Shepherd Canyon: Type localities of Oakland rocks

29 June 2015

Yesterday I led a walk for the group Wild Oakland that took in the rocks of lower Shepherd Canyon, which are the westernmost outcrops of the Great Valley Sequence. These are the same kinds of rocks that make up the monumental set of ranges marching up the western side of the Central Valley from Taft to Redding. The map below shows our planned route. The red dots mark the beginning and end of the route plus mileages. (In fact, for lack of time I cut the walk short where the 3-mile mark is, so we didn’t see the loop on the right side. I leave that as an exercise—and it is exercise—for the reader.)

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This is the geology along that route.

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The starting point, where we climbed up to the unpaved start of the Montclair Railroad Trail, offers a nice view over the valley of the Hayward fault, here at Montclair Playground . . .

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. . . and looking northwest up the fault valley.

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This outcrop, above the curved cut in the railbed, shows the Oakland Conglomerate to advantage.

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And maybe 100 feet away, the rock abruptly changes to shale of the Shephard Creek Formation.

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This spot corresponds to the symbol on the geologic map with the number “73” on it, which means that the bedding here is tilted 73 degrees from the horizontal.

Farther up the valley, we examined this outcrop of the Redwood Canyon Formation.

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I pointed out the thin set of shale beds running up the center of the image and showed how the sandstone beds on the left side had been laid down on top of the shale—that is, the internal evidence shows that this whole set of rocks here is tilted up beyond vertical and is upside-down. This spot corresponds to the symbol on the geologic map labeled “78”.

The last spot is in the Shephard Creek Formation where a large sandstone bed sits amid the shale. The location is just about where the word “Park” is along the walk route. On the underside of that sandstone bed is a splendid set of sole marks. This shot shows how the underlying shale is bent by the pressure of the overflowing sand avalanche that built this sandstone bed.

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And this shot looks up at the underside. When there are a sufficient number of these marks, the geologist can work out what direction the avalanche flowed.

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As I said, we cut the walk short at this point and came down through Shepherd Canyon Park along this stream valley, which is filled with a peculiarly flat deposit that I strongly believe is landfill. It forms the higher terrace in this view looking back from the soccer field.

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Does anyone know the history of this piece of land?

If anyone would like a copy of the handout I prepared, I’ll send you the doc file. Just write to geology at andrew-alden dotcom.

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.

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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.

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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.

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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.

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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.


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