Tuesday, December 3, 2013

Geo 365: Dec. 3, Day 337: Approaching Explication

A couple of other geobloggers chimed in yesterday on that pestiferous fold. Callan Bently offered this annotation "as my attempt at generating a hypothesis that I'd prefer to test on the site itself... :)" Indeed. More to be said on that in a moment. In essence, he has recognized that the larger pattern around the small, tight recumbent fold involves a surrounding sandy layer, which in turn is surrounded by a coarser, conglomeratic layer of variable thickness. I was struggling trying to make sense of this, because, in my mind, it still begged the question, "how did it happen?" How could presumably solid rock, in a relatively low temperature/pressure metamorphic regime, deform this readily and plastically?

Enter Brian Romans: "I'm thinkin' that your puzzling structure could be uber-complex deformation w/in a debris flow." And continuing, the "texture of that deposit, which includes 'floating' clasts in a poorly sorted matrix leads me to debris-flow deposit interp." Aha! There's the problem. As is often the case, my assumptions are getting me into trouble. I'm generally pretty good at recognizing examples of soft sediment deformation, but I've never seen anything like this- at least, knowingly (though now I'm wondering about this other location along the Umpqua River). Brian commented further, "there's an entire spectrum from coherent blocks that have slid/tumbled to disaggregated/mixed debris flows." And, "the term now used for all is 'mass transport deposit' with debris flow being an end member." So presumably, at the other end, one would find a more or less coherent, undisrupted, slump block. In addition, he sent a photo of this new-to-me type of deposit from Patagonia:
...and, on the middle-left, you can see a sweet (not-so) little fold, very similar to the one that's been troubling me.

Now, what's the take-away? First, this is not "the answer." It's an informed hypothesis, one that, to be persuasive, would need more investigation and observations, perhaps even (gasp!) measurements. As I've mentioned before, it would have been really nice if I'd noticed that fold on the spot, rather than nearly seven months afterward. But would that have helped me make this interpretation at the time? No; I've never heard or thought of such a thing as "mass transport deposits" before, and the idea wouldn't have occurred to me. In retrospect, it makes perfectly good sense, but either this concept was beyond the scope of a good BS in geology during the 1980's, or it wasn't a widely discussed/known/recognized aspect of stratigraphy and sedimentation. Another possibility is that it was simply dismissed or ignored by our strat and sed prof, who was notoriously out-of-date. The point is, I couldn't have made this interpretation, no matter how thoroughly I had studied this spot. What I could have done, though, was to more thoroughly document this area, including photos at different scales and distances, to better capture relationships between the different sediment packages.

Hopefully, when and if a similar head-bangingly tough problem like this comes up, I will have a better array of information with which to tackle it.

Original photo taken May 7, 2013, annotated by Callan Bently. FlashEarth location. Second photo by Brian Romans, unknown location in Patagonia.

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