Saturday, December 7, 2013

Saturd80's: Miscellaneous Finds Edition

Back on the Chain Gang, by The Pretenders, via Krugman.

Ah, Leah, by Donnie Iris, via Balloon Juice.

Money for Nothing / Beverly Hillbillies by "Weird Al" Yankovic, via I forget who reminded me of this one.
Oh yes, I do remember, I was reminding myself of what went on in UHF at Wikipedia, and that reminded me of it.

Geo 365: Dec. 7, Day 341: Cracked Pebbles

Looking carefully at this block- the same one as yesterday- you can see a set of joints or very small faults dipping to the right at about 25 degrees. These fractures cut across the pebbles they intersect, which is the defining characteristic of a cracked pebble conglomerate. Now if they were really joints, I'd expect them to be caused by tensional stress, which I think is unlikely in this setting. Furthermore, it seems likely that the fractures would have followed the pebble edges rather than plowing through them. On the other hand, looking carefully at full resolution, I can't make out any offset. So while I suspect these were caused by shearing, it's not observable macroscopically. Below are a couple of crops (run through autolevel routine in to bring out details) highlighting said fractures.
Note also that the white mineral surrounding the clasts- likely calcite- is more abundant to the upper left and lower right of the pebbles. I think this phenomenon is called a "pressure shadow."

Photo unmodified. May 7, 2013. FlashEarth location.

Friday, December 6, 2013

Geo 365: Dec. 6, Day 340: Conglomerate

I didn't notice this block of rock along the path from the parking area to the beach- or more accurately, paid no attention to it- because I was eager to go look at the conglomerate. This does not show the characteristic fracturing that pervades much of the Humbug Mountain Conglomerate at the beach level. It was clearly intentionally placed to block vehicular access, and I suspect it comes from elsewhere on the mountain.

Granitoid rocks are not common in Oregon. There are a few stocks exposed in the Western Cascades, for example, in the Quartzville area. Other than that, it's almost entirely restricted to the Klamath Mountains in southwest Oregon and the Blue Mountains in the northeast to north central portion of the state. In the sample above, you can pick out a number of granitic pebbles, so at least at some point in the past, such rocks were exposed in this drainage basin.

Photo unmodified. May 7, 2013. FlashEarth location.

Thursday, December 5, 2013

Geo 365: Dec. 5, Day 339: Joints or Faults?

Like the mudstone above the hammer, the surrounding sandstones show either joints or very small offset faults that are consistent with shear to the right in an upward direction, and to the left downward. This is an outcrop I'd like to get back to- hopefully on a warmer day. I'm not very proud of the quality of my observations from this spot, but I know part of the issue was that I wanted to get the heck out of the wind and cold.

Photo unmodified. May 7, 2013. FlashEarth location.

Wednesday, December 4, 2013

Geo 365: Dec. 4, Day 338: Sheared Mudstone and Calcite

Though I've not really mentioned it much, if at all, secondary calcite is present in abundance in this outcrop, forming especially in joints which then break open and reveal the patches. Above the hammer is another example, where it's forming a flattened plate within the sheared mudstone. Above and below the mudstone are coarser, sandy layers.

Photo unmodified. May 7, 2013. FlashEarth location.

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.