Archive for the ‘Oakland rocks’ Category

Two bits of gabbro

20 February 2017

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.

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But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But once last year and once the year before, I found some. The two spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, 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.

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I can’t say the same for the Middleton Street exposure, but on the positive side it’s real easy to visit. Oddly, the last time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

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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.

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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.

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

Museum-quality rocks from Oakland

30 January 2017

I keep saying that Oakland has geological features worthy of being put in textbooks. Today I’m here to show you that Oakland has rocks worthy of being in museums, and I’ve put them there.

In 2012, I was asked to put together a set of teaching rocks for the Chabot Space and Science Center. After all, other planets are made of rocks, right? It took some doing, but some of the rocks were easily available within Oakland’s borders in roadside exposures. The conglomerate of the Orinda Formation was one.

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The red chert from the Franciscan Complex was another.

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And of course there was our serpentinite.

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All told, I made five sets of 15 rock types for the kids.

The next year I got a request from Las Positas College, in Livermore, for a boulder of blueschist. Turns out this little college teaches geology, because every citizen will benefit from a course, and students can get a head start on a 4-year degree there. I struggled one out of this streambed, where it wouldn’t be missed.

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They installed it in their teaching garden as Rock J, on the left. It’s small compared to its mates, but that thing weighs a ton because high-grade blueschist is pretty dense.

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My reward included a visit backstage to see their cool collections.

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Then last year, I got a note out of the blue from the under-construction Maine Mineral and Gem Museum asking my help in building their collection. Maine is well known for its gemstone and mineral mines, but the state has no blueschist. I went to a quiet outcrop where it’s just lying around.

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Got two nice boulders and couldn’t choose between them, so I sent them both. They told me one will go on display and the other will go in their teaching collection.

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None of these are precious collectibles or gemstones. They’re just cool and educational.

I’ve pretty much stopped collecting rocks for myself because I’m not important enough. But museums are important enough.

The Pinehurst Shale

23 January 2017

Much of Oakland’s high hills consists of our local piece of the Great Valley Group, the colossal set of sedimentary rocks that runs the length of the Central Valley along its western wall. (How our piece got over here is, as they say, poorly constrained.) The group is well exposed in Shepherd Canyon and points south, and it continues on the far side of the hills as far as the Upper San Leandro Reservoir. There the Redwood Canyon Formation, the upper unit of the group, is overlain by a little rock unit called the Pinehurst Shale. On the geologic map, it’s the slices of darker green nestled amid the Redwood Canyon Formation (labeled Kr).

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It’s named for Pinehurst Road, naturally, and its type locality is at the Pinehurst Staging Area on the far side of Redwood Regional Park. You know the place if you’ve ever driven out there.

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This exposure shows strong bedding, but it’s all shale — rock with little material coarser than clay size. Some of it’s soft claystone that crumbles and erodes easily. Here it is in a rain-carved rut in the fire road that leads into the park.

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Other parts of the Pinehurst Shale are very hard and resistant because they contain a goodly share of silica. Here’s what that siliceous shale looks like in outcrops along the trail . . .

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. . . and here it is in hand specimen. (For display only; I put it back.)

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Farther up the trail, the shale gives way to siltstone of the Redwood Canyon Formation. It’s quite a dramatic change.

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Both of these rock units contain microfossils from the Campanian Age, making them somewhere around 75 million years old. Whatever once lay above them has been removed by faulting, leaving a gap of maybe 60 million years between them and a little ribbon of Sobrante Formation across the reservoir on EBMUD land.

There’s another cool thing here. Here’s the rock just to the right of the top photo. See that big chunk of rock by the right edge, sticking out crosswise against the bedding?

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That is a sedimentary dike. It marks a place where once upon a time, before this rock was fully solidified, an imbalance in fluid pressures caused the one layer to force a crack upward through the overlying layers and squirt into it. Usually dikes are something you expect to see in lava beds, or producing “sand blows” on the ground after large earthquakes. This next shot is taken from the right side, looking at the top of the dike.

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An exposure like this, on a roadcut, is usually fair game for grooming by pulling out the shrubs and sweeping away some of the rubble. I do that here and there during my outings, but this spot looked a bit chancy.

I contented myself with pulling up some young French broom plants. Now’s the best time of year to do that, when the soil is saturated. If we all did a little of that, it would help fight the invasive problem. The land is worth the effort.

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Visiting our parklands can take you very far away.