Oregon Trip Day 7: The Tuff of Smith Rock

As I mentioned earlier, I'm skipping a few Oregon trip days, primarily because not much happened in the geological realm during that time; we've now gone from Day 4 to Day 7. I'm also going to break Day 7 into two or three posts.

On the seventh day, four of us took a day-long field trip around the Bend-Prineville-Sisters area; our first stop was at Smith Rock State Park.

Throughout the day, we saw lots of volcanic rocks (can you imagine?), and I became fond of taking pictures showing more than one volcano or more than one age of volcanism. The picture above, looking southwest, shows the tuff of Smith Rock in the foreground, a rhyolitic ash-flow tuff about 29 to 30 million years old (McClaughry et al, 2009, page 39). The snow-capped mountain in the background is Broken Top, a complex stratovolcano of the Three Sisters–Broken Top area. Broken Top is much younger than the tuff of Smith Rock, and it's heydey as a composite volcano was probably between about 300,000 and 100,000 years ago. As part of the High Cascades, the Three Sisters–Broken Top area is potentially active.

The black specks in the first two photos are turkey vultures.
Looking along the Crooked River back to the northeast, we can see the tuff of Smith Rock dipping more or less to the south or southeast, with the dips being shown by the tops of the cliffs in the background hill. The dark rocks in the foreground are basalt: columnar-jointed flow rock from the Newberry volcano south of Bend. The basalt is about 500,000 to 300,000 years old.
I had to zoom in on these fine examples of the tops of columnar joints.
We've now moved around and are looking back to the southwest again, with a picture nearly identical to one shown on page 38 of McClaughry et al, 2009, not that I planned it that way. Once again, you can see the buff to brownish ash-flow tuff dipping in a southerly direction (left), marked by layering especially near the top of the buff to brown cliff. You can also still see the dark flows of the much younger Newberry Basalt around the bend of the river. That's 30 to 29 million year old rock (tuff) next to 0.5 to 0.3 million year old rock (basalt).
In a closer view, we can still see a little of the layering in the high cliff on the right side of the photo, with the layering being compaction foliation in this ash-flow tuff. The darker brown, fractured looking rock in the middle ground of the photo, jutting out in spires lower and closer to the river than the layered cliff (behind the bridge from this viewpoint), is a rhyolite dike intruding the tuff of Smith Rock (McClaughry et al, 2009). It looks to me as though some of the rock in the far distance, also darker and showing vertical fracturing, could be dike material, but it was reported as tuff in the 2009 field guide, and I didn't cross the river to examine it. I did, however, align two major fractures or contacts in the rock in the foreground with similar looking (from a distance) fractures in the background, but don't know if they are related. Another field trip and some geologic mapping are in order!

It's now thought that the tuff of Smith Rock at Smith Rock is the intracaldera tuff of the huge Crooked River caldera. For comparison, the aerial size of the Crooked River caldera is given here (page 27 has a nice graphic) as larger than the Long Valley caldera in California and smaller than the McDermitt caldera complex of northern Nevada. This is a caldera of true "supervolcano" size.

As a bonus, this tuff, and other widespread air-fall and ash-flow tuffs that erupted from the Crooked River caldera, correlate with tuff units in the John Day Formation. We saw at least one of the younger ash-flow tuffs at Sheep Rock: the so-called "Picture Gorge Ignimbrite ." From what I understand, McClaughry et al, 2009 correlate the Picture Gorge Ignimbrite with what they call the Member H tuff, which may also be correlative with the tuff of Barnes Butte. (I think one needs to be a little more familiar with the stratigraphy of the John Day Formation to follow these various regional names and correlations.)

A few online reports describe Smith Rock as part of a much smaller rhyolitic tuff ring; apparently that interpretation has been superseded by McClaughry et al, 2009, which provided much of my information about Smith Rock. Their field trip guide is well worth reading, and I wish I had read it prior to going out there!

Main References:
McClaughry, J.D., Ferns, M.L., Gordon, C.L., and Karyn A. Patridge, K.A., 2009, Field trip guide to the Oligocene Crooked River caldera: Central Oregon’s Supervolcano, Crook, Deschutes, and Jefferson Counties, Oregon: Oregon Geology, v. 69, no. 1, p. 25-44

USGS Cascades Volcano Observatory.

Trip report to be continued...