Looking For Detachment: Lida Summit Roadcut

Tuesday, December 1, 2015

Lida Summit Roadcut

Here we are, back at Lida Summit, looking west on S.R. 266.

When I pulled up to Lida Summit two years ago—I was on my way to a property examination and had decided to check out the Lida and Palmetto areas—I stopped right in front of...a roadcut! Well, that’s really no surprise, is it? Where else would a geologist stop, other than at some excellent viewpoint? (Acutally, the spot I selected, as you can see above, also came with a view of the mountains beyond, including the high Sierra.)

Offhand, I can think of a few places where a geologist might park or pull over other than roadcuts or viewpoints. For one, I would park off a paved road! And while down some dirt road away from traffic, I would park in a place that wouldn't allow my truck to roll forward or backward even without chocking the tires with a rock or chock blocks. That spot might be some random wash or ditch of some kind—if I thought the low area was out of the way of any potential flash flood—or it might be on a hill, provided the truck wouldn't roll when I let out the clutch: a site with a natural declivity or some in-place, roll-resistant rocks. Then I'd chock the truck anyway.

Speaking of ditches, that brings me to a second possible parking position: a hidden spot. While in the field I might not want to advertise my presence for a variety of reasons. For example, I might decide to indulge in some sort of exploration espionage and therefore wouldn't want my presence to be suspected. Mostly, though, I'd chose to conceal my vehicle, and thus my presence and general whereabouts as a means of personal protection: in order to hide myself from random strangers who might prove to be a threat.

A view of the summit from the west side: summit ahead, roadcut right.

And here's the roadcut. My truck is alongside for scale.

At first glance this is a rather unprepossessing roadcut: The rocks shine white in the sun, they show some jarositic to goethitic iron-staining, I detect a crude layering (am I imagining this?), and there appears to be a vertical jointing pattern. Also, I see a reddish brown soil or slope-forming layer beneath the resistant mini-cliffs.

Is the rock in our roadcut igneous (granitic rock or rhyolite, perhaps), sedimentary (blocky-weathering siltstone, perhaps), or metamorphic (quartzite, maybe)?

It's ash-flow tuff!

I was a little surprised to discover that the roadcut was in ash-flow tuff: I had been thinking more along the lines of granite or siltstone. Also, an elevation of 7400 feet seems a bit high for ash-flow tuff given the much lower current elevations of likely source calderas (Basin-and-Range faulting has no doubt changed the relative elevations of calderas and associated ash-flow sheets in some areas). Possibly, rather than thinking I could tell what the rock type was before I had even slowed down to look (I was going warp what?), I should have been a responsible geologist and selected a designated driver. or I maybe I should have somehow known to drive slow enough to see the micro-texture. I guess I was focused on the larger picture.

Vertical jointing stands out clearly in these photos. I’m not sure from this one exposure if the jointing qualifies as poorly formed columnar jointing, but it looks quite possible. Columnar jointing is fairly common in some parts of ash-flow tuff sheets, especially in the strongly welded zone. The jointing is often rectangular to square (Ross and Smith, 1961), though it can also be five- or six-sided, as is common in basalts. Consequently, it's not always possible to tell the difference between basalts and dark-weathering, columnar-jointed ash-flow tuffs from a distance.

Hints of the strong eutaxitic texture present in this exposure can be seen cutting subhorizontally across the photos, with the planar feature dipping more or less to the right (that would be in an easterly direction).

In this photo, the eutaxitic texture becomes, perhaps, slightly more visible.

Strongly or densely welded vitric ash-flow tuff.

Most of the rock lying along side the road below the roadcut looked like the rock above: white or very light gray to very pale yellowish gray strongly welded vitric ash-flow tuff, with a eutaxitic texture defined by highly compacted (or collapsed) light pinkish gray pumice fragments: the elongate, lens-shaped fragments with tapered or pointed ends: fiamme. The rock contains tiny crystals not easily seen in the photo, and lithic fragments that are small and sparse (at least in the hand samples I examined).

Back in the semi-dark ages, I learned, somewhat incorrectly, that the term "fiamme" refers to the tiny flame shapes at the end of flattened pumice lenses in rocks like these. I also learned to call the lenses "flattened (or collapsed) pumice," rather than "fiamme." The lenses are definitely fiamme, even though Ross and Smith (1961) used a definition that might seem more specific than the one commonly used today. They defined fiamme as

"the Italian name used to describe black glassy inclusions in piperno and which have a cross section shaped like the tongues of flame. These are often several centimeters in length, but may range from microscopic size to several feet in length,"

where "piperno" is a rock characterized by lenses of glass.

The definition and genetic interpretation of fiamme is addressed at length by Bull and McPhie (2007). They point out that the presence of fiamme is not diagnostic of welded ash-flow tuff (the same way that the absence of fiamme is not diagnostic that you are looking at something besides a welded ash-flow tuff).

Moderately or strongly welded lithic-rich vitric ash-flow tuff.

I'm not sure if this rock is part of the same unit or if I've picked up a piece of some float from some other ash-flow tuff. It's pinkish, rather than whitish, although color doesn't technically matter in correlation; it contains a considerably higher proportion of lithic fragments, although that could be a function of being somewhat less welded, and lithics can be concentrated in layers or swarms within an ash-flow sheet. The matrix of the rock shows some similarity to the matrix of the first rock sample: it consists of devitrified glass with collapsed pumice, but it appears to contain a higher percentage of tiny crystals.

We can examine these rocks a little more closely.