Last summer, August 20, 2011, to be exact, Dana, Intrepid Companion, and
I visited Table Rock, at the south side of the Fort Rock/Christmas Lake
Basin (
FlashEarth location;
crosshairs very close to photo location- zoom in to see details.) Table
Rock is very much like Fort Rock, but much larger, filled with lava
late in its eruption, and not as well known. It also has a slew of
primary sedimentary structures along with secondary soft-sediment
deformation and tectonic structures. While I don't know the details of
its history, I love ogling its little tidbits.
I found the spot
below especially fun, and was at a loss making sense of what had
happened, because I was focused on what had gone where, and not paying
attention to the faults that had allowed those motions...
So I sent it off to
Callan, the
geoblogosphere's structural go-to guy, with some comment about "the rarely seen splort structure."
The larger context- the splort structure is in the middle of the lower left quadrant of this photo.
And here it's on the middle right edge.
Callan
responded later with this terrific annotation, which instantly cleared
up my confusion as to just what the heck had happened here- at least in
terms of structure, if not in terms of exactly what caused it. Regarding
that latter, the environment would have been the distal edge of a very
violent phreatic eruption, and sedimentation of debris would have been
very rapid, at the same time that the pile was being shaken by blasts
from the central vent- roughly 3 miles from here- and frequent
earthquakes. So stress fields would have been changing rapidly in a deep
pile of unconsolidated sediment. It seems to me that in such an
environment, nearly contemporaneous normal and reverse faulting might
not be that surprising. And while I like "splort," Callan tells me this
is more properly called a pop-up.
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