The other day, we looked at some flow-banded rhyolite, beginning at the vertical fractures near the fence on the left side of this photo, and ending on top of the knobby hill overlooking the corral. Warning: we won't see much flow banding, if any, today, but its still there. We will, however, find out why we stopped here while on our way to somewhere else. For scale, the log forming the top of the corral fence is about 5.5 feet above the ground. Location: near Middlegate and The Shoe Tree, just north of Highway 50 (Google Maps, Google Maps Street View, MSRMaps).
This mass of breccia in the center of the exposure immediately caught our attention, but we stopped to examine the caves, and only after stopping noticed the breccia. Note the bird nest, possibly an owl's, on the upper flat surface. And note the several caves, with MOH for scale climbing up to check one of them out. You'll also want to note that about waist high on MOH, and over to the right just before the rightmost vertical fracture, you can see a large dark patch or two. These are breccia fragments.
The inside of the upper cave: a good place to find pack-rat middens and small bones.
The largest, uppermost breccia fragment is about 2.5 feet in length and perhaps a foot or more wide. It appears to be surrounded by small fragments, with another large fragment below it to the left. I didn't try chimneying up the fracture to check them out in more detail, so we'll have to rely on this crude description.
In the main exposure to the right of the caves, large breccia fragments are within reach of even short people like myself. Besides the large, dark brown fragments to the right, you can see several small fragments near my hand. The breccia fragments consist of angular to subrounded pieces of rhyolite, which are set in a matrix of smaller fragments.
This large fragment shows what looks like volcanic flow banding or hydrothermal streaming around the right side of the fragment. I suspect that more than one type of brecciation might be exhibited at this outcrop. Candidates include primary volcanic brecciation, brecciation associated with caldera formation, and tectonic brecciation associated with faulting. Another candidate is hydrothermal brecciation.
Here's another example of what the matrix looks like, closer to that first exposure of flow-banded rhyolite from the other day, and visible as a large bright orange spot near the fence on the left side of today's first photo. At first glance, this pale to dark yellow orange to dark yellow brown mass looks like a clast or breccia fragment, but I think what we're seeing is a weathering phenomena, with a piece of the outer, dark brown weathering rind broken off, exposing relatively fresh rock beneath. We can see some light gray veinlets of silica shooting vertically through the yellow orange mass, and there are hints of large fragments cut by small veinlets. The next picture shows a portion of the rock to the right of the middle silica veinlet.
The rhyolite is fractured, possibly brecciated or at least crackled, and shows moderate iron-oxides, mostly goethite and jarosite.
A closer view shows a lot of very fine-grained to microcrystalline light gray silica that forms a matrix around tiny breccia fragments, or has at least replaced an earlier volcanic matrix. Possibly some of the brecciation seen in this exposure is hydrothermal. I recommend petrographic work to get down to the nitty gritty and to really understand what we're seeing.
From the top of the knob, we see the main road north into the central part of the Clan Alpine Mountains. We'll drive up there later, and make a sharp right turn in about a half mile, not far past the second little bend.
And lo, there's Fairview Peak off to the southwest, visible through two knobs of lichen-covered flow-banded rhyolite and rhyolite breccia. Middlegate Station is behind the knob on the right. Will we make it beyond Middlegate Station to Fairview Peak?