Tuesday, April 16, 2013

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.