Wednesday, October 22, 2008

Saddle Butte Lake v. West Crater Lake

On my last field trip, I collected high precision GPS points on the highest evidence of overflow that I could find on each likely lava dam. In the case of the Saddle Butte lava dam (SBLD), there is evidence of shallow overflow over most of the extant dam crest...I had not realized this before. There are some photos of this in the recent album I posted. The highest overflow evidence was at 1044m (3425 ft). As for West Crater, the elevation of the first evidence for overflow (rounded basalt cobbles on the extant flow margin) is found at 1029 m (3375 ft)(*note that this is reported incorrectly in the figure caption). There is not nearly as clear a lava dam edifice here as there is at Saddle Butte, you may recall.

For what it is worth, the difference is more than expected or could be read from a topo map. As the figure indicates, a 15 m difference in depth has a locally significant effect on the extent of the lake. This is based, or course, on modern topography.

Thermodisappointmence commentary

Figure showing extent of possible lake above West Crater lava dam

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.


Tuesday, October 21, 2008

Back to the Bogus Lake

Here is a more detailed example of how large a lake dammed by the Bogus Rim lava would have been. This image assumes a dam at Iron Point with a crest elevation of 1200 meters. Made in a flash with GlobalMapper.

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* <>>
Date: Tue, Sep 30, 2008 at 3:03 PM
Subject: RE: Owyhee OSL?
To: Kyle House <>>


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

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,


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.

Monday, October 20, 2008

Topo Profiles in Study Area

I whipped up these images while revisiting the mapping in earnest today. These can be made in seconds using GlobalMapper. All offer some insights and are interesting to consider. We need to choose some key ones to illustrate with stratigraphy etc. If you lose your bearings, just click to go to the online album and check the map.

Monday, October 13, 2008

Precision GPS Data from Dogleg Bend

Last week, I spent some time collecting presumably high-precision GPS data with my new Trimble GeoXH unit. The first dataset that I have corrected is from Dogleg. Now we have some pretty robust numbers for incision rate calculations.

Grand Yeehow challenge. Someone out there break down and do the calculations. Post it to the blog and ruminate on it. My guess is that it will be sort of high.