Wednesday, October 22, 2008
Minding and (yes) chuckling about the technology gap, I have added some recent commentary from Yeehow Principals about the OSL results (see below).
I am leaning toward analytical or sample problems based in part on other OSL experiences, but admit that Jim's ideas hadn't occurred to me. I will mull them over. It is notable that the lake would have been fairly large and it may have taken sediment a long time to make it to the dam itself.
Can't blog on the site because I can't remember my user name or password or some such immense Luddite hurdle.
But my immediate thought (assuming that 22ka isn't just the standard OSL result) is that the lake (and its deposits) might owe to changing basin hydrologic characteristics rather than the timing of the blockage. Perhaps the blockage was not sealed sufficiently to form a lake until things got much wetter during the maximum of the last glacial. Or alternatively, significant sediment didn't accumulate until then in a lake that did date back to initial blockage. And recall that when the Alvord basin was full and spilling (at least for several hundred years) at some point loosely around this time during last glacial, there would have been a lot more water coming down the Owyhee from the expanded source area. This could be another twist on the story...
Perhaps the Crooked Creek flood triggered incision (would the terraces ages at Dog Leg jibe with that?).
I'll check on the tephra situation--I have many more samples that have been sitting there even longer. As I said when you sent them my way, you get what you pay for, and when you don't pay anything it's hard to whine.
Sorry, Kyle. I tried to post a comment to the blog, I really did. Same
luddite excuse as Jim. I seem to have set up multiple accounts with
multiple passwords and the matrix of possible matches is too much for me.
I've thought a lot about these OSL results. It's possible that the lowest
deposits are related to a landslide dam downstream, but highly unlikely
that the others are. The deposits on river left can be directly traced up
to the rim on top of the Saddle Butte. Any landslide that blocked the
river that high would have been close to the height of the West Crater dam
itself. I haven't been to the higher site on river left, but it looks like
it is on top of the WC lava itself. Jim's ideas are worth considering, but
my hunch is that something went wrong with the OSL dating itself. The
fine-grained samples we have to work with are not ideal. These ages are at
least 20,000 years younger than the youngest date on the West Crater flow,
and 40ka younger than our best estimate.
Caitlin has started reading up on OSL and the possible reasons behind
erroneous dates, especially those that are younger than the true age.
Next week, after we've discussed this at our Bend summit, I'll contact
Tammy with some specific questions about possible sources of problems with
these results and get her take on them. I have submitted additional OSL
samples from Sand Spring, behind Saddle Butte II flow (could also be
backed up from West Crater), 2 more samples from top and base and middle
of same WC section on river left, and one from sediments way up on rim
behind Bogus Rim flow. We'll see what comes of those. Should shed some
light on the issue, as some of them should be from older damming episodes.
Posted by Dr. Jerque at 9:54 AM