Archive for the ‘Oakland rocks’ 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.

Upper Knowland Park and the Chabot fault

22 February 2016

The upper part of Knowland Park is quite different from the lower part. I made a reconnaissance visit last week. Here’s the geologic map, along with white numerals indicating the localities I took the photos from or at.

upper-knowland-geomap

Whereas the lower part of the park (west of Golf Links Road) is dominated by Franciscan rocks and the Leona rhyolite, the upper part is mapped as completely sedimentary. My main destination was the saddle between the areas mapped as Joaquin Miller Formation (Kjm) and Knoxville Formation (KJk), where the obscure Chabot fault runs. Here’s a view of the saddle and the bare knob of Knoxville beyond it.

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The Joaquin Miller is a fine-grained sandstone here, sometimes with a slightly slickensided texture that makes it almost glossy.

upperknowland-2

The roadbed displays it nicely. The saddle doesn’t display any obvious signs of a fault.

upperknowland-3

But as you approach it, the honey-colored rock in the roadbed . . .

upperknowland-4

. . . gives way to a deep sandy soil with chunks of strange rock floating on it. Not what I expected at all. I thought I’d see a hard, dark shale/conglomerate like what’s in the streambed of Arroyo Viejo. Instead it looked for all the world like a Franciscan assemblage. Here are a few of the stones.

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upperknowland-6

upperknowland-7

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This last specimen, and the first, appear to be bits of the Leona rhyolite. So there is some complexity here that’s not recorded on the map, perhaps a splinter of Franciscan that got mixed up in here.

I didn’t learn much about the Chabot fault, except that the abrupt change in lithology is a sure sign of a fault contact. I’ll have to do more poking around before I can write something coherent about it. (In fact, please ignore site 4 on the geologic map; I’m not showing that this week.) This is the view south from point 3 along the valley that marks the fault trace (1000 px).

upperknowland-9

And here’s your weekly cheesecake shot looking north from point 5 (1000 px). Rabid fans will note Sugarloaf Hill on the skyline.
upperknowland-10

This is a great time to visit, but do watch for the newly sprouting poison oak.

Bike trails, right wrong and ambiguous

15 February 2016

One day a year ago, as I set out to investigate the old Crusher Quarry, I was standing off the fire trail looking at something when a rattling sound came from up the hill. Two mountain bikers burst out of the woods in a spray of dust and gravel and jerked to a halt nearby, rear wheels tipping off the ground. “That was intense!” I heard one say. Then they rode a few yards down the fire trail to Mountain Boulevard, where someone in a pickup had just arrived to meet them, and a minute later they were gone, with no one the wiser except for me.

Where they’d come from, tracks ran up a delta of dirt to a narrow trail.

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It was really steep, and slippery too (that was a dry winter, you’ll recall). A false move might get a rider impaled on young acacia stumps. And if another biker had hurtled down upon me I had precious little space to duck. I climbed the trail a bit nervously. None of us really belonged there.

biketrail-2

Bikers get an intense ride here. An intense ride is a legitimate thing. But isn’t all they see here just a blur of trees during their minute of white knuckles? What I saw was untreated erosion on the ground, an unpermitted trail in the city’s Leona Heights Park, and a community of scofflaws for whom it’s their personal secret.

After a while I struck off the trail to continue my own intense hike, and that was that. Then the other week I found the trail’s top end, near the Merritt College parking lots.

biketrail-3

The trail is pretty nice up there. Not exactly legal, but not in the city park either. Useful. I even approve of it. I saw access to some nice countryside with splendid views, a well-built trail that suits the slope, and a community of avid cyclists for whom it’s their personal secret.

Once again I got sidetracked, so I haven’t traveled the whole trail yet. Clearly, though, the top and bottom segments, Jekyll and Hyde, meet at the fire road in Leona Heights Park. It would be really nice if the lower part were converted to foot traffic only, with erosion control and occasional steps. Mountain bikers could take the fire road down to McDonell Avenue.

At Leona Canyon Open Space Preserve, less than a mile south, “the park is an ideal place for hiking, running, biking, dog walking, and similar activities.” The intense, pellmell riding experience can be had on either the Artemisia or Pyrite trails there — just not the thrill of living outside the law.


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