Saturday, February 15, 2014

Geo 730: Feb. 15, Day 411: Downsection

Here we can see most of the overall section at this stop, without the rimrock basalt. The smaller channel is above and to the left of Dana, on the middle left edge of the photo, and the upper left corner shows the tafoni-weathered homogenous tuff. Near the middle, you can see the contact between the larger lahar and the gently dipping tuff beds, which I believe were water-laid. Above Dana's head, maybe 20 feet, and a bit to the right, you can see a block of what appears to be the same yellowish tuff as the material into which debris flow is entrenched. So we can propose a series of events here to explain what we are seeing:
  1. Deposition of volcanic ash, probably fluvially transported, possibly in a lake or floodplain.
  2. Uplift (or drop in base level), with subsequent erosion of a large channel.
  3. Deposition of a debris flow/lahar into that channel.
  4. Erosion of a smaller channel into that deposit.
  5. Deposition of a smaller lahar into the smaller channel
  6. (There may have been other lahar desposits, but I'm not seeing any clear-cut evidence for them)
  7. Deposition of homogenous tafoni-weathered tuff, either as airfall, or as in #1.
  8. Eruption of basalt, possibly of CRB/SMB affinity, creating rimrock.
This is why I love geology. Every roadcut is a puzzle, some more, some less, complex than this one.

Photo unmodified. August 19, 2011.

Friday, February 14, 2014

Geo 730: Feb. 14, Day 410: Atop The Tuff rimrock that I suspect is Steens Mountain Basalt (SMB), though I'm not positive about that. When I was an undergrad, the SMB was treated as its own rock unit, though compositionally and chronologically, it was clearly similar to, and possibly related to, the Columbia River Basalts (CRB). Sometime over the last 25 years or so, SMB has been regrouped as a part of CRB, so technically (and again, with some uncertainty), this is CRB.

This location is driving me nuts... I know it's between Lakeview and Adel, and everything about the FlashEarth location I've been denoting looks right... except for the gosh-darned power line. There is another spot back near the Plush cut-off that has a power line paralleling the road like this, but the road isn't the right configuration there, and it doesn't look as if there's any tuff exposed in that cut. Oh well, I'll keep looking. Until I'm happy with it, no more location links for the time being.

Photo unmodified. August 19, 2011.

Thursday, February 13, 2014

Geo 730: Feb. 13, Day 409: Tuff to Distinguish

Still farther to the left from yesterday's photo, and looking up the hill, we see some tafoni-pocked weathering in what looks to be a fairly uniform, homogenous, tuff. This kind of has the look of ash fall, but water transported versus ash fall tuff can be difficult to distinguish, even up close, which I didn't get in this case. The power lines are a good landmark to look for in the location department. There are some power lines near the spot that I've been placing the FE location, but not this close. I need to reconsider.

Photo unmodified. August 19, 2011. FlashEarth Location (probably incorrect).

Wednesday, February 12, 2014

Geo 730: Feb. 12, Day 408: Left Side

Above, we see the left side of the channel cut-and-fill shown in the previous three photos from this stop. I'm pretty sure the basalt boulders in the upper middle have rolled down from flows above, and are not part of the original deposit. The angular nature of this channel puzzles me a little- it's almost rectangular in it's appearance. I guess I'd expect it to be a little more gently concave, like a sideways ")." However, I have seen angular channels like this elsewhere. Arroyos cut into alluvial fans often have such a shape: sheer sides and a more or less flat floor. In fact, there's a lot about this outcrop that makes me think the setting was an alluvial fan. This would have been a completely different landscape at the time. I'm not finding a quick and easy reference, but my recollection is that the modern basin and range tectonism and resultant topography didn't initiate until about 5 million years ago, and worked from the south and central portions northward over time. (Please correct me if I've botched that.)

Photo unmodified. August 19, 2011. FlashEarth Location (somewhat uncertain).

Tuesday, February 11, 2014

Geo 730: Feb. 11, Day 407: Channel Close-Up

After placing the hammer, I took a somewhat closer photo of yesterday's channel. You can see it's cut into an older, much more bouldery debris flow, but again, the volcanic nature of the clasts dominates. The light yellow/buff block above and to the left of the hammer appears to be the same lacustrine/floodplain deposits that form the right edge of this older flow (possibly flows).

Photo unmodified. August 19, 2011. FlashEarth Location (somewhat uncertain).

Monday, February 10, 2014

Geo 730: Feb. 10, Day 406: Incised Channel

This appears to be either a channel cut by normal fluvial processes, then filled by a lahar, or possibly a channel actively cut by a lahar, then subsequently buried during the same event. In trying to figure out what's going on here, I did track down some information that looks useful and relevant. In this listing of geologic units of Lake County, Oregon, the rock appears to be the "Andesite and dacite and sedimentary rocks (Miocene? and Oligocene)."Clicking through on that link leads to a more detailed description, which is basically consistent with what we're seeing here, as well as a number of map references. Opening the Adel Quadrangle link (click on the thumbnail to open a viewing window), and zooming and dragging to look at the SW corner of that map, you can see the canyon emerging into the Warner Valley from the west, where the rock unit is labeled "Tts." And looking at the legend, the description of that unit is again consistent with what one can see on the ground and in this photo. The map doesn't quite get as far west as this spot, but I'm pretty confident I've got this nailed down.
The unit's description:
What we're seeing here is clearly not "fine-grained," but we are off to the west, so we're apparently into the pumice lapilli tuffs. And as I mentioned in Thursday's post, we haven't seen them clearly yet, but there are a series of finer-grained tuff beds off to the right.

Photo unmodified. August 19, 2011. FlashEarth Location (somewhat uncertain).