Posted by: Alan Pitts | April 25, 2011

Folded and faulted in Wyoming

A few days ago I wrote about some really old plutonic rocks I encountered last summer at Split Rock, Wyoming while on a cross country road trip.  I mentioned that the rocks were of an extremely old age and some of the oldest on the continent at somewhere around 2,500 million years.

After leaving Split Rock, we continued west towards the Grand Tetons

Along the way I performed an emergency roadside stop to investigate some rocks.  I saw several  interesting structures in these gneissic rocks which I think are metamorphosed pieces of the Wyoming Craton, but since they are not in situ I’m not sure.  Just west of this location,  the mountains are mapped as the “oldest Gneiss complex“.   I think that through glacial/fluvial transport/deposition it is possible for these rocks to have made their way to the current location. If I am correct, then they are even older than the plutonic rocks at Split Rock and could be greater than 3,000 million years old.  This would make them among the oldest rocks in the world and definitely the oldest Ive ever seen in the field.

Here are some of the boulders that I saw:

Apologies for the blinding red hot quarter for scale, I’m pretty sure its alright to view without eye protection, but just to be safe I wouldn’t stare  for too long.

Here is a closer shot:

A folded felsic vein in perhaps Amphibolite.

And here is another close up of the bottom of the original photo:

And with some annotations to illustrate the kinematics of the fault:

Ive started using new software to annotate photos and I am still learning all the tricks, like creating proper “geology arrows” for now these will  have to suffice.  I plan on updating the above photo as soon as  I create some neat and uniform geology arrows.

The above image demonstrates a left-lateral sense of shear, top to the left, no matter which way you look at it.  There are so many structures here, a fault plane along the dotted line offsetting the more or less linear felsic vein, which is quite different above and below the fault plane.  The same fault also cuts through a folded vein which seems to have similar composition to the left vein, but is thicker and folded.  Also the offset to the object on the right appears to be greater than on the left.  I think this is actually an optical illusion, there is weathering involved and the right side of the photo is farther away than the left side of the photo.

Or, another current theory I have is that the felsic vein on the left of the above photo is offset but the  object on the right ( the folded vein) is not offset at all, rather has a section missing.  Here is an annotation to illustrate what I mean.  The dashed yellow line represents a part of the fold that has been weathered away

Here is the same outcrop face rotated 90 degrees:

And now a closer look at the right side of this image

Another pretty complex story in the photo above.  A ptigmatic (intestine-like) fold in the center encased in what could be a refolded fold.

And this:

And another fold with some boudinaged fesldpars:

And this really sweet fold partially camouflaged by lichen:

Whoa, not so fast there Field-Alan, the angle you are trying to demonstrate on that S-fold looks a little off.  Good thing Computer-Alan is here to help you out with some more appropriately angled fingers.

And then I found this guy. Yikes !

This last outcrop was really folded and approaching migmatitic in some areas, as it got extremely hot during metamorphism nearly to the point of remelting.

I’m kind of kicking myself in rear for lack of adequate and complete photographic coverage of these boulders.  I attribute this to two causes;

1)  The overwhelming availability of sunlight out west.  It was incredibly sunny throughout Wyoming, making it hard to see in the field, and even harder to see the intended object through the LCD-screen of my camera.  Many of the pictures looked okay when I took them in the field, but didn’t make the cut after an initial inspection.

2)  Last summer I didn’t see the outcrops the same way I do now.  I’m a student of geology and still developing my skills in geologic interpretation, as I learn more I start to see more and understand which parts are important.   Looking back at my photos I immediately recognize several areas that I wish I had covered better.

Lesson learned:  Time in the field is precious, so make good use of it. ( I’m talking to you Field-Alan)


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