Friday, March 8, 2013

Geo 365: March 8, Day 67: In the Neighborhood of the Splort

As I've been emphasizing the last couple weeks, there's a heck of a lot going on at Table Rock. There's plenty going on at Fort Rock, too, but the latter is well known, commonly if not heavily visited, while the former is largely unknown outside of a few intrepid, geologically-inclined, souls. And Table Rock records fine details, as well as a plethora of volcanoclastic sedimentary features, that Fort Rock simply does not. Table Rock deserves more attention, and this past couple week's photos have been my attempt to give it a bit of time in the spotlight.

Let's consider this environment as the eruption and deposition occured:
  • Sedimentation and erosion were both occurring quickly, as shown by poor sorting, angular grains, rapid changes in dominant grain sizes, and common disconformities.
  • Overall, though, the pile was accumulating much faster than consolidation and lithification; it probably wasn't until long after the eruption stopped that much of the material could be considered to be "rock."
  • During the episode, the pile of debris would be subject to frequent, likely locally powerful, seismic shock from two sources: earthquakes related to magma moving underground, and frequent phreatic blasts as ground and surface water interacted with hot basaltic magma and lava.
  • I picture this situation as a rapidly growing "circular delta," with the central vent as the metaphorical sediment source. It seems likely that with such rapid accumulation, oversteepening and slumping were frequent. This would lead to tensional regimes in the topset beds, normal faulting in the foresets, and compression in the bottomsets. Because sedimentation was so rapid, the stress regime was almost certainly changing rapidly and constantly. This may have been exacerbated by rapid shifts from loading to unloading, and back, as sites of deposition became sites of erosion.
  • Add on top of all this that the alteration of basalt to palagonite results in a net increase in volume. I'm not sure by how much, but in a pile like this, that effect probably shouldn't be ignored.
Given the above, it's not really surprising to me that the rock is as disrupted as can be seen above, but still, very mesmerizing. And below, I've annotated most of the major features that pop out at me. (I just realized I neglected to mark the common examples of graded bedding, but see the concluding link in this paragraph for a good example of that.) Clear, sharp faults are marked with solid yellow lines. Faults that look more like ductile shear, or are so minor as to make it difficult to tell where they begin and end are marked with dotted yellow lines. A sequence of strata that I'm pretty sure is correlative across the largest fault, running diagonally through the photo, is marked at the top and the base with solid blue lines. And finally, I'm not really going to say much about the "splort," highlighted with a black circle, because I've already written it up- you can see that post here.
Photo unmodified. August 20, 2011. FlashEarth Location.

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