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.

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