Archive for the ‘Oakland rocks’ Category

The Pinehurst Shale

23 January 2017

Much of Oakland’s high hills consists of our local piece of the Great Valley Group, the colossal set of sedimentary rocks that runs the length of the Central Valley along its western wall. (How our piece got over here is, as they say, poorly constrained.) The group is well exposed in Shepherd Canyon and points south, and it continues on the far side of the hills as far as the Upper San Leandro Reservoir. There the Redwood Canyon Formation, the upper unit of the group, is overlain by a little rock unit called the Pinehurst Shale. On the geologic map, it’s the slices of darker green nestled amid the Redwood Canyon Formation (labeled Kr).

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It’s named for Pinehurst Road, naturally, and its type locality is at the Pinehurst Staging Area on the far side of Redwood Regional Park. You know the place if you’ve ever driven out there.

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This exposure shows strong bedding, but it’s all shale — rock with little material coarser than clay size. Some of it’s soft claystone that crumbles and erodes easily. Here it is in a rain-carved rut in the fire road that leads into the park.

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Other parts of the Pinehurst Shale are very hard and resistant because they contain a goodly share of silica. Here’s what that siliceous shale looks like in outcrops along the trail . . .

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. . . and here it is in hand specimen. (For display only; I put it back.)

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Farther up the trail, the shale gives way to siltstone of the Redwood Canyon Formation. It’s quite a dramatic change.

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Both of these rock units contain microfossils from the Campanian Age, making them somewhere around 75 million years old. Whatever once lay above them has been removed by faulting, leaving a gap of maybe 60 million years between them and a little ribbon of Sobrante Formation across the reservoir on EBMUD land.

There’s another cool thing here. Here’s the rock just to the right of the top photo. See that big chunk of rock by the right edge, sticking out crosswise against the bedding?

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That is a sedimentary dike. It marks a place where once upon a time, before this rock was fully solidified, an imbalance in fluid pressures caused the one layer to force a crack upward through the overlying layers and squirt into it. Usually dikes are something you expect to see in lava beds, or producing “sand blows” on the ground after large earthquakes. This next shot is taken from the right side, looking at the top of the dike.

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An exposure like this, on a roadcut, is usually fair game for grooming by pulling out the shrubs and sweeping away some of the rubble. I do that here and there during my outings, but this spot looked a bit chancy.

I contented myself with pulling up some young French broom plants. Now’s the best time of year to do that, when the soil is saturated. If we all did a little of that, it would help fight the invasive problem. The land is worth the effort.

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Visiting our parklands can take you very far away.

A hunt for silica-carbonate

12 December 2016

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.

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So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.

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Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.

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It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.

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Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.

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And at the north end, during a strenuous climb, some Franciscan chert from the melange.

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The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.

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This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.

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Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)

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What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.

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That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.

Oakland geology ramble 2, Rockridge to Orinda

15 August 2016

The second geology ramble — my name for a long walk that starts in one place and ends in another — is a long and rugged one, just to show you I’m not kidding about these. From the Rockridge BART station to the Orinda BART station is a walk of more than 9 miles with a thousand-foot climb in the middle.

There are several ways to do this. This summer I’ve pioneered what I’ll call the middle route on two separate outings. Here they are, first on Google Maps and then on the geologic map (both images are 1000 pixels). The photos are a mixture from both traverses.

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From the Rockridge station, the route along the south side of Route 24 is more direct while the alternative, up Chabot Road to Roanoke Road to The Uplands to Tunnel Road (dashing across Tunnel to the uphill side), takes you through more shade and past more rocks, starting on Roanoke.

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The mixed lithologies of Franciscan melange (KJfm) give way to rugged outcrops of the Leona rhyolite (pink color) as you cross Vicente Creek on Tunnel Road. Admire them at the century-old estate called The Rocks. Beyond the Fire Garden is a short stretch without sidewalks that passes a long, excellent exposure of Leona Rhyolite. This was quarried in the 1930s and again in the 1950s during construction of upper Broadway, the Caldecott Tunnel and Route 24.

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The rocks change to mudstone of the Great Valley Sequence (Ku), then the much younger Sobrante Formation (tan color), as you ascend Tunnel Road’s steady, gentle grade.

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The change to the Claremont chert is dramatic as you near the ridgetop and enter Sibley Preserve.

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Cross the park on the Round Top Loop trail, which goes through the coarse-grained sedimentary rocks of the Orinda Formation (Tor), although you won’t see much of them. Take the Volcanic Trail left, which leads into the structurally overlying basalt of the Moraga Formation (Tmb). The quarry that carved up these golden hilltops extracted that basalt. In 0.2 miles, at the right edge of this photo, is a road with a cattle gate that exits the park.

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The views change dramatically on the east side of the hills, whether you’re looking to the left up Siesta Valley . . .

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. . . or to the right toward Mount Diablo.

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Straight ahead lies the unbuilt Wilder Ranch subdivision of Orinda. The valley it sits in is the continuation of Siesta Valley, and it’s underlain by nonmarine sedimentary rocks of the Siesta Formation.

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Both valleys owe their shape to the large fold, or syncline (“sloping together”), in the Siesta Formation that’s noted on the geologic map. The Moraga Formation basalt is also downfolded by this syncline, and it crops out again in the hill with the quarry scar.

This subdivision looks bleak, but the developers are doing the job right. The lots are gray because they’re sealed with some tough, pliant substance that prevents all dust and weeds. And as you cross, the route takes the dirt road running from the intersection of Wilder and Bigleaf Roads to the big bend in Rabble Road. You’ll pass several vegetated catch basins designed to hold the extra runoff from the new properties.

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This is another example of the flood-control practices I mentioned last week.

The route goes from Rabble Road to Boeger Ranch Road, but take the straight spur between them and follow it to the end, where a footpath connects with the end of Oak Road. All of this area is mapped as mudstone of the Mulholland Formation, of which I know nothing beyond its (young) age, Miocene and Pliocene. From Oak, take Stein Way down to busy-busy Moraga Way and from there head to the BART station. Sidestep as much of Moraga Way as possible by taking Camino Encinas.

If time permits, stop for a beer at The Fourth Bore in Theatre Square. If you take this ramble the other way, stop for a beer at Ben & Nick’s. Either way, you’ve earned it.

The first time I made this trek, many years ago, I took the northern route: up Claremont Canyon, north on the Skyline Trail, then down through the Lomas Cantadas maze to Camino Pablo. That was work. I’ve hiked up the canyon on Claremont Avenue several times, but the traffic is nerve-racking. The alternatives, through the Hiller Highlands or Grandview neighborhoods, are steep, sunny trudges. On the Orinda side it would be more fun to descend through the East Bay MUD land from the Skyline Trail (for which you need a hiking permit). I plan to attempt the northern route again when the weather cools. I have a vague scheme for a southern route, too.

See ramble 1 here.