Things You Find in the Field: Screwy Contacts?

While wandering up one of our hiking hills back in mid-March, I noticed some orangey brown rocks that at least in part seemed to be cutting across the general trend of beds in the dark, brownish gray quartzite. Are these dikes? Maybe just some slumping or sliding of large boulders? Or maybe it's just the angle of my view.

For the first photo, I'm standing between the first switchback and the first large outcrop of quartzite (next photo). The first switchback is at Location 1. Locations 1 through 8 are identified in the final Google Earth image at the bottom of the post.

View from higher on the hill, looking across the canyon.

Now, having rounded the bend of the first switchback and made it up a very rocky jeep trail, I'm standing in the shadow of a large, jutting outcrop of quartzite [Location 2]. A narrow bed of quartzite holding up a ridge across the canyon runs parallel to some vaguely orange-brown, scrabbly-looking or blocky-weathering material [at Location 8].

We wander past more quartzite, hoping to find our mysterious, unknown rock around the bend. Really. That's what we do on hikes.

Aha! A pale yellowish to light brown rock in contact with our darker, brownish gray quartzite! Right there in the road!!

The rock above the road on the skyline is made of the same stuff. What is it? [Contact is at Location 3.]

It's a dike rock!

What I hold in my hands is a white to pale grayish yellow to moderate yellowish orange and light brown rhyolite porphyry with conspicuous quartz phenocrysts.

Now I'm standing on the dike above the drill pad [Location 4] from a day or two ago, looking south across Water Canyon [Location 8]. A fuller explanation of what we're looking at can be read here, with contacts drawn in.

From that same perch just above Location 4, I look west across a field of orangey brown outcrops of more of the same rhyolite porphyry, toward the dark brown quartzite knobs [Location 5] near the west end of the road.

Here's the view from the quartzite knobs [Location 5] seen in the last photo, looking east up Water Canyon and across the same field of dikes and quartzite outcrops [toward Locations 3 and 4].

Most of the exposures in this photo, saving ones in the foreground and a few on the far left and right (and on hills in the far distance), are of rhyolite porphyry.

An unexpected rock wall [Location 6] cuts diagonally across the photo, running from the foreground outcrops of quartztite to a couple middle-ground outcrops of porphyry. In the previous photo, the wall had looked more like a dike. Trust me when I say it isn't! (More on that later.)

Having reached our hiking and geologizing goals, we'll now head back down the hill.

From the middle of the hill, we look down toward Water Canyon across a few bouldery outcrops of orange-brown porphyry and dark gray quartzite [at about Location 7], with more massive exposures of quartzite lower on the hill.

Now we've arrived back at the first switchback [Location 1], and I realize the outcrops below it are made of porphyry.

What we see directly across the canyon at Location 8 is mostly porphyry, with a couple thin beds of quartzite running straight up the ridge.

Looking back to the north from just across the canyon.
(This photo is from a hike on a more recent day).

The first switchback [Location 1] is just right and uphill of the bold porphyry outcrops on the slope just above the trees. These outcrops of mostly porphyry seem to contain a few blocks or beds of quartzite, although I haven't walked the area to know for sure. The orange-brown outcrops on the far skyline left of center include the exposure at the drill pad [Location 4] that we saw in the earlier post. A couple irregular dikes cut across the middle hill, connecting the switchback exposures to the drill pad exposures.

The contacts don't seem so screwy when you know what's going on!

Google Earth image of our area, with north to the right.

Numbers of locations referenced in the post.

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).

References:
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.