Wednesday, February 10, 2010

Road to Jarbidge: South into Nevada!

And so, I relaxed at the Jarbidge Forks, before turning back to my truck. It was the second day on my journey to Jarbidge, and I was going to make it this time!
But first (from the first day), we'll take a quick look to the north, downstream in the canyon of the Jarbidge River. The geology of the canyon walls is a bit deceptive. The rocks are dark-looking from a distance, so it's easy to imagine they are all basalt. It's not that simple, though. The thin flow on the distant canyon rim in the above picture is composed of a basalt mapped as Tbf, basalt of Big Flat, on this map, a portion of which is shown below. All the rocks you see in the foreground on the right side of the canyon (on the east)—all those chocolate-colored brown rocks—are part of a thick, widespread, high-volume rhyolite mapped as Tdc: the rhyolite of Dorsey Creek, sometimes called the Dorsey Creek Rhyolite. It erupted from the Bruneau-Jarbidge eruptive center.
1 This southeast part of the mapped area just barely shows the Jarbidge Forks area: it's the easternmost canyon junction, located just a little below the center of this map snippet, below a "Tbf" label.
Looking upstream to the southeast (a view from the second day) we see a similar geologic situation in the East Fork of the Jarbidge River. The map shows that the uppermost flow is still the basalt of Big Flat. The basalt is underlain by a section of Tsl—lower undivided sediments—with the rhyolite of Dorsey Creek below that, underlain by another section of Tsl. The photo doesn't really show these units that well, but was taken more as an overview shot.
And finally, at about 12:30 pm on the afternoon of the second day, I left the Jarbdige Forks and forayed south toward Jarbidge. The flow on the canyon rim, if we are still inside the mapped area, may be Tda: the basalt of the Diamond A Desert. Like the basalt of Big Flat, this younger baslat is underlain in this area by the rhyolite of Dorsey Creek. The rhyolite pinches out rapidly to the south and is replaced by a thick section of Tsl, likely the slope former in this photo. I took the photo because of the cirrus clouds, and wasn't really thinking about the rocks at the time. Location of the photo is inexact due to a failure of my GPS-track–saving system.

The basalts are Pliocene or Miocene, the lower undivided sediments are Pliocene or Miocene (and Miocene where below the rhyolite of Dorsey Creek), and the rhyolite of Dorsey Creek is Miocene, dated at 8.1 Ma.
I then left Idaho and entered Nevada. There was, for once, a nice pullout—so I pulled over and took a photo of the Idaho sign, looking back to the north. The road was wider in Nevada. Yay!
While at the pullout, I shot a couple pictures of the hoodoos on the west side of the canyon—unknown formation, unknown rock type.
It was fall...
...beautiful, beautiful fall.
At 1:00 pm on the second day, I arrived in Jarbidge. The sign says: *JARBIDGE* PLEASE SLOW DOWN, THIS IS OUR TOWN.

Some References:
Bonnichsen, Bill, 1982, The Bruneau-Jarbidge Eruptive Center, Southwestern Idaho, in Bill Bonnichsen, and Roy M. Breckenridge, eds, Cenozoic Geology of Idaho: Idaho Geological Survey Bulletin B-26. p. 237-254.

Bonnichsen, Bill, and Breckenridge, R. M., eds., 1982, Cenozoic Geology of Idaho 1982: Idaho Geological Survey Bulletin B-26, 725 pp.

Bonnichsen, Bill and Jenks, M.D., 1990, Geologic map of the Jarbidge River Wilderness Study Area, Owyhee County, Idaho: U.S. Geological Survey, Map MF-2127, scale 1:50000.

Cathey, H. E., and Nash. B. P., 2009, Pyroxene thermometry of rhyolite lavas of the Bruneau–Jarbidge eruptive center, Central Snake River Plain [abs.]: Journal of Volcanology and Geothermal Research, Volume 188, Issues 1-3, p. 173-185.


Gaelyn said...

You sure do get around in some gorgeous country. But it sure looks like the middle of nowhere and makes me wonder if the "town" of Jarbridge really exists.

tony edger said...

Really liked the picture of the hoodoos. Hope this question makes sense. Is the picture showing "decapitated" hoodoos in the foreground with a continuation of complete ones in the background?

Silver Fox said...

Gaelyn - it is most assuredly the middle of nowhere (although not that far by miles to Twin Falls).

Silver Fox said...

Tony, the picture shows two beds, lower and upper, both are probably volcanic flows or tuff beds, or they are part of a thick, mostly volcanic section. The center slope is probably a softer layer that doesn't crop out.

The lower bed has had less time that the upper bed to weather into hoodoo shapes, because it was exposed by downcutting of the Jarbidge River more recently than the upper one was. If downcutting continues, or just more erosion of the slope in general, the lower bed could develop into hoodoos in the geologic future, especially if it is as similar to the upper bed as it looks without a better examination (with rock hammer!).

So the lower bed doesn't consist of decapitated hoodoos the way it kind of looks like; instead, possible or future hoodoo shapes haven't developed yet. I don't really know how long it takes for hoodoo shapes to form, and the time needed would depend on various things including rock type.