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.
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.