Archive for the ‘Franciscan rocks’ Category

The Dunsmuir-Chabot trail

1 October 2018

The most remote part of Oakland will be opened to public access fairly soon, when the East Bay Regional Parks District finally gets around to constructing a trail between Dunsmuir Ridge and Lake Chabot. I made my way into part of the route a few weeks back. It’s interesting and inviting territory, set above an untouched oak-filled stream valley with Fairmont Ridge beyond.

The land has divided ownership, with parcels belonging to the city, EBMUD and the East Bay Regional Parks District. They’re all public agencies, so the bureaucracy must have been difficult. Here’s the setting, as laid out in a 2009 EBRPD map.

The solid blue line is a trail in Anthony Chabot Regional Park that’s been closed for years. The dashed blue line is the proposed Dunsmuir Heights to Chabot Regional Trail.

This summer the EBRPD board was shown this map of the approximate route. Again, the solid line is an existing (deteriorated) roadway and the dashed part will be built from scratch. The photo at the top of this post is the view from the EBMUD water tank at top center. The part I’ll be showing is between there and the “P” mark at the city golf course.

A couple more maps to help you see what’s here. First is the bare land as shown in Google Maps terrain view.

The centerpiece of the trail’s route is the valley in the lower right quadrant. Note its depth and steepness. The permanent stream in that valley has no formal name, so I will hereby dub it Chabot Creek. And finally here’s the same area in the 1947 topographic map, which shows the old roads that will become the trail.

If you look at the upper part of Chabot Creek valley, you’ll see it turns sharply from southeast to southwest as you go downhill. On the Google map, though, the streamcourse is interrupted by a flat area. That’s landfill made of waste from the Cypress Viaduct, which collapsed in the 1989 earthquake. It’s sterile and weedy, but the view is nice.

At its edge is a curious structure, visible in Google Earth, that turns out to be a spillway, made for the event of a large rainstorm during a very wet winter. Presumably the landfill was capped with a layer of clay to stop any contaminants from leaching into Chabot Creek, and thus the site would fill with rainwater quickly and have a risk of spillover.

If you looked closely at the second map, you saw the intriguing pointers to an “old foundation” and a “1936 WPA rock chimney.” The chimney is a massive stone fireplace, suitable for a hunting lodge. But the building it once occupied is gone. Foundations around it show that it was a group facility of some kind. I’m hoping that local historians can say more about it in the comments.

A stone in the entryway is carved with the date 1935, so the map is slightly in error.

In any case, the stonework is indeed classic WPA masonry, of the same vintage, material and durability as the Woodminster Cascades in Joaquin Miller Park.

And speaking of stone, what are the rocks like around here, you ask. Here’s what’s mapped in the area.

“Jsv” is the Leona volcanics, “KJk” is the sedimentary Knoxville Formation, and the blue field is Franciscan melange, the same body of rock underlying Knowland Park. When I visited, I checked out a roadcut right where the Franciscan and Knoxville meet and found an assortment of rocks.

All of these are appropriate for the Franciscan, but the brown sandstone could just as easily be from the Knoxville. A return visit is in order during the upcoming wet season, when the ground is firmer. I hope to see more signs of the Franciscan “knockers” so well exposed in Knowland Park — and on the golf course, like this blueschist knocker cropping out in the rough.

The plans for the Dunsmuir Heights to Chabot Trail are supposed to come up for public comment this fall, with the work to be completed by 2021.

Franciscan landscaping

9 July 2018

This house in Piedmont caught my eye not long ago. Homeowners who live in conspicuous places do their neighborhoods a service by making their properties shine. I appreciated the care the owners of this home displayed not just in their plantings, but also in their choice of rocks.

The site (110 Scenic Avenue) is in the middle of the block of Franciscan sandstone that underlies most of Piedmont and some adjacent parts of Oakland. The massive sandstone, of an unobtrusive tan color and undistinguished structure, makes a serviceable setting for some of the Franciscan’s other, more colorful rock types.

The exposure of bedrock is discreetly patched with concrete, which may well conceal rock bolts set into the hillside. The section of concrete on the right side, below, is surfaced with the same blue serpentinite found at Elks Peak in Mountain View Cemetery, the old pit at Serpentine Prairie, around Butters Canyon, and elsewhere.

There are several basins built onto the exposure. The bluish high-grade metamorphic river rock is carefully chosen, too. It comes from outside the Bay area, most likely somewhere on either flank of the Sacramento Valley.

And just beneath it is this little jewel of high-grade blueschist.

Of course a geologist’s first focus is on the stone. But the true beauty of a yard like this is how the rocks converse with the plants set among them over the course of a California year. I’ll be back to see that.

When did everything Franciscan begin?

25 June 2018

A paper I read last week led me a long way in an interesting direction that started in El Cerrito, just up the ridgetop from Oakland, at the property fondly known as the Mira Vista Golf Course (restored in 2011 to its original glory and rechristened with its original name, the Berkeley Country Club).

I first visited Mira Vista in 1999 to see the Hayward fault, which runs through the fairways and helps give the land its picturesque form. Trenching studies there have added to our knowledge of this threatening feature.

But what brought Mira Vista to mind last week was a paper in the journal Tectonics titled “Early Onset of Franciscan Subduction.” A handful of rock harvested here gave the authors a new answer to an old and vexing question.

California is known to geologists around the world as the type example of a subduction zone. Its rocks preserve a record — a messy one — of a long period of geologic time when a tectonic plate consisting of ocean crust was moving toward and diving (subducting) underneath North America, which consists of continental crust. That went on for some 150 million years, interrupted when the San Andreas fault system first formed about 30 million years ago and turned the plate boundary into the sideways-moving setup we have today.

During the subduction period, North America scraped off parts of the top of the oceanic plate. That collection of stuff, analogous to a pile of dirt on the blade of a bulldozer, is a mixed-up lithological scrapple called the Franciscan Complex.

One of the most basic questions we have about the Franciscan is, how old is it? That is, when did subduction begin?

The authors of the paper, led by Sean Mulcahy of the University of Washington, looked for special samples from “high-grade blocks,” lumps of rock that have survived being stirred by the bulldozer blade deep below the crust and returned intact. Most promising of these are the highest-grade rock type I’ve found in Oakland: blueschist. The authors studied just two rock samples, one from the high-grade blueschists of the Tiburon Peninsula and the other from Mira Vista, where high-grade blocks crop out of a matrix of serpentinite along with other Franciscan “knockers.”

The high-grade blocks are part of the golf course as well as the surrounding terrain.

The course is right next to The Arlington. It was easy to get there on the 7 bus line. Getting off near Madera Circle, I spotted telltale boulders at the roadside: Blueschist.

Mulcahy got his team’s sample from this knocker behind the fire station. Their International Geo Sample register says it was collected by “hammer and chisel.”

Close up, a fresh exposure of the rock — maybe the actual collection site — glistened with blue and green crystals: the high-pressure metamorphic minerals glaucophane and omphacite.

The real work began under the petrographic microscope, where Mulcahy had to untangle the complex set of high-pressure events that affected this rock from the geometry of these thin sections.

Briefly, the big garnet grains preserved grains of very old material inside them, protecting it from later reactions. Those grains were about 176 million years old; the same minerals outside the garnets were about 160. Their ages had been re-set during a later episode of high pressure.

Moreover, the earlier episode had a much higher pressure than the later one, high enough that the rock had been not blueschist, but eclogite (ECK-la-jite). It had been carried at least 50 kilometers deep, a depth only attainable by subduction, and just a few tiny shreds protected inside garnet grains, measured in microns, preserved the evidence.

That’s pretty cool. This finding sets a new record back in deep time for the beginning of Franciscan subduction, in the early Jurassic.

It also offers a telling clue about another California geology mystery: What’s the exact relationship between the Franciscan Complex and the Coast Range ophiolite? In brief, there are three main hypotheses, and this evidence weighs against two of them and favors the simplest one. That will change the book I’m in the middle of writing.

By the way, stairs and footpaths lead from here down through Motorcycle Hill to the El Cerrito del Norte BART station. Take that hike some time.

The St. James Drive roadcut

11 December 2017

Recent work in far east Piedmont has exposed some excellent bedrock worth a close inspection. Because the town government won’t put an interpretive sign there, this post will have to do.

To my knowledge, there are only two sites of powerline towers in Piedmont, one at the mouth of Estates Drive and the other at 298 St. James Drive, near Park Boulevard. Last year the power company replaced the latter pair with shiny new towers, and as part of the work it cleared the roadcut of its cover of acacia trees (Google Maps shows the site as a thicket going back to 2007). Shortly afterward I discovered it and had high hopes, though it wasn’t much to look at in late October.

By January, the exposure had been stripped of loose rock. Already it was clear that it would be a showcase of slickensides.

By August, a strong concrete wall had been put in place and landscape plantings made.

The slickensides turned out to be fabulous. These are the polished marks made as movement along faults grinds rocks against each other.

And here’s a closeup.

Also visible is evidence of brecciation, the geologist’s word for shattering rocks and cementing the pieces together.

The rock here is sandstone of the Franciscan Complex, specifically part of the Novato Quarry nappe. This is a thick slice of fine-grained sandstone that was laid down off the ancient coast of California, then shoved against the continent’s edge and pulled apart into lumps by the San Andreas fault system. A bunch of it underlies Marin County, and more makes up Point Richmond and El Cerrito as well as Piedmont and points south. This tectonic history probably accounts for the wear and tear visible in the roadcut.

When I visited the roadcut again last week, I annotated and recorded the site in the ROCKD smartphone app and announced it on Twitter. I’m trying out the app just for fun as a way to make some of my observations public. I’m looking at other apps for more rigorous mapping purposes.