Archive for the ‘Oakland geology views’ 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.

Oakland panorama

29 February 2016

Even though it’s called Leap Day, February 29 feels like a good day to stop and look around. Here’s a fine panorama from 10 February 2015, the first day I explored the Ridgemont neighborhood high above the old Leona Quarry (1200 px). The view is almost due south.

ridgemont-pano

Most of the California landscape is oriented like this, northwest-to-southeast, thanks to its plate-tectonic position between the North America and Pacific plates.

What are we looking at? Here’s an annotated version, with the following landmarks noted.

ridgemont-pano-anno

1: Mission Peak, overlooking Fremont and San Jose.

2: Knowland Park hill with its green meadows.

3: Fairmont Ridge, overlooking San Leandro.

4: Loma Prieta.

5: Mount Umunhum and the spine of the Sierra Azul.

6: Coyote Hills on this side of the bay, Stevens Creek canyon on the other, where route 17 leads to Santa Cruz.

7: King Estates Open Space, looking its emerald best.

8: Black Mountain overlooking Mountain View (and hey what a coincidence).

All of these are great places to visit.

The red Vs mark the Hayward fault, left to right: at the foot of Mission Peak, in the saddle of Fairmont Ridge, in the Oakland Zoo and running down the valley of Arroyo Viejo past Holy Redeemer College.

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).

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And here’s your weekly cheesecake shot looking north from point 5 (1000 px). Rabid fans will note Sugarloaf Hill on the skyline.
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This is a great time to visit, but do watch for the newly sprouting poison oak.


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