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.

Lobe 8 of the Fan: Evergreen Cemetery hill

15 June 2015

Cemeteries occupy some of Oakland’s nicest hillsides. Evergreen Cemetery, unlike the others, has a whole hill to itself. The hill consists entirely of sand, silt and gravel laid down in a large alluvial fan (the Fan) back some time in the Pleistocene. Later, stream erosion cut the Fan into pieces, which I have numbered west to east from 1 to 8. This one’s the smallest and, all things considered, the sweetest.

lobe8-evergreencem

This is the view from 64th Avenue, the hill’s gently sloping side. The NL bus runs past here, too.

Below is the terrain view in google Maps, where I’ve marked it along with lobes 6 and 7.

lobe8-topo

This is the view from across MacArthur Boulevard at 68th Avenue/Church Street. The hill is pretty secluded, with trees screening it on most sides. When you’re there, your eyes are lifted to distant views, which is fitting. The homes on the far side of 68th probably excavated into the edge of the hill when they were built.

lobe8-from-church

When the weather’s right, you can get a good view of it from Outlook hill, for instance from Delmont Avenue:

lobe8-millsmont

or from the overlook at the end of Simson Street. These days the cemetery is doing its part in the drought. Most homeowners are, too.

lobe3-from-simson

The geologic map (as seen in this earlier post) shows a little more Fan alluvium to the north and east of the hill, but it’s topographically subdued.


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