Three bits of gabbro

I’ve noted that while the San Leandro Gabbro has a presence in easternmost Oakland, it’s hard to find. The geologic map shows what seems like a lot of it, marked “Jgb” for Jurassic gabbro.

But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But I’ve found some. The three spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, the middle one is in the woods along the route of Ramble 3, and the southern one is in Sheffield Village at the north end of Middleton Street where it meets Marlow Drive.

The northern site, Seneca Reservoir, was once the upper pit of the old Catucci quarry. (The lower pit was repurposed as the site of Bishop O’Dowd High School.) Not much of it is accessible, but here and there you can spot pieces of the quarry waste. It looks like nothing else in town and everything like San Leandro’s namesake stone.

The middle site lies along the trail and is kind of unexpected, but the rock is definitely the same stuff.

This too is an out-of-the-way place. You’d have better luck farther south at Oakland’s utter end in the Dunsmuir Hills.

The Middleton Street exposure, at least, is real easy to visit. Oddly, the first time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

This site too is very near a strand of the Hayward fault, so it’s been rattled and squeezed for quite some time. It has a battered appearance, even a little fried.

And no matter how close you get, it doesn’t show much detail. As a whole, though, it has the typical color of the gabbro: light gray with a slight blue-green tinge. This resulted from petrochemical disruption at the time of its eruption, some 165 million years ago, when a pulse of younger magma sent up fluids that changed its black pyroxene minerals into green amphibole and some other greenish minerals — an obscure process known as uralitization.

I mentioned the Hayward fault being next to the reservoir. We probably would think twice before building a hilltop reservoir there today.

One Response to “Three bits of gabbro”

  1. brian oregan Says:

    For those who wish to stay within Oakland, there is a great exposure of Jgb on the north side of San Leandro Creek , about 100 yards upstream of the east end of “san leandro chabot park”. You can spot the outcrop easily on google satellite view. The east end of the outcrop has a large exposure of apparent fault gouge. Might be the contact with Jsv but I didn’t have time to fully investigate that. The fault gouge contains all the weird stuff you might want. blue serpentinite? green serpentinite, and pure white fluffy carbonate that not only fizzes in 14% Hcl, but falls to pieces. A green very soft material that might be talc? You can even spot the whiter material of the fault gouge on google satellite view, just above the dirt road along the bottom of the outcrop. With the recent rains quite a bit of soil has sluffed off making the exposure prettier. Of course, you can’t legally go there and I’m not advocating it.

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