Wednesday, November 12, 2008
Cascadian Pyroclastic Goodness
The results are in. Nick Foit's lab at WSU has analyzed some Owyhee tephra samples with a new probe. The delay in getting the data owes to the time it takes to set up such a device. Turns out that it was worth the wait. The tephra id's match Yeehow conventional wisdom in so much as they match the most likely candidates at each site.
I don't have time to work up anything fancy for this post (leaving for different field area in AM...not ready yet, but here is the dirt:
Mt. St Helens Set C, approximate age 47.2 ka found in lacustrine seds on river right and left above West Crater flow (image above is from sample on river right). These are the samples that we both related to OSL ages of approximately 20 ka. The occurrence of this tephra on both sides of the river and at disparate elevations is interesting and is evidence for sedimentation at two different levels here. One associated with the Owyhee, and one associated with the perched paleo Ryegrass Creek.
Mazama Tephra was found in auger holes in depressions on the Bogus Point landslide and on one of the Heaven's Gate landslides. The Trego Hot Springs tephra was also found deep in the core of the Bogus Point landslide sag. This bed (from Newberry Volcano, I believe) dates to approximately 23.2 ka.
We also found Mazama (6850 c14 yrs BP; ~7600 real years) tephra in an arroyo cut formed in a breached sag on a landslide on river left in Heavens Gate.
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Posted by Dr. Jerque at 10:24 AM