoest]

Looks like a piece of heavily vesiculated volcanic rock (the color would lead me to say basalt but that is not always 100% diagnostic) with secondary mineral growth in the vesicles. I suspect those vesicles would be filled with calcite which you could test with some pure warm vinegar (or if you had some pool acid) - if you put the vinegar on the crystals/white filling of the vesicles if it starts fizzing and bubbling it is calcite, if not it is something else probably quartz.

[Gee.S]

Thank you, good sir. Appreciate it, very helpful!

[Gee.S]

Some more interesting geology. I happened across this large boulder yesterday. Some kind of fossil worm stuff?

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[mcvansoest]

Could be remains of bioturbation - ie. the traces of worm or other non-vertebrate creatures as they burrowed into the sediment, with their tubes being filled with a slightly different kind of material.

However it is a little hard to see from this picture if it is not a 'planar' surface feature that extends laterally along the surface you are seeing, in that case it might be that you are looking at a vein filling that got exposed and is currently being weathered at slightly different rates in different places away. If this was near a mine area then I would probably go with the vein filling.

Any more close up pictures?

[Gee.S]

There are mines all over the area, but this was about as far from a mine as anyplace there. This boulder was unique in the area in which I found it, a large boulder field toward the bottom of a raucous, slopey wash, so there may be others high above.

This is the same snap, only zoomed in a bit. Best I can do.

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[mcvansoest]

Could you hazard a guess as to the type of rock the boulder is made out of? For it to be any kind of bioturbation of fossil something it would need to have a sedimentary rock. If it was say a granite type kind of rock then that would rule out any kind of fossil related possibility.

The stuff on the boulder gives me a sense it is travertine or some similar kind of carbonate deposit which could associated with mineralizing fluids, but it is really hard to tell without being there or holding it in one's hand.

We need to get you a basic field geology kit: hand lens, dropper bottle with ~10% HCl, rock hammer, rock ID chart.

Were was this? Ie. which mountain range - that can potentially be elucidating given that the geology of AZ is mapped quite well.

[Gee.S]

Harquahala. The boulder pile and surrounding area was a mix of granite and limestone. Lots of limestone.

I know size is difficult to gauge from this snap, but this is a large boulder, easily a meter across.

I actually have several rock field guides, but I find them largely useless without more hands-on direction out in the field.

[jam]

I know size is difficult to gauge from this snap, but this is a large boulder, easily a meter across.

...it is difficult to gauge. At first I thought the boulder was a rock wall 15-20 meters across. Nevertheless, never seen anything like that. Isn't it funny how people into xeric plants sooner or later get also into rocks/geology as a side affect? To some extend anyway. Some only become rock aware and some take it a step further I find field geology kit under their xmas tree

[Gee.S]

I know size is difficult to gauge from this snap, but this is a large boulder, easily a meter across.

...it is difficult to gauge. At first I thought the boulder was a rock wall 15-20 meters across. Nevertheless, never seen anything like that. Isn't it funny how people into xeric plants sooner or later get also into rocks/geology as a side affect? To some extend anyway. Some only become rock aware and some take it a step further I find field geology kit under their xmas tree

Last time I saw something like that was in a mirror after an ergot encounter.

[mcvansoest]

I have been looking into if there are some decent/easily available reads on the geology of Harquehala area - it has/had lots of mining activity. So far the things I have found have been pretty generic. If I find something worth reading I will pass it along.

As to the deposit in the picture, I, for a second, thought maybe fossil stromatolites, but I am not sure the picture can be used to positively determine that to be the case.

To me it still most looks like it was a somewhat planar feature that might at one point have covered the whole boulder and it still has the appearance of a weathered travertine-like deposit, that would suggest either a vein filling or even evidence of run off from an ancient CO2 rich spring which can be associated with mineralization going on at depth/nearby, but to get any more sense/certainty, I'd probably have to be on the outcrop. Photo geology tends to only get you so far.

The Harquehala mountains are part of what is called a metamorphic core complex associated with Basin and Range extension frequently juxtaposing old pre-Cambrian high grade metamorphic rocks with much younger mesozoic/cenozoic igneous intrusive and sometimes associated sedimentary and volcanic cover rocks generally at what are currently extremely shallow dipping fault/movement planes. There is still quite a bit of discussion about the exact mechanism of how these form exactly, but generally these are great targets for resource exploration/exploitation as the frequent old and currently operational mines attest to.

The South Mountains, Catalina Mountains, White Tank Mountains and several other ranges visited by you, Gee.S, are also examples of such core complexes. I am actually talking about trying to get funding for a small research project on rocks from the Catalina mountains.

[Gee.S]

In my experience, Harquahala is almost entirely unique in central AZ. But I only say that because of all the limestone (also, tho to a lesser extent at South Mountain), which often promotes a notably distinct plant community.

[mcvansoest]

Yup it entirely possible that one place has a lot of limestone while the others have less or no limestone (exposed). The specific rock types occurring can be different, the nomer 'metamorphic core complex' implies more about the formation mechanism of the mountains, than what kind of rocks are specifically there though usually there will some igneous intrusive rocks associated with these, which is indeed the case at Harquehala.

[Meangreen94z]

I’ve seen similar “boring tunnels , I can’t quite put my finger where. Here are a few pictures from central Texas of limestone, dolemite , etc.

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[mickthecactus]

My cactus friend Ralph Martin is in China lecturing at Tsinghua University (he’s a retired computer professor) and has sent me these pictures of natural stones installed there. The Chinese revere stones like this and on his last visit he went to a stone museum with some remarkable examples, all natural.

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