Archive for the ‘oakland geology walks’ Category

Shepherd Canyon: Type localities of Oakland rocks

29 June 2015

Yesterday I led a walk for the group Wild Oakland that took in the rocks of lower Shepherd Canyon, which are the westernmost outcrops of the Great Valley Sequence. These are the same kinds of rocks that make up the monumental set of ranges marching up the western side of the Central Valley from Taft to Redding. The map below shows our planned route. The red dots mark the beginning and end of the route plus mileages. (In fact, for lack of time I cut the walk short where the 3-mile mark is, so we didn’t see the loop on the right side. I leave that as an exercise—and it is exercise—for the reader.)

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This is the geology along that route.

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The starting point, where we climbed up to the unpaved start of the Montclair Railroad Trail, offers a nice view over the valley of the Hayward fault, here at Montclair Playground . . .

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. . . and looking northwest up the fault valley.

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This outcrop, above the curved cut in the railbed, shows the Oakland Conglomerate to advantage.

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And maybe 100 feet away, the rock abruptly changes to shale of the Shephard Creek Formation.

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This spot corresponds to the symbol on the geologic map with the number “73” on it, which means that the bedding here is tilted 73 degrees from the horizontal.

Farther up the valley, we examined this outcrop of the Redwood Canyon Formation.

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I pointed out the thin set of shale beds running up the center of the image and showed how the sandstone beds on the left side had been laid down on top of the shale—that is, the internal evidence shows that this whole set of rocks here is tilted up beyond vertical and is upside-down. This spot corresponds to the symbol on the geologic map labeled “78”.

The last spot is in the Shephard Creek Formation where a large sandstone bed sits amid the shale. The location is just about where the word “Park” is along the walk route. On the underside of that sandstone bed is a splendid set of sole marks. This shot shows how the underlying shale is bent by the pressure of the overflowing sand avalanche that built this sandstone bed.

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And this shot looks up at the underside. When there are a sufficient number of these marks, the geologist can work out what direction the avalanche flowed.

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As I said, we cut the walk short at this point and came down through Shepherd Canyon Park along this stream valley, which is filled with a peculiarly flat deposit that I strongly believe is landfill. It forms the higher terrace in this view looking back from the soccer field.

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Does anyone know the history of this piece of land?

If anyone would like a copy of the handout I prepared, I’ll send you the doc file. Just write to geology at andrew-alden dotcom.

The Wild Oakland walk on the Hayward fault

9 November 2014

Saturday I led a walk for the members and friends of Wild Oakland to show off one of Oakland’s most striking places to encounter the Hayward fault. There was a nice turnout, about 60 people. I was glad to see so much interest. I hope that this post will enable those people, as well as all of you readers, to visit in person and learn more.

Here’s the route we took. It was just over 3 miles, although the altitude gain in the middle made some people bail out. Next time I’ll try to have alternative routes for their benefit.

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The numbers refer to the stops during the walk. The asterisks refer to direct evidence of the fault’s activity, both on and off the day’s route.

Next is the same map with topography added. The thrust of the day’s exercise was to tour some distinctive features that the Hayward fault has left on the landscape.

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Stop 1, where we started, is where Arroyo Viejo does its abrupt 90-degree turn on its way from the hills to the bay. The right-lateral Hayward fault has dragged the Bay side of the landscape to the northwest, and the creek has had to bend in response.

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It’s a vivid example of how plate tectonics works in California, caught between the Pacific and North America plates. As the Pacific plate moves northwestward, pulled in that direction by subduction zones off Japan and Siberia and Alaska, it moves sideways—right-laterally—with respect to North America. That distorts the courses of streams that cross the boundary between the two plates. That plate boundary is a wide zone with three main sets of major faults running along it. The Hayward fault is in the middle set.

At Stop 2 I was able to point out a good example of creep offset, where the curbs on both sides of Encina Avenue have been cracked and shifted by creep (slow motion, less than 10 millimeters per year, without earthquakes) on the fault.

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Stop 3 gave us a decent elevated view of the fault zone from the Oakland Zoo grounds through that offset valley of Arroyo Viejo. (Here’s an earlier post showing the other direction.)

At Stop 4 I pointed out another probable example of creep offset, and everybody turned on cue to look at it.

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Stop 5 was at a highly disturbed bit of ground on Ney Avenue. The scarp crossing the road appears to be the head of a landslide right on the active trace of the fault.

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Stop 6 was on a hilltop in the King Estates Open Space with a high view over the fault zone and the rest of the Bay area. The set of smooth-topped ridges extending into the valley of Arroyo Viejo have been cut off to form shoulders, and the dramatic shutter ridge on the right is the landform that has forced the stream to run sideways around it instead of straight to the Bay as it would prefer. (Here’s an earlier post about this same view.)

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I am quite taken with King Estates, and I believe it to be the largest piece left of the East Bay hills’ original landscape of grasslands. As the rains come this winter, I hope some Wild Oaklanders will poke around and examine it closely. For most or all of the people who came, it was their first visit.

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Along the way is this nice little example of a landslide.

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The King Estates hills are mapped as alluvium of an earlier generation than the Pleistocene alluvium that makes up East Oakland’s low hills. I wonder two things about them. Are they a pressure ridge, pushed up by compression across the Hayward fault? (I noted that the fault’s motion is 90 percent strike-slip and 10 percent compression.) And what is to be learned from the blend of stones that practically forms a pavement on the hills?

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Stop 7 was a view into the watersheds of the three creeks above the fault: Rifle Range Branch, Country Club Branch, and Arroyo Viejo. And Stop 8 was in the valley of Country Club Branch, so close to its neighboring streams but so well separated from them by elevated divides. I blame the fault, which keeps jolting the countryside out of the equilibrium it seeks.

For dessert, I present a portion of Jim Lienkaemper’s 1992 map of the fault, which has annotations about the detailed evidence along it.

WildOakwalk11-14-features

The features marked G are geomorphic—things geologists notice in the landscape. Those marked C are hard evidence of creep—offset curbs (rc), sidewalks (rs) and fences (rf), and at Stop 2, echelon cracks (ec) across the road that have been erased by road repairs since 1991 when the map was compiled. You can download the whole map and consult the updated version from 2008 if you like.


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