Saturday, November 30, 2013

Geo 365: Nov. 30, Day 334: Puzzling Structure

I have to admit, I didn't see this until I started looking at the photo this morning. I missed it at the outcrop where I might have looked for clues more carefully. On top of that, I neglected to get some mid-range photos, between today's close-up and Thursday's distant shot, so unless I get back to this spot, and can find this structure again, as far as I can tell, the problem is insoluble.

Do you see it? Check below for an annotation of the fold that has been tormenting me for several hours now:
At first, I thought it was just an illusion, not real. But I convinced myself that it does look real, and was faced with the problem "how did it happen?" And I just don't know. I have three very tentative guesses, none of them satisfactory, as far as I'm concerned.
  1. This is an extraordinary example of a flame structure. Problem: I've never seen any example of this phenomenon even approaching this length and quality of development.
  2. This a drag fold over a thrust fault, with the upper portion of the rock moving to the left. Problem: I see no other supporting evidence for this supposed fault- in fact, no clear faulting of any kind in the photo. (There are, however, some "maybe..." faults in here, just nothing that's clearly convincing.)
  3. This represents an overturn of the entire sequence. Problem: Lots. I'd expect to see much more pervasive deformation, fracturing, and faulting in such a situation, unless this rock had been subjected to much higher confining pressures and temperature than was the case. As noted above, I don't have enough supporting photos to even check if that's a viable guess, but it doesn't seem likely. Another fail: I can't spot any geopetal structures to give me any sense of "which way's up" anywhere in the photo, near the fold or elsewhere.
I liked the 3-D effect at this spot enough that I took six photos to make a "wobble gif" of it, but when I opened the first one this morning, and started looking at it carefully, I spotted this problem and got distracted from that task. Maybe later. Right now, I just don't want to think about it anymore.

Photo run through Paint.Net's autolevel routine for contrast and clarity, and annotated there as well. May 7, 2013. FlashEarth location.

1 comment:

Hollis said...

thanks for sharing the thought process! Seems like most of what I read is about situations where people know (or claim to) what happened.