Friday, September 26, 2008

Friday Fault Photos #4

As you drive east on Highway 50 through central Nevada, between Fallon and Austin, you will eventually have to cross the Desatoya Mountains, and you will do so in a narrow canyon called New Pass, which leads up to New Pass Summit. A couple short miles after entering the canyon you will come upon this magnificent, hoodoo-like roadcut exposure of a fault. As you can see, it's located just a bit east of Churchill County Milepost 104.
Here's a view from just a little bit closer. The top of the hoodoo, from the ground up, is about 20-30 feet high. Don't be afraid! Just because it's hoodoo-like doesn't mean it will bite you! In the above photo you can see a fairly sharp fault plane, a white line that appears to bisect that lonely pinon tree.
The fault dips moderately steeply away from the road and toward the tree (northeasterly), and the fault plane strikes sub-parallel to the road (about northwesterly). The hangingwall - the side to the left above the fault plane - is composed of light-colored, almost white ash-flow tuff; the footwall - the side to the right below the fault plane - is composed of dark greenish or green-stained rock. The rock really is a kind of yucky green.
Now, just drive around until you find a place to park, and walk up to the fault, which is seen in the photo above from the other side.
The white hangingwall tuff, this time on the right side, has been fractured with joints parallel and perpendicular to the fault. The green footwall rock is brecciated, with the breccia composed of pieces of ash-flow tuff. All the ash-flow tuff in this canyon is supposed to be New Pass Tuff, which is about 22 million years old, or early Miocene.
Above, a close-up of the footwall breccia.
While you are standing there, you will be able to follow the fault down to ground level, as seen above, by following the color contrast between the light-colored hangingwall and the greenish footwall. As the fault approaches the roadside ground level, the footwall breccia becomes a little more chaotic and sheared looking.

NBMG Report 40 - Sediment-Hosted Precious-Metal Deposits of Northern Nevada - Road log/trip guide for SEG precious metals field trip, fall 1984, by J. V. Tingley, H. F. Bonham, Jr.
NBMG Bulletin 88 - Geology and mineral deposits of Lander County, Nevada; Part I, Geology, by John H. Stewart and Edwin H. McKee, Part II, Mineral deposits, by Harold K. Stager, 1977.


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Callan Bentley said...

What's the green stuff?

Silver Fox said...

I'm not really sure. I was expecting something like celadonite, and then after examination thought the color was wrong - but apparently it can be a range of greens. It's very fine-grained and seems to have soaked into the rock in large, round liesegang banded masses, though sometimes it looks like it is dripping down outcrops or coating fractures like a stain.

It could be celadonite, maybe chlorite, some greenish clay, or something else. The color in places is remindful of As- and Sb-oxide stains I've seen in gold country, and certainly this rock could contain high amounts of arsenic. Those oxides could combine with Fe-oxides for quite a range of colors, I think.

Probably need some geochem and x-rays!

Andrew Alden, Oakland Geology blog said...

What a beautiful example of a fault. I'm looking for an excuse to drive route 50 next month, but I'll probably visit the Needles area instead.

Did you say what the sense of displacement is?

Silver Fox said...

Andrew, slicks on a nearby more-or-less parallel fault (or offset portion of the same fault?) exposed lower on the hill and just a bit west (still east of the 104 Milepost) are dip-slip. I don't know the sense of offset, although pluck marks make it look normal, down-to-the-northeast.

Let me know if you come by on 50 and when, if you don't go to Needles. I could be driving through that canyon again in just about a month.