Archive for the ‘Oakland rock types’ Category

Lake Temescal, the west side

12 April 2021

For one of Oakland’s most rugged places, the west side of Lake Temescal doesn’t expose a lot of rock. But what’s there is unusual for Oakland, and interesting.

I’m talking about this ridge — tectonically, a shutter ridge — across the lake from the swimming beach.

The map of Lake Temescal Regional Park shows two trails there, the low one along the water and the high one up in the woods. An even higher trail, not marked on the map, is off limits and doesn’t expose much rock anyway.

I often wish I’d lived here in the 19th century when Oakland was new. It was in 1868 when Anthony Chabot acquired a steep little canyon back of the hills and built a dam to supply the young city with dependable water service. His technique, perfected in the gold fields of the Sierra, was to take a high-pressure water hose and wash down the sand and gravel from the hillsides to build the dam. I wish I could have inspected the scrubbed slopes at that time, but there were almost no trained geologists in the whole state, let alone me.

The canyon has a flat floor now, after decades of sedimentation, but you can see from the high trail that it’s still steep and narrow.

And the action didn’t end when Chabot finished the dam. He built a control tower in the new reservoir, but a landslide soon took it out. I’m guessing that was probably near the sluicegate where the beach house is today, and I’m guessing that the rainy winters of 1868-69 and 1869-70 plus afterslip and aftershocks of the big 1868 earthquake on the Hayward fault had something to do with it.

Speaking of which, two major strands of the Hayward fault run right through the reservoir. They’re helpfully shown on the map above. This is where the main strand crosses the dam. (Don’t worry, the massive dam will not fail even under the largest possible quake on this fault.)

The dam itself was raised and then lowered during the next few decades, and presumably the lake rose and fell too. Meanwhile trees and brush moved in upon the slopes where the Ohlone had previously maintained grassy meadows, and the rocks decayed and soil built up.

And the rocks themselves embody the complicated history of starting out in a vigorous subduction zone, being deeply buried and exhumed probably more than once, then being torn up and shoved around by the San Andreas fault system of which the Hayward fault is part.

All that is to say that Lake Temescal is a dynamic area at all time scales. It’s more complex than the small-scale geologic maps can show, even though it’s complex enough on that map.

KJfm, Franciscan melange; ch, chert block; af, artificial fill; sc, silica-carbonate rock; sp, serpentinite; Jsv, Leona volcanics; fs, Franciscan sandstone; KJkm, Knoxville Formation

The west side is mapped as melange, which is basically a mess of mashed-up sandstone with big blocks of other rocks, each with its own separate history, suspended in it. The little blip labeled “ch” is this block of chert at the top of Hill Road. So when I visited the west side trails last week, I expected to find things I didn’t expect. And most of the rock along the trails appears to be nondescript sandstone — I say appears because since hammering is forbidden, it’s hard to find a fresh surface. But lo and behold, along the high path coming down from Broadway Terrace, there’s the telltale gleam of blueschist in the exposed subsoil.

The color really comes out when you wet a piece.

Other apparently high-grade metamorphic rocks include this hard, glittering schist. Not having access to thin sections and petrographic microscopes, I can’t check for the presence of jadeite, which has been reported in blocks from this melange.

And over at the north end of the train is a distinctive outcrop of another schist. Hard rock supports slow-growing lichens, and the species differ depending on the rock’s chemistry.

Underneath the lichens, the rock is a bluish-gray mixture with a strongly folded texture, both signs of a rock that’s been through a lot of distortion at high pressure and temperature. These photos are from exposures by the lawn. The first shows the folding and the second shows fault-related crushing on the left side. The crushed material is called gouge, and bits of it are common in this sector.

My authority on Oakland’s Franciscan rocks, John Wakabayashi, holds that the west side of the lake hides the same ancient major thrust fault — a megathrust, in fact — that’s famously exposed in El Cerrito above the recycling center, where high-grade rocks have been pushed above lower-grade rocks. Unfortunately the fault itself appears to be in “a brush-filled gully with no exposure,” so it may be a while before we ever learn more.

But I did learn something more last week, about the beach house.

It is made with the local rock, namely the Leona volcanics. Whether the stones came from the hill just upslope to the east, the one that collapsed in the 1870s landslide, or from across the freeway in the great Tunnel Road cut during the 1930s, I do not know. I’m guessing the former, but I would be happy to be wrong if it means I can be certain. Putting the building and the roadcut in the same photo, there is a resemblance.

The fun thing is, both locations can be closely inspected. That sounds like a good afternoon project.

Rock garden coming to Lake Merritt

29 March 2021

The Gardens at Lake Merritt are building a rock garden in the heart of the grounds. They have plenty of gardens with stones in them already, but this will be a proper rockery. This post is about the work in progress. I know almost nothing about the plants they’ll be featuring, but I do know a little about rocks.

As we enter yet another year of drought, it’s important to note that rock gardens are made to conserve water. The stones and gravel offer solid shade to the underlying soil, and the typical plantings are small, hardy species from alpine or desert settings. As you bend down to admire these plants, have a look at the stones.

The location is between the community plots and the Torii gate, the crossroads of our remarkable garden complex. Just across the path is the hill-and-pond garden, where the turtles hang out.

This view toward the lake shows the layout. In the background is the entry to the Sensory Garden, which has had a thorough going-over during the shutdown.

The foreground containers in both photos showcase rounded river stones, blue-green argillite most likely from the northern Coast Range. I would not be surprised if some of the rock nuts of the Suiseki Societies of Northern California contributed to this project.

The center of the garden is a mound of sandstone tablets, with some accent stones, oriented north-south for optimal sun. Rings of different colored gravel surround it. Note the “do not climb” sign.

Some of the basins echo the brown sandstone of the central mound, offering textural contrast.

Others contrast more strongly. Here rough greenstone is set in crushed marble.

And what would a Northern California rock garden be without some red chert?

All of these rock types are typical of the Franciscan Complex, a lithological scrapple that makes up the bulk of the northern Coast Range, including San Francisco and the hill that Piedmont sits on. Get to know them, and you’ll see them all over the place.

The Gardens at Lake Merritt have several sectors that artfully mix plants and stones. The water garden I mentioned earlier is one, and there’s the cactus garden and the bonsai garden (which has just added a suiseki section) and the enclosed Japanese garden by the Community Center building. They’re all looking great right now, but check the hours before you go; weekends are still closed.

A few years back I wrote about the remarkable rock garden assembled by Ruth Asawa in San Francisco. As the world reopens, I hope to visit it again.

The Paleocene blob revisited

15 February 2021

It may seem to readers like I’ve been out in the field during the past year, but in fact I’ve been holding back since the lockdown last March, taking my own advice. Last week, on the verge of receiving the Covid vaccine, I decided to formally resume geologizing, and start by giving a small patch of rocks a new, more thorough inspection.

It was thirteen years ago when I first reconnoitered this odd little area of rocks, shown on the geologic map as unit Ta, “unnamed glauconitic sandstone (Paleocene).” It’s described as “coarse-grained, green, glauconite-rich, lithic sandstone with well-preserved coral fossils. Locally interbedded with gray mudstone and hard, fine-grained, mica-bearing quartz sandstone.”

Here’s a closeup in Google Maps with the outline of the “Ta” unit. It manifests as a ridge-forming substrate that is undermined by an active landslide scar (part of which is the notorious Snake Road/Armour Drive landslide) on its northwest end.

In a systematic approach, I sought out the three places marked on the map with strike-and-dip symbols. (I used this same strategy a couple years ago with the overlying Eocene mudstone unit.)

The northernmost site, at the stub end of Armour Drive, is hopeless; it’s been thoroughly disrupted by the Snake Road landslide, and the fortress houses being built on the scar will disturb it more as the owners landscape their grounds. There were no good exposures at all, let alone one showing beds dipping 80 degrees south. But this is what some of the rock looks like: a dark siltstone with a greenish tinge and a bit of clay in it.

The middle locality was where my hopes were highest — an aborted foundation pit on Saroni Drive where the “well-preserved coral fossils” had been documented. In fact, I had asked Russ Graymer, compiler of the geologic map, about this pit. That was in 2009, which by his account was 14 years after he’d visited it (or a good 25 years ago today). He replied that his notes from the site were as follows: “The rock here is massive, black, coarse-grained, glauconitic sandstone and pebbly sandstone. There are many fossils here, including pecten, coral (Paleocene?), shark teeth, and snail. There is also pink-brown siltstone and brown mudstone.”

All I can say is I wish I’d been here 25 years ago.

I gave the site a thorough look, without hammering anything as is my practice. I saw no pebbly mudstone, not even any coarse-grained sand. I noted clayey siltstone and silty shale, hard here and soft there, with fine to massive bedding. On the lefthand side the shale beds were vertical, with the upper side to the east. Nothing that I could possibly interpret as overturned beds with a 60-degree dip.

Elsewhere the rocks had no reliable bedding. Down in front were some crumbling mudstone boulders. One of them had some vague fossil-like shapes that fizzed in acid, but the eyes can be fooled and our rocks commonly have some lime in them. It’s not always meaningful, though I always check for it.

You may wonder how this rock unit was determined to be of Paleocene age, unique in Oakland. As I recall our conversation, Graymer was accompanied that day by Earl Brabb, who said the corals reminded him of Paleocene corals he knew from the Santa Cruz Mountains. In fact I wrote Brabb for more detail and he replied with the location of the roadcut he had in mind. But I never got over there, the email was lost, and Earl Brabb died a few years later. Now I would never gainsay Brabb’s judgment — he was a top-tier field geologist — but that’s the main line of evidence behind this age assignment.

I wish he had been with me at the third site. It’s under a power-line tower north of a bend in Balboa Drive and consists of thin-bedded siltstone, nicely tilted. This spot, at least, is still good.

The roadcut on Balboa Drive was where I hit paydirt. Bedding surfaces were exposed that included sole marks. These occur on the underside of beds, and they indicate that here the rocks are overturned, contrary to what the map shows.

And in the gutter of the curve, buffed by errant car tires, were a couple of these round, laminated objects nestled in situ among the siltstone beds. They responded to acid, indicating that the laminations included calcite. And the rocks nearby displayed a fine vein of solid calcite about 4 millimeters thick.

I would peg these as some sort of fossil, but Earl Brabb might well have said they were just like the Paleocene corals he knew from the Santa Cruz Mountains. The setting could have been a cold seep, such as are known elsewhere in the Great Valley Sequence.

The rocks of the Oakland Hills are poorly organized and poorly exposed, and hence not really well mapped. They’ve been overturned and broken and shuffled around. Whenever I try to make sense of them I doubt my senses; that’s the way the Earth just is here. A geologic map is as much an exercise in imagination as in observation. The pros are certainly better mappers than I am, but they aren’t superhuman and their work can be interrogated; the rocks can speak differently with each visit. The outline on the map, as far as I can tell by checking around its edges, is fairly correct — you’ll notice that every line is dashed, meaning it’s inferred, not firmly nailed down.

The “Ta” rock unit hasn’t revealed itself to me as a coarse-grained green lithic sandstone, more like a fine-grained sorta greenish lithic siltstone. Geologists train themselves and have tools to specify rock colors, but to me green is always suspect; our woods favor mosses and algae, and our weathering environment favors rusty colors.

The rock here is definitely something other than the Redwood Canyon Formation to the south and the Eocene mudstone to the north. It’s a little piece of somewhere different.

Mapping rocks never ends

1 February 2021

A few days ago I took part in the latest monthly meeting of my local geological society — we do it via Zoom these days — and our speaker, Christie Rowe of McGill University, reported on three research projects her grad students are doing in the Bay area, specifically the Franciscan Complex. The Franciscan is a scramble of different rocks that has challenged geologists since they first came to California.

Fifty years ago Stanford’s Gary Ernst recognized that the Franciscan represents the mess of material that gathers around a subduction zone, where oceanic crust (a now-extinct neighbor of the Pacific plate, in our case) slides beneath continental crust (the North America plate). So now we know what it is — the tectonic equivalent of the dirt in a bulldozer’s blade — and prompted by that knowledge we can try to unscramble the mixed-up pieces and learn what they might tell us about California’s geologic history or what happens in subduction zones.

Rowe is a Marin County native who’s been working since her PhD days on the latter problem, in the Franciscan rocks of her home ground. Specifically, she’s been looking for preserved bits of ancient earthquake faults. Normally these are buried deep underground, but they’re important because subduction-related earthquakes, so-called megathrust events, are the largest on the planet. Think of the 2011 Tohoku earthquake in Japan, the magnitude-9 monster whose tenth anniversary is coming up on 11 March. The Marin Headlands are full of them, broken in pieces.

Rare bits of the Franciscan have survived being subducted deeper than 25 kilometers and then returned to the surface, without totally wiping out what happened to them down there. The work requires dogged persistence. You have to look hard to find these “high-grade blocks” in the first place, then put your face close to them, magnifiers out, detect signs of slippage, then bring samples to the lab and determine what that slippage means — whether it happened on the way down, on the way up or afterward as the San Andreas fault system wrenched it all sideways.

Heart Rock, at Jenner Beach up the Sonoma coast, is small enough to fit inside a living room. One of Rowe’s grad students is mapping it at centimeter scale, spending a master’s thesis worth of effort on this one outcrop looking at rocks like this.

Seeing all this during Rowe’s talk took my mind, among other places, out to the rocks of Shepherd Canyon and Redwood Peak. The last person to give those strata a PhD-level scrutiny, using all available tools of the time, was a Berkeley grad student named Jim Case around 1960. Yes, 1960, a time when researchers were stuck in a mental framework of now-forgotten concepts and plate tectonics was still years in the future, when optical microscopes, brass seives, fossil correlations and test-tube chemistry were the best tools we had.

Case got his PhD, demonstrating that he’d mastered these tools as well as the literature, but he didn’t accomplish much more than correct a couple of ideas from earlier studies, establish a few new rock units on the map and tentatively correlate them with other units scattered around the East Bay. He put his little brick into the Wall of Science, then went on to a long research and teaching career doing other things.

Since then, other distinguished geologists have been over this territory. Case collaborated with Dorothy Radbruch of the USGS, a sharp and able field geologist. And in the late 1990s when Russ Graymer was putting together the East Bay geologic map that I rely on, he tramped the area with the late Earl Brabb and was ably advised by the late David Jones. Each of these workers found new things and revised their predecessors’ achievements. It always paid to reinspect the rocks. Nevertheless, none of them pulled out all the stops and pioneered a new in-depth reassessment of this interesting area.

We could do much better today. Every tool has advanced. The jigsaw puzzle of ancient California is far enough finished that any piece, if studied closely enough, can be placed on the table near — or even exactly on — its correct position and joined to other pieces. This would be more satisfying than what Case could accomplish in his time. We just need another grad student to take it on, another local who has imprinted on his or her home ground.

Mapping never ends, and geologic mapping always improves. New bits of rock are being exposed all the time. Fresh eyes see new things, and persistence furthers.