Tuesday, December 14, 2010

Geology on the Road: Flow-Banded Rhyolite with Breccia

This hydrothermally altered flow-banded rhyolite, with thin layers or bands dipping gently away from us to the east, is in the eastern arm of the Clan Alpine Range of central Nevada, within walking distance of Highway 50 and not far from The Shoe Tree. To get there, from Highway 50 just west of The Shoe Tree, turn north at the sign that says Bench Creek Ranch 13 miles. The dark brown jutting outcrops showing cavernous weathering are immediately to your right as you pass through a corral, just east of the large bend in the graveled road (Google Maps, Street View from Highway 50). The main graveled road will take you farther into the Clan Alpine Range and to Bench Creek Ranch; side roads lead off in many interesting directions. This locality is about 500 meters west of Middlegate the geographic place (MSR Maps).

For more on flow banding, see this Short Primer at the relatively new geoblog, Chaotically Flow-Banded.
Looking upward from my shoe (which I didn't leave at The Shoe Tree), you'll see an irregular contact between the lower, whitish to pale orange flow-banded rhyolite and a mass of dark, iron-oxide-stained rhyolite breccia. The contact is marked by green grass. Above the breccia near the top of the photo, we get back into light brown flow-banded rhyolite, this time with flow foliation dipping off to the southwest.
The contact between the lower flow-banded rhyolite and the rhyolite breccia is irregular but generally vertical, seen best when looking along the contact toward the north, where it runs into the near vertical crack on the right side of this photo. The vertical nature of the contact suggests a possible fault, as does the presence of breccia. We need to remember, however, that volcanic rocks can do weird things — they can show strange and unexpected fabrics — because they are created in the tumultuous environment of... volcanoes.

In this particular instance, we happen to be near the southern margin of the large volcano-tectonic trough that runs east-southeast from the Stillwater Range, through the Clan Alpine Range, through the Desatoya Mountains, and into the Shoshone Mountains. The trough is about 40 km wide and 100 km long, although it has been extended somewhat by Basin and Range faulting and wasn't originally that long. This southernmost of two volcano-tectonic troughs in central Nevada formed in the late Oligocene to early Miocene, with the likely generation of several large or very large overlapping calderas (likely VEI 7 to 8). Volcanic rocks in the Clan Alpine Range erupted in two pulses between 29 and 30 and 24 to 25 million years ago (Burke and McKee, 1979). A better place to view the southern margin of this volcano-tectonic trough would be south of Eastgate in a western arm of the Desatoya Mountains and south of Highway 50 in the main part of the Desatoya Mountains.

The breccia could be both volcanic and tectonic in origin, and I suspect that at least some of it is volcanic. Fragments range in size from a less than a couple centimeters to over a meter wide.
MOH and I were out exploring, already wandering off our intended path, so we took the time to climb (I mean climb, not hike) to the top of the brownish hills. We found more volcanic breccia and a lot of flow-banded rhyolite. Here the lichen-covered flow banding is dipping to the southwest.
And here in this small cliff, the banding is entirely vertical, possibly giving way upward to brecciated rhyolite (cheat grass in the lower left for scale).

Burke, D.B., and McKee, E.H., 1979, Mid-Cenozoic volcano-tectonic troughs in central Nevada: Geological Society of America Bulletin, v. 90, p. 181-184.

Steward, J.H., and Carlson, J.E., 1976, Cenozoic rocks of Nevada--Four maps and brief description of distribution, lithology, age, and centers of volcanism: Nevada Bureau of Mines and Geology Map 52.

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