Archive for the ‘oakland rocks’ Category

On Oakland’s blueschist

22 June 2015

It may seem like I have a fixation on blueschist. I’ll admit that. I have a fixation on every rock type. Here’s a fine blueschist boulder at the very north end of Castle Drive, in the Piedmont Pines neighborhood.

higradeblock-mountaingateway

This qualifies as a knocker, and it also qualifies as a high-grade block. “Knocker” is local geologists’ slang for a small block of resistant rock that protrudes out of an area of melange (like those in Mountain View Cemetery that I feature here in the “cemetery knockers” category). By default that’s understood to mean Franciscan melange, because melange—a collection of geological bric-a-brac mixed in a matrix of shale—practically defines the Franciscan complex. However this high-grade block is in the serpentinite patch, part of the Coast Range ophiolite, which also qualifies as melange.

Oakland’s serpentine patch contains a goodly share of blueschist. This is a piece of it near Redwood Road that was lying right next to a piece of classic serpentinite.

blueschist-redwoodrd

The key indicator for me is the color, which is typical of the mineral glaucophane. Glaucophane is described as various shades of blue, while serpentine is described as varous shades of green. They differ in their luster and hardness as well. My gold standard is the classic occurrence at Ward Creek near Cazadero, which I visited in 2005. Here are two photos from there.

wardcreek1

So glaucophane tends to a dusky, blue-jean blue or gray-blue. Green minerals like chlorite, epidote and omphacite may accompany it as they do here.

wardcreek2

Technically, blueschist is a metamorphic facies rather than a specific rock type—it’s a set of typical minerals that form at a specific combination of heat and pressure. Glaucophane and lawsonite indicate the blueschist facies in metamorphosed rocks of mafic (MAY-fic) composition, like basalt. In metasedimentary rocks, the indicator minerals are phengite, chlorite and quartz. Those won’t make a blue rock (they’ll be greenish). So amateurs like you and me shouldn’t read “blueschist” and envision something blue. But in Oakland, we do have real blue blueschist. (Some of the Franciscan sandstone also has blueschist-level minerals in it.)

Serpentinite doesn’t change much with pressure. It’s a cryptic rock that doesn’t retain many traces of its history. But the blueschist that accompanies it in the Oakland serpentine patch testifies to fast, deep burial and equally rapid exhumation.

Serpentinite tentacle at Merritt College

1 June 2015

My quest to get my arms around Oakland’s serpentinite patch took me to the grounds of Merritt College, where a long tentacle of this rock is mapped. It’s a complicated area, and the topography on the ground no longer matches what’s shown in the geologic map. Long story short (or in the new hip lingo, “tl;dr”), I think it’s best to show the Google Map map first, with the outline of the tentacle superimposed on it.

serpmerritttopomap

I’ll show photos from the four numbered locations. And then here’s the geologic map of the same area. Remember that most of Oakland’s rocks have been shoved around, crumpled, sliced by a major fault and tilted on end. It’s OK if the map doesn’t make sense.

serpmerrittgeomap

Here’s the overall scene looking northwest from hill 1175. (Do the locals have a name for this hill? I need to know.) Redwood Peak in the background. Two ballfields and a solar array have wiped out the earlier contours of the land, which was previously a rock quarry.

serpmerritt1

The road skirting the two ballfields is cut into the serpentinite. The first locality is that light-colored patch behind the six sentinel trees. And here it is.

serpmerritt2a

This is a very informative exposure showing a clear contact between the serpentinite and the adjoining shaly rocks of the Knoxville Formation (KJk). I think this is the key locality that led to this contact being interpreted as a thrust fault (signified by the black triangles, “teeth on upthrown side”). I’ll show you the evidence. First, here’s a look uphill along that contact, at the front edge of the exposure.

serpmerritt2b

The rock along the contact is dark, hard and altered. Farther uphill, the serpentinite is thoroughly shattered—brecciated. The shock and shifting due to earthquakes, many thousands of them, would do this, just as they cracked the Oakland Conglomerate. The next three shots show the breccia at ever-closer range. There are lovely blue-green pebbles of the original serpentinite floating in this broken matrix.

serpmerritt2c

serpmerritt2d

serpmerritt2e

The map’s author, Russ Graymer, interpreted this contact as a thrust fault because that’s practically the default in the East Bay hills, which have been compressed from the west for several million years and counting. He also surely noted that outside the contact zone there’s no sign of heat or chemically active fluids associated with the brecciation. That points to cool conditions near the Earth’s surface rather than some truly ancient activity when these rocks were more deeply buried. So during the recent geologic past, as our hills were being pushed upward, the serpentinite rode up relative to the Knoxville Formation mudstone, sliding on the surface between them.

The whole roadcut, wherever the rock peeks out through the grass, is serpentinite. Farther along the road is this darker example.

serpmerritt3

It’s full of swirly detail and veins of white mineral, probably calcite, that attest to hotter deep-seated activity. That’s an older story not related to the recent thrusting.

Beyond the roadcut a stream valley full of thick brush cuts through, hiding any more rock. And beyond that the ground beneath the solar array is untrustworthy, because such things are usually set upon beds of imported gravel. But on the other side, where bedrock is exposed again, there is enough dark serpentinite to justify drawing the map as Graymer did.

serpmerritt4

Next I want to trace out the uphill edge of the tentacle, which is conveniently mapped right along Fernhoff Road. Some other time.

By the way I’m finally going to subdivide the “Oakland rocks” category into a hierarchy. After nearly 8 years of this blog I’ve tagged 140 posts with that label, which is silly.


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