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.

More Crestmont serpentinite

25 May 2015

I’ve done some more poking around in the fat part of the serpentinite patch. Today I’ll show photos from three localities, as marked below on the geologic map (where “sp” means serpentinite).

crestmont-geomap

The first locality is a truly spectacular exposure at the intersection of Crestmont and Kimberlin Heights Drives.

crestmont-kimberlin

The neighbors have made an effort to love this inhospitable stone, as you can see in this closeup.

kimheights-closeup

But serpentinite is not only very low in calcium and extremely rich in magnesium; it also tends to have high levels of toxic metals such as nickel, cadmium and chromium. Serpentine soils tend to be thin and poor (see this helpful Forest Service page). You can see that the little palms are almost dead, and on the upper slope even the pampas grass, one of California’s toughest invasives, is looking peaked.

I didn’t climb on this exposure—there wasn’t time and the rock has treacherous footing. It’s full of texture, though, and this exposure higher up on Kimberlin Heights suggests that it’s pretty chunky.

kimheights-exposure

Locality 2 is on the next street down, Colgett Drive, which is too new to be shown on the base map above. It exposes serpentinite all the way to the end, where I collected this fragment to take home.

crestmont-blue-specimen

I wanted to inspect it as closely as I know how. I have several hand lenses as well as a teeny pocket microscope that gets me up to 45X. With those tools, I perceive that the rock consists of blue bits the color of glaucophane in a matrix of light-colored serpentine. I continue to think of it as blueschist, but I’ll save that discussion for a later post. The point is, just because this area is mapped as serpentinite doesn’t mean it’s 100 percent one thing or another. But for all practical purposes this area is serpentine rock.

Locality 3 is the spot on Crestmont Drive I showed in my very first post, back in 2007. It looks a lot better now. You could creep around the fence and give it a good examination.

crestmont-ur-exposure

There are a couple of interesting things going on here. First is that just around the corner on the left side, where Butters Drive intersects Crestmont, the rock abruptly changes to the Knoxville Formation (KJk), a nice brown mudstone. The geologic map puts a thrust fault there separating the two rocks.

Second is that on the righthand side you can see signs of the pavement being disrupted by some sort of ground movement. Cracking related to it extends across the road. That has taken place in the 7-1/2 years since I last came here. I went to inspect it more closely, but my attention was drawn instead to an unusual sight: a swarm of bees gathered on open ground.

crestmont-bees

I lingered long enough to take two shots, but after the first one I sensed dozens of bees zipping past both my ears, so I let the poor critters be. It was a raw day, and I hope they found a new space to set up housekeeping.


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