Archive for the ‘Franciscan rocks’ Category

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.

Bedrock in the headwaters of Trestle Glen Creek

13 November 2017

Surprisingly, the town of Piedmont has its share of woodland trails — well they’re paved sidewalks, but they’re unused, covered with duff and overhung with untended shrubbery. On a weekday afternoon, you can walk quietly on miles of these soft paths and encounter only a handful of property workers. It’s in that spirit that I recommend a geologizing stroll around the highest part of Trestle Glen Creek’s eastern watershed.

The shaded-relief map below shows the creek valley in eastern Piedmont, with Dimond Canyon on the right edge. The area I’m featuring is the triangle just right of center bounded by Crest Road, Pershing and Estates Drives and Hampton Road.

And here’s a closeup just to display the street names. During weekdays, the 33 bus stops at Lexford and Hampton, where the two valleys in this little watershed join.

The geologic map shows that this area is solidly within the block of Franciscan sandstone (Kfn) that underlies most of Piedmont. The hilltop above it consists of Franciscan melange (KJfm) that includes bodies of chert (fc), notably the one on Pershing that I’ve called the best bedrock in Oakland.

The neighborhood is gracious. This view looks up Huntleigh Road, which runs on the valley floor. As I traversed the streets, I used sidewalks that almost never feel a human foot. At times it was easy to imagine being in a Tolkien novel.

Lexford Road, in its own valley, is more secluded and more whimsical architecturally.

For the geologist, these streets are valuable because they aren’t as tightly landscaped as in most of Piedmont, and the Franciscan bedrock can be seen and studied at leisure in several places where the road builders exposed it. That’s unusual for this town.

Plenty of hand specimens are available too, if that rings your chimes.

There are even a few empty lots here. Unlike the existing homesites, these are especially challenging due to the steepness of the terrain and the strength of the rock — not just on the surface, where the weathered sandstone has fractured into rubble, but also deeper down where foundations would need to be dug into the hard, unweathered bedrock. When this lot was cleared recently, it had shed enough rubble to nearly cover the sidewalk.

A large house has been proposed here for many years, and the record of the intricate wrangling needed to invite and address everyone’s concerns is mind-numbing. However, the record does include geotechnical reports that give us a glimpse underground.

What’s on the site now is this set of what are called story poles, which serve to outline the planned building.

Geologists acquire a certain ability to see the ground through everything growing or sitting on it. It’s an ability to visualize the landscape as if it were covered with story poles instead of vegetation and structures. This bit of watershed is a good field site to practice.