Archive for the ‘Oakland geology views’ Category

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.

upperknowland-1

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.

upperknowland-5

upperknowland-6

upperknowland-7

upperknowland-8

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.

Chimes Creek headwaters

1 February 2016

I’ve mentioned how tempting the uppermost catchment of Chimes Creek looks, perched above the Leona Quarry scar:

chimesCktop

Finally got up there last week. Access is difficult and not for casual visits.

Here’s the valley in Google Earth, looking obliquely at it. At first glance it looks natural, but it’s heavily engineered.

Chimeshead-view

I’ll show photos going from top to bottom, between the two dots on the above image. The valley above the upper dot was filled in by the Ridgemont developers, using material cut from the ridge to its left. This is the view downhill from the position of the upper dot.

Chimeshead-1

The floor of the valley here is crisscrossed with concrete ditches, which converge about a third of the way down the transect at a culvert. Along the way you pass a large outcrop. All the rock here consists of the Leona “rhyolite.” The outcrops are tempting, but the slope is steep and treacherous.

Chimeshead-2

Here’s the culvert, possibly the only one in Oakland without graffiti, joined from the right side by the concrete ditch. I think it must carry runoff from the Ridgemont streets. But what’s that ugly orange?

Chimeshead-outlet

Why, it’s acid drainage from one particular part of the subsurface here. I would love to see the geotechnical reports from the time they built this development.

Chimeshead-pipes

This is one of many places where I’ve seen “yellowboy” in the Leona; the most notorious is the old sulfur mine, of course, and I noted another last week. By now I think that every excavation in this rock unit, old or new, should be treated as a potential hazard.

The next couple hundred feet downhill from here is a lovely tree-shaded, undisturbed steep cascade over large boulders. Here are just two of them.

Chimeshead-3

This beautiful rock is extensively slickensided (polished by underground movements) and coated with the iron hydroxide minerals that result from natural weathering. It is not stained orange by the pollution from upstream; in fact the water at this point is only slightly milky. It’s very much like the rocks in the Redwood Road boulder pile.

Chimeshead-4

And what to say about this one? It’s an unusual lithology within the Leona. My impression just from eyeballing it was that it’s an autobreccia — a ground-up body of lava or tuff consisting of lumps of the original rock in a matrix of pulverized (and relithified) rock. But that’s not the only possibility.

Another outcrop up on the valley wall exposes a slickenside that covers a good square meter.

Chimeshead-5

At the base of the cascade is more engineered land, a small catchment housing a screened culvert entrance. Chimes Creek is trapped here and conveyed beneath the old quarry and across the freeway to emerge in the Millsmont neighborhood. As I say, it’s engineered land, but it’s planted with trees and rather pleasant. It also catches runaway rocks before they can take out a townhome down below.

Chimeshead-6

The view up from the lip of the catchment shows the two outcrops and the shape of the land.

Chimeshead-7

I can’t wait to return for a closer look, though with so many other places to see it might be a while.


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