Archive for the ‘Oakland hazards’ Category

A reconnaissance of San Leandro geology

14 March 2016

San Leandro is a much smaller city than Oakland, but it has its share of interesting rocks and features. ‘Twas a cloudy day when I visited, but the worst day geologizing is better than the best day working. Here’s the geologic map with the photo locations numbered on it.

SLgabbrogeomap

The purple area marked Jgb is underlain by the San Leandro Gabbro, of Jurassic age, a crystalline rock similar to granite that belongs to the Coast Range Ophiolite. It’s about 160 million years old and was once a deep-seated part of the oceanic crust. Unfortunately the color, while it follows the official U.S. government geologic color guidelines for Mesozoic plutonic rocks, makes the map hard to read. The blob of brown marked Jpb represents Jurassic pillow basalt, which I thought would be very interesting to see. And the solid black line down the middle of the map is the Hayward fault — it’s solid black because the fault is very well mapped there.

The San Leandro Rock Quarry has been closed for a few years.

SL-quarry

The land is for sale — 58 acres of it, right on the Hayward fault — but I didn’t feel up to impersonating a possible buyer, so it was off limits. But the view the other way is pretty cool, overlooking the gorge of San Leandro Creek below the Chabot Reservoir dam. It’s the biggest canyon between Niles Canyon and Wildcat Canyon and pretty intimidating.

SLCreekcanyon

A little ways west on Lake Chabot Road, where it meets Astor Drive, is a saddle in the hillside where the Hayward fault crosses the road. A steep gulch descends to the north along the fault trace. To the south, the Bay-O-Vista Swim and Tennis Club has nestled on the fault unscathed for almost 60 years.

SL-bayovistaclub

We’ll visit the fault on the other side of the club. But first, the gabbro! It’s exposed in various places in the Bay-o-Vista neighborhood, where it’s mostly shattered from being next to the fault for millions of years.

SLGabbro-exposure

Gabbro is made up mostly of dark pyroxene and light plagioclase feldspar. Like granite, it likes to weather into decent soil. The excavations of residential areas are helpful in bringing it into view. And up close, this gabbro is pretty.

SLGabbro-specimen

Studies of this area using airborne gravity meters and magnetic instruments suggest that this gabbro extends well north and south of here in a big slab about 3 kilometers thick lying between the Hayward and Chabot faults, tilted almost straight up and down. This figure is from a 2003 study led by Dave Ponce of the U.S. Geological Survey.

SLgabbroprofile

In Oakland, the gabbro shows up in stringers and blobs along the fault as far north as Chimes Creek. I’ve picked up pieces on Eastmont hill by the reservoir. But the geophysical study suggests that it underlies a much larger area as far as Merritt College, beneath the surface rocks.

The gabbro is strong enough that it bends the Hayward fault slightly off course. But during the 1997-98 El Niño, a big hunk of hillside gave way just below the place where I shot the outcrop. Two homes were lost.

SLslideface

The top of the slide displays some pretty rotten stone.

SLslidescarp

Farther south, Fairmont Road swings around the county juvenile justice center past the Hayward fault. This is looking north from there up the fault trace.

SL-faultvalley

Our active faults grind up the rocks so fine that they’re easily eroded into gulches, gullies and valleys, and that’s what this one is. It last ruptured on October 21, 1868, so any trace of that is long gone. It takes careful trenching studies to find it. We’ll have to wait until the next big one to see where it decides to rip up the ground.

Sulfur mine mitigation: An El Niño preview

4 January 2016

SmineDec15-1

Earlier this year I took a peek at the mitigation work going on at the former pyrite mine at the head of McDonell Street, in the Leona Heights neighborhood. The workers had laid down lovely geofabric, seeded grass and paved the creek bed with boulders. They were also patching infant gullies. I predicted it wouldn’t last. Sure enough, on Saturday large expanses of black plastic were shielding parts of the locality.

Farther down, the patchwork had massively failed.

SminDec15-2

This was after a normal December rainfall. We’re looking at El Niño rainfall during the rest of the winter that will be substantially above average.

On the good side, I can report that the water downstream at Twitter Court wasn’t an ugly orange. Whether that’s due to larger runoff volume or success in muffling the acid drainage, I’m not qualified to say. Think positively as you walk past the mine site on your way up the fire road into Leona Heights Park.

McDonell-fireroadup

It will always be an instructive and rewarding hike.

Claremont chert closeup; or, Oakland hills are falling down

23 November 2015

On Grizzly Peak Boulevard, pretty much right above the Caldecott Tunnel, there’s a little old fire road that heads downhill to the west. I poked my nose down it the other day. The whole area has excellent exposures of the Claremont chert, starting with the roadside.

Grizchert1

It’s real nice right now. The ground is moist and makes for quiet walking. Pine needles smell great. The rock is pretty.

Grizchert2

There’s a spot where a lot of loose rock has tumbled down. The Claremont can be crumbly, because it’s so brittle, even though the stone itself is rather hard. The loose stuff is good for collecting a specimen if you’re into that. Unlike the bleached stone exposed along the ridgetop, there’s some variety here, including the black, kerogen-rich stuff that has made this formation, like its larger cousin the Monterey Shale, good petroleum source rock.

Grizchert3

Grizchert4

Grizchert5

During the Caldecott Tunnel dig, this formation leaked significant amounts of oil and gas into the working space. Precautions had to be taken. The same black Claremont crops out at Alum Rock, as I showed you a few years ago, as well as at the Calaveras Dam site.

That’s all fun. But the road’s cut off by a washout ripped into the hillside, a twisted galvanized drainpipe sprawled along its path. At some risk, I scrambled across it and noted that at its floor lies the Claremont chert, which has its bedding planes oriented only slightly steeper than the gully. Treacherous ground. I don’t recommend that you follow me.

Grizchert6

And just beyond it is another gully, somewhat bigger but not eroding as actively. Giving up on the fire road, I scrambled up to Grizzly Peak Boulevard and this is what’s at the top of that gully.

Grizchert7

At the top of the active washout is this innocuous-looking street drain.

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As far as I can tell, every one of these cute drains is carving gouges into the hillside. This one points toward the Parkwood condos.

Grizchert9

Can’t we do better than this?

Perhaps our children can revise the old playground song to “Oakland hills are falling down.”

A lot of my outings are like this — mixtures of pleasure and concern.


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