Tuesday, October 21, 2008

OSL Results....Thermodisappointmence?

Before my last field outing, I contacted the USU OSL lab about our samples. The good news: preliminary data are available. The bad news: wtf do they mean?

From: *Tammy Rittenour* <tammy.rittenour@usu.edu
tammy.rittenour@usu.edu>>
Date: Tue, Sep 30, 2008 at 3:03 PM
Subject: RE: Owyhee OSL?
To: Kyle House <pkhouse@gmail.com pkhouse@gmail.com>>

Kyle-

Ive attached the preliminary results from your Owyhee River samples.
They are about 75% complete at this moment. I am working on sending
them in for further processing because of some initial feldspar
contamination, but you should have the final results by the end of the
month.

The samples are all ~22-23ka, if they were taken from the same deposit,
then OSL seems to be working quite well.

Errors on the ages will go down when they are completed, but I dont
expect them to change much at all.

Good luck in the field,

Tammy

What do we make of this information? Well. We suspected that these lacustrine seds were related to damming by the West Crater flow. Given their distribution with respect to the upstream face of that flow this seems like a valid assumption. However, these ages are far younger than the likely age of the West Crater flow. That is a problem. Also, the samples cover a large range of elevation. I expected that there would possibly be a correlation between the elevation of the sample below the likely dam and its age. This is not the case. Check the figure below for a visual cross-check:This figure is a profile from the 10 m DEM data. The profile hits the four sample spots in the area. The geology is schematic (obviously) and hastily drawn (more obviouslyer).


Note the large elevation range of the three sample areas*. Each sample is denoted by a yellow circle (excpet the highest circle, that is a gravel deposit that wasn't sampled). We are still awaiting an identification of an obvious tephra bed that was sampled below the lowest OSL sample. Sent that (and all others) to WSU in early August....USGS has been sitting on it for over a year. If that comes back with a confident ID, then we will have some additional perspective on the OSL data. Any thoughts? Break down and add to the blog....

*I have wondered aloud on numerous occassions about the lowest one...possibly related to landslide dam? Its elevation is pretty low relative to the sublava paleochannel of Ryegrass Creek. The paleotopography implied by this (if related to West Crater) is pretty deep.
Posted by at

4 comments:

  1. Liz SafranOctober 22, 2008 at 1:35 PM

    Kyle et al.,

    Ugh, those are indeed problematic results. Not only are they 20 ka younger than the youngest ages we have for West Crater (and I thought the best ages were more like 60-90, no?), and not only are they uniform with respect to elevation, but they don't make sense with our Dogleg terrace ages. Maybe I'm mis-remembering, but don't the cosmo data tell us that incision was already occurring by ~38 ka at Dogleg? T4, which is ~35 ka, is 10 m below T5 (~38 ka), and at 22 ka the river should have been happily sawing its way down to T3, which is 40 m below the West Crater surface and dates to ~15 ka. If we believe those numbers, then Jim's explanations don't get us out of our difficulty. I tend to wonder, like Lisa, whether the OSL dates are meaningful. It is now crucial to get a date on that tephra. If the problem is that our sample is low priority because it's a freebie, then let's commit some funds to getting it done. We're not out of dating money yet, are we? There is, of course, the other problem of finding a lab that will actually turn around our samples in a timely fashion. Has anyone gotten any leads on that since the UNLV experiment?

    ReplyDelete
  2. Dr. JerqueOctober 22, 2008 at 1:56 PM

    I sent that tephra to Nick Foit's lab at WSU in July. I expect that it will come back fairly soon. I inquired about it yesterday but have not yet heard back.

    I have had good luck with that lab in the past.

    ReplyDelete
  3. Liz SafranOctober 22, 2008 at 2:33 PM

    Upon further reflection, maybe the dates at Dogleg don't rule out a 22 ka age for the lake seds at the upstream edge of the dam. The implication then would be that it took at least 20 ka for a knickpoint to propagate (if that's how the river responded) from Dogleg to the upstream end of the dam. That might actually make sense, given that it potentially took that long for incision to initiate at Dogleg (assuming West Crater is at least 60 ka old).

    ReplyDelete
  4. Liz SafranDecember 1, 2008 at 10:23 PM

    What's Qrg, again?

    ReplyDelete

Note: Only a member of this blog may post a comment.

Subscribe to: Post Comments (Atom)