Wednesday, August 4, 2010

Oregon Trip Day 2: A Hike near Jarbidge across Two Rhyolites

Photo: Jarbidge mine dumps and fireweed.

On our long trip to and from Oregon, we dedicated our only full day in Jarbidge to hiking, although we also walked around town and up canyon in the evening. Overall, it was a good day, with clouds providing some relief from the hot sun, while also contributing to an unusual dampish humidity.
The steepness and plant overgrowth at the base of the hillslopes in the canyon of the Jarbidge River forced us onto a road — the road shown as a jeep trail on this map. We hiked from the base of Moore Gulch, past Cottonwood Spring, to the saddle overlooking Jenny Creek. The road was steep and rocky, especially over the first mile, and partly followed an old powerline.
Photo: Jarbidge Rhyolite in the the road.

The layering in the slabby bedrock sticking out of the road is probably flow layering; the rock is the Jarbidge Rhyolite — a crystal-rich rhyolite, often hydrothermally altered, which hosts the gold-bearing quartz-adularia veins of the Jarbidge mining district. The Jarbidge Rhyolite is about 15.5 to 17 Ma (from two dates by Coats, 1987, from elsewhere in northeastern Nevada).

Notice the rocks scattered over the road. The rhyolite weathers to a coarse grus-like material that made going uphill interesting and going back downhill downright slippery.
Photo: Hand sample of the Jarbidge Rhyolite.

This is the culprit! The yellow-brown to reddish-orange to brownish weathering, pink and white, crystal-rich rock weathers into sub-rounded chunks, creating a road surface remindful of a bunch of ball bearings.
We stopped for many breathers, especially after surmounting the lower, scrabbly part of the road. This view, across the canyon of the Jarbidge River toward the west, looks along an old, defunct powerline.
Oh, even more steep and rocky clambering. Is that shade up ahead?
About an hour into the hike, we came to a patch of aspens and other trees. The road flattened out a little, and the rocky ball-bearings disappeared.
The linear groves of aspens, sub-alpine fir, limber pine, whitebark pine, and possibly white fir (a second, unidentified fir-like tree downhill from us, not reported online as part of the Jarbidge flora) were just rich with wildflowers.
Needless to say, we took some time walking through this green, shady scene.
Mariposa lily, possibly Calochortus bruneaunis AKA Bruneau mariposa lily.
Unknown, but seen especially on the lower slopes.
Beyond the trees, we go ever upward, but at a more leisurely pace.
Wildrye, what I used to know as Elymus, probably now Leymus cinereus or Basin wildrye.
Photo: Partly devitrified, flow- or compaction-foliated rhyolite.

As we approached the saddle, the soil became dark, apparently from weathering of a dark gray to brown, glassy, flow-banded rhyolite, the Cougar Point Tuff, which is about 10.5 to 13 Ma. The strongly flow-banded rhyolite was variably devitrified, locally forming gray, subrounded balls similar to Apache tears, but dull and much less glassy in character. The banding or foliation could be the result of compaction or flow or both.
Photo: Folded flow-foliation in the Cougar Point Tuff near the top of the saddle.
Photo: Flow lineations in the flow-banded rhyolite.

Flow lineations can be seen above my boot and below the whitish-looking lichen. This kind of flow foliation in extremely welded or high grade ash-flow tuff is common in high-silica to peraluminous welded tuff formations, making it difficult to distinguish these types of tuffs from rhyolite flows in the field.
Photo: Brownish outcrop of Cougar Point Tuff.

After walking to the saddle, in hopes of seeing a grand view to the north or west but instead finding only another canyon, we sat down and rested against some brown-weathering flow-banded rhyolite.
We took in the view looking southwest toward Jarbidge, and then walked down. You can see a couple of the linear, downhill-oriented patches of evergreen trees. Surprisingly (at least to me), a few of the subalpine fir make it almost all the way down to the canyon floor.

Elevation gain: about 850 feet. Distance total: about 5 miles. Time from start to finish: about 3 hours. Both gain and distance include walking to and from the motel. Other plants found besides those mentioned: sagebrush, Woods' rose, antelope bitterbrush, alder or water birch., and many, many unidentified or unmentioned others.

Trip report to be continued...

Some References:
Bonnichsen, Bill, 1991, Geology of Scenic Jarbidge Canyon Near Murphy Hot Springs, Idaho: Idaho Geological Survey, GeoNote 16, 2 p.

Cathey, Henrietta E, and Nash, Barbara P., 2004, The Cougar Point Tuff: Implications for Thermochemical Zonation and Longevity of High-Temperature, Large-Volume Silicic Magmas of the Miocene Yellowstone Hotspot: Journal of Petrology, v. 45, no. 1, p. 27-58.

Coats, R. R., 1987, Geology of Elko County, Nevada: Nevada Bur. Mines and Geology, Bulletin 101, 112 pp (links to sales info).

Jewell, P. W., Rahn, T. A., and Bowman, J. R., 1994, Hydrology and Chemistry of Thermal Waters Near Wells, Nevada: Ground Water, v. 32, no. 4, p. 657 - 665 (links to abstract).

Flow banding and rheomorphic deformation at Magma Cum Laude, January, 2011.


Gaelyn said...

Such great contorted shapes with this Rhyolite. I remember walking on pumice into the crater that was like marbles 100s of feet deep. Yet the views and flowers are stunning, and I'm sure the shade much appreciated.

Julia said...

Fantastic walk - I can see why people compare the northwest USA with parts of the UK.

I think your unidentified pink flower is a Geranium species, but I'm not that good on stinkin' flowers, so that's as far as I'll go!

Silver Fox said...

Julia, thanks for the clue on Geranium. I think it's probably either Geranium richardsonii or Geranium viscosissimum.