Tuesday, April 25, 2017

From the Road: Some Nice Gneiss

Every time I drive Highway 6 between Idaho Springs and Golden, Colorado—and I almost always take that route when heading east, rather than the section of I-70 between Idaho Springs and the hogbacks—I try to get a few photos of the great gneiss forming the canyon walls. If I'm going eastward, it can be hard to stop because I'm almost there. This time, in the fall of last year, I was heading west, but I still found it hard to stop, partly because of traffic and unexpected road work. Nevertheless, I managed to make a few pullouts that were rewarding.

The first shot is typical of the biotite gneiss that underlies much of the region: darker bands with more mafic minerals, and lighter bands composed almost entirely of felsic minerals.
Here's a little more of that same biotite gneiss showing complicated deformation.
The geology of the area, seen below in a cutout from a USGS map, looks a little complex! Our first two photos (the two easternmost dots that are rather close together) are in what is mapped here as Xb: Proterozoic biotite gneiss.
Map courtesy USGS (Kellogg et al, 2008).
A little farther into Clear Creek Canyon, I pulled over and grabbed a couple shots of this second roadcut (single dot to the west, above), primarily because of all the light-colored dikes and masses. These rocks are probably what is mapped as Xh: hornblende-plagioclase gneiss and amphibolite.

All of the rocks we're looking at today are thought to have originated as sedimentary and volcanic rocks that were deposited in a basin 1780 to sometime after 1750 Ma (million years ago), as per this pamphlet accompanying the map.
A shot of the entire roadcut. 
Somewhat darker gneiss is above somewhat lighter gneiss in this roadcut. The darker portions may qualify as amphibolite, which is part of the Xh unit.
The same gneiss, zoomed in a bit.
I noticed what looks like a little folding while I was processing these photos, so I drew in a few lines. The rocks look darker inside the fold nose, and there has been a lot of injection of the light-colored material, which might include felsic dikes like aplite or pegmatite, and also might include some quartz veins. I really don't know how common quartz veins are within these gneisses, and the roadcut is too close to the often busy, narrow and windy, two-laned U.S. Route 6 for safe examination.

If you want to see more of these rocks, here is Robin Rohrback's set of gigapans of the roadcuts along this stretch of Highway 6, along with gigapans of hand samples of the gneiss.

Kellogg, K.S., Shroba, R.R., Bryant, Bruce, and Premo, W.R., 2008, Geologic map of the Denver West 30’ x 60’ quadrangle, north-central Colorado: U.S. Geological Survey Scientific Investigations Map 3000, scale 1:100,000, 48-p. pamphlet.

Tuesday, April 11, 2017

From the Road: The Book Cliffs in Western Colorado (Mount Garfield)

Earlier that day (the same day I stopped in Parachute and Rulison, resulting in three posts about the Roan Cliffs), I had pulled over at the convenient viewing pullout on I-70, which is east of Grand Junction and a bit west of Palisade. It's hard to pass by this cliff, part of the Book Cliffs, without taking a picture or two. This time, unlike most other times, I took only one photo!

The Book Cliffs are capped by sandstone and lesser shale of the Upper Cretaceous Mesaverde Group, which overlies shale and minor siltstone and sandstone of the Upper Cretaceous Mancos Shale. The Mesaverde Group (sometimes Formation, see Geolex) usually consists of several recognized sandstone tongues, members, and formations, with intertongues of Mancos Shale in its lower part. In the area of these photos, the mapped sandstone formation—the buff-colored cliff-former above the Mancos—is the Mount Garfield Formation, obviously named for Mount Garfield, the highest point above the cliffs, on the upper left.
I cropped the photo to zoom in, partly because the nearly white layer just below the sandstone cliff caught my eye. While here, we might as well take a peek at the geologic contacts.
Kmg is the Mount Garfield Formation; Km is the Mancos Shale. The contact I’ve drawn in is largely from the Geologic Map of the Clifton Quadrangle, Mesa County, Colorado (Carrara, 2001), with extrapolation from the Geologic Map of the Palisade Quadrangle, Mesa County, Colorado (Carrara, 2000). The contact very roughly approximates the one shown on the Macrostat online geologic map.

Other photos of mine of the Book Cliffs can be seen here (photos from 2006) and here (photos from 2008). Also check out Ron Schott's GigaPan of the cliffs.

Tuesday, April 4, 2017

From the Road: Talus-y Goodness and More of the Roan Cliffs, and a Question (at the end)

I was looking back through some photos and realized I had some of the Roan Cliffs from the spring of 2006. These first two photos are taken from an impromptu campsite MOH and I found not far from the Rulison exit on I-70, and they somewhat approximate the views seen in Ron Schott's excellent gigapan of the cliffs. I'd embed the gigapan here, but Blogger seems to have some problem with either the iframe coding or the flash implied within the code (or maybe it's Chrome, and I'm being deluded as to what's possible). In either case, we're looking northwest toward the cliffs in the first photo (above), and northeast toward the cliffs in the second photo (below).
It was a very green spring, according to these photos, so the color balance doesn't match with my fall 2016 photos below, which were taken from near or at the Rulison exit. Also, my Nikon tends towards blue.
The reason I stopped for these particular photos during my semi-mega roadtrip of last fall was the talus. I found the striping caused by alternating zones of vegetated and non-vegetated slope areas to be fascinating in the way that perhaps only a geologist, geomorphologist, or photographer can. So I grabbed a few quick shots focusing largely on the talus slopes. More recently, I decided some approximated geologic contacts were in order.
In this photo (above), the cliff and talus slope below the cliff are formed on the Tertiary Green River Formation (Tg), and the colorful beds near the bottom of the photo consist of the Tertiary Wasatch Formation (Tw). The thin cyan line near the base of the cliff marks (hopefully) the top of the Mahogany ledge as extrapolated from this USGS preliminary geologic map, as deduced from this report (Fig. 9, p. 16)—which points out the top of the ledge in a cliff to the west—and as mapped from Ron's gigapan of the cliffs. A few other reports were helpful for exploring the general geology and reading some background info about the ledge and zone—it's called the Mahogany zone when intercepted in drill holes.
In this set of two photos, I've zoomed in on the striped talus section that was on the far right of the previous two photos. The vertical striping of the talus contrasts nicely with the horizontal layering of the Wasatch beds, don't you think? I've labeled the photos below. This time, a tiny bit of the Tertiary Uinta Formation (Tu) is barely visible at the top right. The stratigraphic contact between the Tg and Tw is crudely outlined in faded cyan: It crops out behind the foreground slope where we can't see it.

This set of two photos is not that different from the previous set, but here (below) I've sketched in the Mahogany ledge (as approximately extrapolated), and I've broadly labeled the geologic formations without drawing in the contacts. The circle is where an old mine, the Rulison Oil Shale Mine is shown on the topo map of the area  (USGS TNM 2.0 Viewer link). I can't see it there in the photo—though maybe it's just not apparent—but there is a tiny black area that *might* be an adit just beneath the arrow.
The cliffs in these 2016 photos lie northwest of the Rulison exit and occur between the first two 2006 photos.
In this last set of two photos, we're looking northeast from the Rulison exit, zooming in on cliffs that approximate what we can see in the second of the two 2006 photos. I once again focused in on striped talus, but then I really zoomed in on some switchbacked roads that I thought might be drill roads. It turns out that the switchbacks climb up to some underground workings into the Mahogany ledge, which I've marked approximately with a thin cyan line (below, with thicker lines marking formational contacts; Tu = Uinta Formation; Tg = Green River Formation). The line is taken directly from the previously linked-to preliminary geologic map of the Anvil Points Quadrangle (O'Sullivan, 1986). The map can be viewed directly in Google Earth, an option I always appreciate.

The underground workings here consist of several adits comprising the Anvil Points Oil Shale Mines. The mine was active intermittently from 1925 to 1982 (the last link includes some undated photos), with some clean-up operations running from 2008 to 2013.
How do you spell Talus-y? I see 3 options: talus-y, talusey, and talusy. I went with the first option in the title but prefer the last. Yes, I know it's not a *real* word.