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.

Friday, November 29, 2013

Geo 365: Nov. 29, Day 333: Rip-Ups

Looking at the base of the outcrop in yesterday's photo, the main rock type is sandstone, but there are several rip-ups to the left and above the hammer. In particular, the one immediately left of the chisel tip is very pretty. There are fractures all over this rock, too, but looking closely, I don't see offsets on any of them, so they're joints, not faults.

Photo unmodified. May 7, 2013. FlashEarth location.

Thursday, November 28, 2013

Geo 365: Nov. 28, Day 332: Thankful for the Rocks We Could Get At

While I was quite frustrated and disappointed that we couldn't get at the conglomerate I remembered so fondly, when we turned back north, there was an outcrop that made the stop very much worthwhile. This is an overview, so there's not too much detail. It's much finer-grained material, mostly sands and silts. As I mentioned in paragraph two of Tuesday's post, there is a fault running roughly along Brush Creek. Though I didn't piece that together while we were here, it makes very good sense; the conglomerate and these finer sediments are clearly separated by a significant stratigraphic distance. Their overall tone and look are similar, so I suspect they're of the same provenance, but the environment of deposition in this outcrop is much lower energy.

Another question to confront here is whether this is sedimentary or metamorphic rock. My puckish answer is "yes." That is, there has clearly been some recrystallization and alteration of the rock; on the other hand, the clast composition, bedding and sedimentary structures appear essentially pristine. I could see a metamorphic petrologist studying the degree of recrystallization, at the same time a sedimentary petrologist was working out the details of this environment's shifts through time. This outcrop is in that gray area in between the two, and I resolutely refuse to accept that it must be one or the other. It is what it is. So much geology falls into gray areas that if you can't accept that our human definitions are arbitrary, and the rocks don't pay attention to what we think, it's probably a discipline that won't bring you happiness or contentment. On the other hand, if you find joy in the fact that the world is far more messy than we'd like to think, this sort of thing can make one very happy indeed.

 Photo unmodified. May 7, 2013. FlashEarth location.

Wednesday, November 27, 2013

Geo 365: Nov. 27, Day 331: Cracked Pebbles

Cracked pebble conglomerates form in situations where, first, there's conglomerate, i.e. sedimentary rocks the main component of which are clasts larger than sand. Second, that rock is exposed to shear stress great enough to deform it, but, critically, not enough confining pressure (or heat) to allow it to deform plastically. In more straightforward terms, it deforms by breaking. These fractures, very small faults, cut across the clasts, giving the appearance of cracks, hence the name "cracked pebble conglomerate." From the hand sample I once had from here, my recollection is that typical offsets were very small, a millimeter or less. While the resolution of my camera is better than my eyes, we're simply too far from the outcrop for me to spot any of the small faults in this photo.

What I can see, on the other hand, is a very poorly sorted, texturally immature conglomerate. This tells me that it's a high energy environment,  and that the sediment has not been transported far from its source. While I was hesitant to accept it at first, I've more or less convinced myself that the prominent lineations dipping about 35-40 degrees to the right are bedding planes. The sorting is just so poor, though, that I'm not certain just how the bedding planes are so clearly defined. However, they don't look tectonic in origin.

Photo unmodified. May 7, 2013. FlashEarth location.

Tuesday, November 26, 2013

Geo 365: Nov. 26, Day 330: Unreachable Conglomerate

There's a very nice cracked pebble conglomerate exposed in this cliff face. It's an outcrop I had visited only once before, on a field trip in the mid 80's. On that trip, we had to clamber over driftwood to get across Brush Creek, but there was a decent beach we could walk up to look at the exposure. On this trip in May, winter storms had removed most of the driftwood, and the creek, still high with spring runoff, had meandered up against the cliff. In addition, it was too swollen to wade without removing shoes and hiking up our pants to our knees. As I've mentioned a number of times, the coast portion of our trip was nasty cold, and removing articles of clothing was not a practical option. So, much to my disappointment, we couldn't really get a look at it.

In my poking around the innernetz, I came across a Master's thesis on the geology of the area from 1960 (8.9 Mb, link to archive, not direct to PDF), which shows a significant fault roughly along Brush Creek (See fig. 4, pages 14 and 15 as numbered by the PDF, rather than as paginated in the original thesis). Since I hadn't really looked at the geology that carefully on either visit, I hadn't dinked that out for myself, but in retrospect, it's pretty obvious. As we'll see in days to come, there's quite difference in the rocks on either side, and a fault is an obvious explanation for that discrepancy. As a side note, I haven't read that document in any detail, but I'd take much of the interpretations carefully. With utterly no disrespect to the author, our understanding of pretty much all of Oregon was overturned- at least greatly modified- by the plate tectonics revolution of the 60's and 70's, with much of our "flake tectonic" understanding developed during the 80's. On the other hand, most of what I have read is mostly descriptive, and is very useful. What the rocks are doesn't change (though nomenclature can change their names). It's our interpretation of them than can change dramatically.

Also, the picture I'd thought of using today was one I'd already posted, at the end of a "teaser" post during the trip (direct link to photo). I'd expected to expand on that description, but after reviewing it, I decided there wasn't really much more to say. It's still a cool rock, though.

Photo unmodified. May 7, 2013. FlashEarth location.

Monday, November 25, 2013

Geo 365: Nov. 25, Day 329: Not Geopetal

I've mentioned or alluded to "geopetal" structures a number of times in this series (2/14, 2/15, 2/16, 2/28, 3/1, 3/2, 3/8), structures or features in a rock that provide an indication of which direction was stratigraphic "up." In the context of sedimentary rocks, this tells you which beds are younger and which are older. It may not seem like a big deal, but it's the difference between telling the story accurately or backwards, and is of enormous importance. At first glance, this might appear to be an example. It would be easy to assume this represents a suddenly decreasing flow regime, from high-energy conglomerate to lower-energy sandstone. The problem is that, at least at a cursory look, one can't tell with any confidence that it wasn't the opposite situation, representing a sudden increase in energy. In the end, this is an appealing cobble, but it doesn't tell me a whole lot more than "there was an abrupt change in energy in this depositional environment." Of course, that's just looking at texture. Had I looked more carefully at the clast compositions, I could say much more about their source. But time, which the earth has in nearly limitless amounts, is limited for us poor humans, and this day was rushing by much too quickly.

Photo unmodified. May 7, 2013. FlashEarth location.