Archive for the ‘Oakland rocks’ Category

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.

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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.

A hunt for silica-carbonate

12 December 2016

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.

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So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.

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Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.

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It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.

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Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.

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And at the north end, during a strenuous climb, some Franciscan chert from the melange.

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The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.

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This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.

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Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)

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What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.

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That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.