Wednesday, November 23, 2011

The Contact at Contact

We continued on toward the small bordertown of Jackpot, Nevada, having passed out of the Great Basin by dropping into the drainage of Willow Creek – which joins with Dry Creek, which joins Jakes Creek, which enters Salmon Falls Creek near the railroad siding of Henry. Salmon Falls Creek then flows north, eventually emptying into the Snake River below Twin Falls. Skies were clearing somewhat, and snow was no longer thretening.

A few miles south of Henry, China Mountain rises into view east of Highway 93. China Mountain is a good place to see an intrusive-sedimentary contact at 70 mph. The generally rubbly or knobby looking rocks on the far north side of China Mountain are outcrops of the granodiorite of the Jurassic Contact pluton. It’s intruding late Paleozoic limestone and marble (and possibly other associated sediments and metasediments): the layered and moderately dipping beds across most of the mountain.
Here I’ve drawn the approximate contact in hot pink. Granodiorite is to the left (Jgd on this map); limestone and marble are to the right (PMl on the same map). Where I’ve ended the contact, it dives under undivided Miocene to possibly Pliocene tuff, ash, and sediments (mapped as Ts3).
The granodiorite of the Contact pluton shows some spectacular spheroidal weathering between Henry and the small outpost of Contact. This view to the west of the highway, taken just a couple minutes beyond that last photo, shows the granodiorite in the foreground, with Ellen D Mountain in the background. Ellen D is underlain by the same late Paleozoic sediments seen on China Mountain; the contact between the granodiorite and the sediments runs behind the wonderfully knobby mass of granodiorite and the base of Ellen D Mountain.
What isn’t clear from the highway, is that the intrusive contact passes eastward from the present site of Contact, runs west toward Ellen D Mountain, then curves southward. All along that contact, the topo map of the area (from MSRMaps) shows a series of old mines and prospects dug into mineralized, contact-metamorphosed rocks right along the contact. These are some of the mines and prospects of the Contact mining district, which were prospected originally for gold, but have been mined mostly for copper. The original mining town of Contact was moved from its original site fairly early on prior to being relocated a second time at its present location.
While we drive by Ellen D Mountain, the highway passes through some roadcuts in the Contact pluton. The knobby knob on the left and the low mass on the right are in granodiorite. The reddish brown hill to the far right is in Tts: rhyolitic ash-flow tuffs probably about 10 to 12 million years old, possibly erupted from the Bruneau-Jarbidge eruptive center to the west.
After stopping briefly in Jackpot, to gas up and grab a snack, we crossed the Idaho line at 1:20 pm PDT, 2:20 Mountain Time. Jackpot is not the only town in Nevada that operates on Mountain Time.

Oh, by the way, here's my take on the naming of the various contacts. First, the Jurassic granodiorite intruded the Paleozoic limestone. Later, uplift exposed the contact between the Jurassic and Paleozoic rock formations. Much later, people came along prospecting for gold and copper; they were attracted to the contact for various reasons, not the least of which was probably some nice float of copper oxides and copper carbonates. When they staked claims, they listed the claims as being in the Contact mining district, which they named after the mineralized contact they were so hot on. They set up a town and called it Contact, after the mineralized contact and the mining district. Eventually, all the nearby mining districts in the area were consolidated into one district called Contact. The order of the naming and founding of the latter two (Contact the mining district and Contact the town) is entirely speculative on my part.

I have a bit more information on this part of Highway 93, from south of Wells to the stateline, in an earlier post, here.

Map References:
Hope, R.A. and Coats, R.R., 1976, Preliminary geologic map of Elko County, Nevada: U.S. Geological Survey, Open-File Report OF-76-779, 1:100,000

Sheet 3 of the above, covering our area of interest in the upper left.

Sheet 6 of the above, the explanation for the entire map.

Sunday, November 20, 2011

Sand Mountain for Sand Dune Week

Two dune posts in a row in a row creates a geoblogosphere meme, and Sand Dune Week has been proclaimed!
Sand Mountain — located on Highway 50 about 25 miles east of Fallon, NV — is a perfect candidate for Dune Week, being one of those booming or singing dunes described here on Cocktail Party Physics.

View angle, time of year and day (sun angle), and weather all affect the seemingly shifting appearance of Sand Mountain, sometimes dramatically. The first photo (above)was taken from the west-southwest during the middle of an early March day last spring, 2011.
This second photo was taken from the south in the late afternoon in late May, also 2011. Enlargement of the second photo will reveal a barely visible white line on the east side of the dune: the RV city usually sitting in the lee of the dune, where ATVers gather to run up and and down the face.
Here's the mountain in a wind storm, a late afternoon in mid-May of 2009. Any RVs present are getting sand blasted!
Lighting for pictures can be quite variable, and I often get flat pictures just driving by at random times of the day. But even mid-summer can be good for shadows and details, as shown by this photo taken early one morning in early July, 2010. Enlargement of this photo shows two tiny-looking ATVs on the west side (far left) of the dune, behind and a little to the right of a foreground dark spot that is possibly a rock or large bush. Look carefully!

Although I've driven up to the RV city and to the half buried Sand Springs Pony Express station, I've never gone on the dune to try to make it boom.

One thing that can be done fairly easily, as part of this mini virtual field trip, is to follow the sand from Sand Mountain back to its original source, back to the southwest across a mountain or two, then across Highway 95 south of Fallon, southwest toward the delta of the Walker River, where the river emptied into ancient Lake Lahontan. Start with this map and this air photo, then go here (scale change), then here, and then here (direction change). The last air photo is mostly below the high stand of Lake Lahontan, but shows the final current destination of the Walker River: Walker Lake.

Meme participants so far:
Galileo's Pendulum
Clastic Detritus
Cocktail Party Physics
In the Company of Plants and Rocks
Agile* - Journal
Ron Schott's Geology Home Companion Blog
Research at a snail's pace
Earth Science Picture of the Day
Pools and Riffles
Earth-like Planet
En Tequila Es Verdad
Through The Sandglass
EarthSky // Blogs // Space (Deborah Byrd)

Some References:
La Pointe, D.D., 2000, Earth Science Week 2000: Geology along America's Loneliest Highway. A field trip for families and rockhounds: Nevada Bureau of Mines and Geology Educational Series E-37.

Trexler, D.T., and Melhorn, W.N., 1986, Singing and Booming Sand Dunes of California and Nevada: California Geology, v. 39, no. 7, p. 147-152. Also reprinted here on Tom Sweich's webpages.

NOTE: The California Geology search page is quite finicky or counterintuitive. I finally searched using only the year, 1986. What also worked was %California% %Nevada% with the percent symbols, entered as keywords under "Title of Article."

Friday, November 18, 2011

What I'm Considering as an Xmas Gift to myself... replace my aging Canon A650 IS camera:

A650 IS specs at Canon

G12 specs at Canon

D10 specs at Canon

ABCDeals refurbished A650 $450

Refurbished A650 "from China" $290

A650 at Amazon, varied prices

G12 at Amazon $430

I really like my A650 IS, I have had it cleaned or fixed once (dust or spots internally on the lens or mirror, a glitch that somehow develops during airline flights). It's a great camera that has already survived one E18 or "Lens error, restart camera" event, which I fixed by carefully whacking the camera, as per the percussion advice on some forum (maybe this one; other options were send in for repair or take the camera apart and mess around). I'm now facing a growing spot of no pixels on the vari-angle LCD screen (vari-angle is a higher requirement for me than AA batts). I'm sure Canon can fix this, but I don't know at what cost (yet). In the meantime, before I send my camera off, I'll need another camera (though I could pull out my old A620, like I did in the spring of 2010 when I last had the A650 in for repair). eBay currently has A650's that generally sound no better than the one I already have.

A note: Xmas always comes early when I buy something like this for myself.

UPDATE 20Nov2011: Nikon Coolpix P7100 review

UPDATE 28Jan2012: Just ordered a Canon G12. This option retains the vari-angle LCD screen, adds slight weight and height, aperture drops from max 4.8 to 4.5, total zoom drops from about 210 mm to 140 mm, wide angle increases from about 35 mm to 28mm, megapixels drop from 12 to 10, have to use Canon battery packs, gain High Dynamic Range shooting.

Tuesday, November 15, 2011

A Wintry Trip North

Over the weekend, thinking that it wasn't really wintry enough already, we took off to the north...
...eventually heading directly into the fast-moving storm that was just clipping the northeast part of the state, not leaving a lot of snow in it's wake, but blowing like crazy.
Light and dark patches northeast of Highway 93: the storm moves across Clover Valley south of Wells, Nevada.
Sunlight comes through light snow showers on the west side of Spruce Mountain Ridge, a narrow basin-and-range ridge east of the East Humboldt Range.
Still south of Wells, somewhwere near the turnoff to Ruby Mountain Brewery, we drove by a small herd of elk grazing on the other side of the right-of-way fence.
Over the last couple years, NDOT has installed a couple wildlife overpasses between Wells and Jackpot. This is the one at HD Summit (MSRMaps location).

At this point, Highway 93 is still inside the Great Basin, within the watershed of miles-long Thousand Springs Creek, which eventually leaves Nevada for Utah and the Great Salt Lake Desert northeast of the small town of Montello.

About 10 miles north of HD Summit, Highway 93 enters the drainage of the Snake River after crossing an unimpressive drainage divide atop a small set of east-west hills, south of the "93" on the MSRMaps image below, and here looking north on Google Street View. I couldn't quite locate this major drainage divide while we were driving south, but instead probably thought it was in the small hills just north of the "93" on the MSRMaps view.
MSRMaps image courtesy of the USGS. One mile is shown by the black section line running east-west between the red, north-south township border to the left of Highway 93 and the black, north-south line to the right of Highway 93.

At this seemingly insignificant drainage divide, the internally draining Great Basin ends. The Basin and Range Province, which overlaps the smaller Great Basin, continues a ways to the north, merging gradually with the plateau country on the southern edge of the Snake River Plain.
Scenes like this always make me want to drive off road, but we didn't. Instead, the pavement continued north, and so did we...