Tuesday, August 23, 2016

Tertiary Tuesday: Hoodoos along Old 8A in Northwestern Nevada

Last week we saw some hoodoo-ish rock formations on a hill behind the road maintenance station at Vya. Let's take a closer look:
When we zoom in on these particular hoodoos, we see that they are shaped much like the "tent rocks" in the Kasha-Katuwe Tent Rocks National Monument in New Mexico, and though close to the ground, they also look a lot like the hoodoos, tent rocks, and "fairy chimneys" at Cappadocia in Turkey. Notice the softer, pinkish part of the section near the base of the the central and most fully exposed hoodoo, the white caprocks or "tents" all across the hill, and the darker-colored outcrops near the top of the hill. This outcrop pattern of light-colored rock beneath darker rock is typical of many ash-flow sheets in Nevada, and that's what this most likely is: a poorly welded (bottom) to maybe moderately welded (top) ash-flow-tuff cooling unit.

From Vya, as you remember, we drove south then west onto old Highway 8A (now a county road, possibly still called 8A), and started climbing over an unnamed pass through the Hays Canyon Range. MOH and I hoped we would see some of these hoodoos up close, and sure enough, when we rounded a bend one was sticking out right next to the road!
We pulled over quickly and walked up the hill.
These rock formations (above and below) have an incipient tent-rock shape, with a tiny capping of a harder, more weather-resistant layer near the top. I suspect that these budding hoodoos correlate with the "tent" part of the hill we saw near Vya, where more of the geologic section was exposed.

The weathered form above is similar to some shapes one might see in weathered granite. This particular type of shape—gently rounded or sub-spheroidal—is not uncommon in some ash-flow tuff units, hence the name "granite-weathering tuff" for part of at least one regional ash-flow sheet in south-central Nevada.

Let's get closer:
Here's a fairly large, subrounded lithic fragment within the tuff.
Zooming in a bit, we see that the matrix of the tuff is fairly fine-grained, fairly well-sorted, and some grains look sharp and appear to be interlocking.
It would be fairly easy from this one photo (above) to convince oneself that the matrix was pumiceous and glassy.
Here's another view of the rock: Rounded to sub-rounded lithic fragments and white pumice float in a fine-grained matrix.
I'm pointing to one of the white pumice fragments in this enlarged view.
I had a harder time, after looking at this second example of the exposure, convincing myself that this was unwelded or poorly welded pyroclastic flow rock (ash-flow tuff—same diff). The second closeup shows scattered rounded to subangular dark-colored and white lithic fragments and fairly small to fairly large white pumice in a dirt-colored matrix of subrounded, fine-grained pebbles, lithics, or pumice and glassy-looking particles. This photo could be of a reworked tuff or a lithic-rich or lapilli tuff of unknown origin. Are the pale orange, subrounded fragments pebbly lithics or rounded pumice? If pumice, are they primary, deposited in either an air-fall or ash-flow type of environment, or have they been rounded by reworking in some type of fluvial or colluvial environment? (Your thoughts are welcome.) The two photos, the first with the large lithic and the second with the white pumice, were taken within a few feet of each other.

Standing back and looking at the overall mien of the outcrop, I'm going to have to stick with poorly welded ash-flow tuff.
I tried to find out about the geology of the area and found this one geologic map by Egger (2010), a regional map focusing largely on the Warner Range to the west but also including part of the Hays Canyon Range. According to the map and accompanying text (both are available for download here), these hoodoos and rock formations occur in what has been called the Fortynine Tuff by Carmichael et al (2006) in an article I didn't access (paywall).
I noticed (after the fact) a couple intriguing joints within the tuff, including two that might have veins of some sort or slickensides (the upper, center one in shadow, and one just below it and to the right, also in shadow).
Egger references an age of 26.26 ± 0.13 Ma (Egger, 2010; Egger and Miller, 2011) for rock just a little north of this exposure, rock included in the Fortynine Tuff of Carmichael et al (2006) and mapped as unit Tovu by Egger (2010): This tuff is Oligocene in age.

A Few References:
Carmichael, I.S.E., Lange, R.A., Hall, C.M. and Renne, P.R., 2006. Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range [abs link]: Geological Society of America Bulletin, v. 118, no. 9-10, p. 1196-1211.

Egger, A.E., 2010, Geological history and structural evolution of the Warner Range and Surprise Valley, northwestern margin of the Basin and Range Province [Ph.D. thesis]: Stanford University, 180 p. Plate1.

Egger, A.E., and Miller, E.L., 2011, Evolution of the northwestern margin of the Basin and Range: The geology and extensional history of the Warner Range and environs, northeastern California: Geosphere, v. 7, no. 3, p. 756-773.

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