Wednesday, January 9, 2008

Rock of the (mid) week: mylonitic quartzite

I'm going to jump on the rock-pictures bandwagon, and try to post a rock photo (or outcrop, or thin section - rocks are interesting at all scales) once a week. I was almost ready to post these photos Saturday night, but then my power went out.

I've got two photos this week, of the same rock - one from the side, and one looking down from the top. (I just told my Structural Geology class how important it is to look at an outcrop from all sides. I figure I can try to model that behavior in public.)

These are the rocks that I blogged about in my first post here (which is going to be included in The Open Lab 2007). At first (and probably second and third) glance it looks like a typical quartzite - quartz, and maybe some thin layers of hematite or magnetite or some other heavy mineral, and not much else. And, well, that's what it is... except that it's been thinned to somewhere in the neighborhood of 1/10 its original thickness. Those black lines were probably originally cross-bedding, but I certainly can't tell it from the outcrop. (The evidence for the stretching comes from a comparison with undeformed rocks from the same unit, the Prospect Mountain Quartzite, in nearby ranges, and from the shapes of deformed pebbles in a conglomerate below this unit.)

The top surface shows the black streaks that define the lineation in the rock - the evidence of the direction in which the rock was stretched. It's subtle in this photo, and is often a really subtle feature. (Thermochronic's garnet-sillimanite gneiss also shows a fabulous stretching lineation, maybe more obvious than this one.)

The third piece, which I guess I'm just going to have to supply after I cut a thin section of the rock, would be to show the microscopic texture.

If you want to visit this location, follow the directions from either of these field guides:

Gans, Phillip B and Miller, Elizabeth L, 1983, Field trip 6; Style of mid-Tertiary extension in east-central Nevada, in Gurgel, Klaus D., ed., Geologic excursions in the Overthrust Belt and metamorphic core complexes of the Intermountain region; Guidebook, Part I: Special Studies - Utah Geological and Mineral Survey, vol.59, pp.107-160.

Miller, Elizabeth L, Gans, Phillip B, and Lee, Jeffrey, 1987, The Snake Range decollement, eastern Nevada, in the GSA Cordilleran Section's DNAG field trip guide.

In both guides, it's the Hendry's Creek stop. Out in the middle of nowhere, but still a great field trip stop.


Thermochronic said...

Excellent photo! I've wanted to get to this area for a while now, but never made it. One of my thesis chapters was on some work I did in the Deep Creeks just to the south, but (and I am ashamed to admit it) I worked with samples other folks collected, I've never been out there.

Kim said...

It's surprisingly easy to get there - well, once you've driven US 50 to the Nevada/Utah state line.

I should do my ELM impression and give you a hard time about not mapping every place you collected a sample. :D But I won't. I don't think I've pulled out that impression in a long time.

Andrew Alden, Oakland Geology blog said...

This is Stanford's EL Miller? I'll bet she could really make a student squirm.

Route 50 is on my big road-trip list. When's the best time to go?

Kim said...

Andrew - yep, that's her. She was my PhD advisor.

I went to the Snake Range in late May (Memorial Day weekend). It was already hot at low elevations, and I didn't go up high. Stanford's field camp used to map there in July, but they worked up high a lot.