Oakland’s conglomerate cousin

26 October 2015

Last Saturday I took part in a field trip with the Northern California Geological Society, looking at conglomerates down at the San Luis Reservoir, in the mountains between Gilroy and Santa Nella. It sounded kind of dry and remote and obscure, but I always have a good time, and I got new insight into Oakland’s rocks.

First a quick refresher: the Oakland Conglomerate is made largely of cobbles in a matrix of sandstone. It crops out in a long belt starting in Montclair, crossing Shephard Canyon and stretching along Skyline Drive across Lake Chabot into Hayward and beyond. Here’s an exposure above the Montclair Railroad Trail, about as good as you’ll see it.


The Oakland Conglomerate dates from late in the Cenomanian Age of the Upper Cretaceous Epoch, a period of geologic time between 100.5 and 93.9 million years ago. At that time the west coast was a large subduction zone, rather like the west side of South America today. Eastern California and Nevada was boiling with volcanoes, with big bodies of granite rising up beneath them.

Huge amounts of sediment eroded off these rocks were delivered down rushing rivers and dumped into a deep-sea trench near the coast. Conglomerate is a typical product of this high-energy setting as coarse-grained sediments are deeply buried.

At the same time, volcanic island chains and seamounts and other bits of the earth’s crust were being carried into the trench from the west, on the back of the oceanic plate, as it approached and plunged beneath the North American continent. These also shed large amounts of coarse-grained sediment into the trench, along with their mangled remains. (Our Leona “rhyolite” is one of those remains.)

What I didn’t know before taking the field trip is that the same kind of rocks were being made during Cenomanian time down at the San Luis Reservoir. (Probably a bit earlier than in Oakland, although we can’t get very precise ages from rocks like these.)

Here’s the geology of the area, from the online state geologic map. The reservoir is at the center.


The three dashed lines in the reservoir are strands of the Ortigalita fault. We walked along the shore just east of the middle strand, starting at the Romero Visitor Center. Here, just like in Oakland, the set of rocks known as the Great Valley Sequence (light green, labeled “Ku”) is tilted toward the east and exposed edge-on in the hillsides. That means the oldest rocks, at the bottom of the pile, are exposed along the western edge. The tilted beds of these rocks are subtly visible in the grassy hillsides in this view north, just as they used to be in Oakland’s hills before we let the trees take over.


Also, just like in Oakland, the Great Valley Sequence is shoulder to shoulder with Franciscan rocks across a major fault. In Oakland the two bodies of rock are interesting examples of their type. At San Luis Reservoir, the two bodies are world-class examples of their type. The Franciscan rocks there are jadeite-bearing meta-graywackes that are famous in the literature. The Great Valley rocks there are the biggest conglomerates in the whole state. They aren’t yet famous in the literature, but our field-trip leader wants to raise their profile.

San Luis Reservoir is a storage tank, not a lake. It’s pumped full and drained under human control to regulate irrigation for the San Joaquin Valley. As the water has risen and fallen over the last 50-plus years, it’s washed the soil off the rocks and left fabulous exposures. Here’s the view from the visitor center, with the dam on the left (east).


You can walk all over these yourself, just as we did.


Oakland’s conglomerate generally contains stones of pebble size (16 to 64 millimeters) or cobble size (64 to 256 mm, plum to basketball size). The San Luis conglomerate has stones of boulder size, which is pretty phenomenal. The biggest of these are measured in meters.


Our trip leader, Todd Greene of Cal State Chico, helped us picture this area as sitting at the mouth of a big, long-lived submarine canyon like the Monterey Canyon off our coast today. Torrents of well-rounded river rocks roared down its channels to pile up in a deep-sea basin. So the biggest rough-edged boulders, like the one he’s sitting on, represent crags that have fallen from the sides of the canyon.

We worked our way along the shore, moving down through more than a kilometer of conglomerate beds. Near the bottom was a thick unit of modest-sized stones, something very much like our Oakland Conglomerate.


Imagine if we could see exposures of this quality in Oakland. Oh well, everyone needs a daydream.

Two lessons about floods

19 October 2015

As we anticipate the strong possibility of heavy El Niño rains, my attention will be on Oakland’s streams this winter. Last week parts of southern California were hit by “thousand-year” rainfall events, cloudbursts that washed thick sheets of mud over roads and properties. We can expect such things here too, in any given thousand-year period.

Arroyo Viejo, the stream that crosses Knowland Park, offers two lessons about floods. The scene below is at the northern edge of the park, looking upstream: a streambed piled with boulders, some as large as sofas. (All photos 800 px)


Notice: these rocks have been tumbled by the stream. How much water would it take to do that? Let’s make a rough, arm-waving estimate.

The rainfall in last week’s cloudburst was almost 4 inches in one hour. Had it fallen on the watershed of Arroyo Viejo above this point — say, half a square kilometer — it would represent an input of roughly 30 cubic meters of rainwater every second.

Picture in your mind that volume of water — no, it would be mud and therefore that much greater — funnelled through this narrow valley. Do a little geometry and it’s easy to see the floodwater would be well above the tops of the boulders.

Hidden in plain sight in this photo, then, is a single hour of tumult that might have happened a thousand years ago or five hundered years ago — or perhaps during the dreadful winter of 1861-62, when it rained for 43 straight days and much of the Central Valley became a lake.

The lesson is that most of geology’s hard work gets done in rare spurts of extraordinary activity.

Okay, the second lesson is hidden in these rocks. All of them, like this boulder as tall as me, are made of conglomerate.


These rocks, assigned to the Knoxville Formation of Late Jurassic to Early Cretaceous age, were laid down by ancient floods in a nearshore or terrestrial setting. I’ll show you three different specimens. Notice the large clasts and the fine-grained matrix that surrounds them.


This boulder displays a wide range of clast sizes. It was probably laid down by what’s called a hyperpycnal flow, a slurry of sediment that carries everything along with it. We’ve watched them happen offshore in Monterey Canyon. Here’s another example.


Instead of an underwater landslide, as seen in the first specimen, this represents something gentler and more organized, like a mudflow, or like the mudslides we saw in the news. The clasts are aligned with the current that carried them here.

The key observation in both cases is that the large clasts are floating in the matrix. In geologist’s terms, they are matrix-supported conglomerates.

Then we have this.


Here’s a beautiful clast-supported conglomerate. It represents a clean bed of well-rounded cobbles, all touching each other, like you’d see in a rushing stream or a rocky beach, nicely infiltrated with clean silt or clay after it was laid down.

None of these stones were made by ordinary sediment wafting down streams during ordinary rainy seasons. They were assembled by floods of all sizes.

Knowland Park knockers I: High-grade chert

12 October 2015

As I review my discoveries at Knowland Park since that day in July when I first set foot there, it makes me want to linger over its rocks. The large exposure of the Franciscan Complex is probably the park’s most significant geologic feature.

Oakland has two exposures of Franciscan rocks. The big one underlies Piedmont and the adjoining neighborhoods of Oakland, which means it’s covered with houses, roads and landscaping. The small one underlies Knowland Park and the uplands of the Chabot Park and Chabot Park Highlands neighborhoods. The portion in Knowland Park is the largest open exposure of this rock unit in the city and perhaps the whole East Bay.

It’s Franciscan melange, but distinctly different from the Piedmont block. Melange is a mixture of rock types, largely mudstone of various types, with big chunks of harder stuff floating in it. In the landscape, the chunks emerge as the surroundings erode away, and generations of California geologists have called them knockers.

Knowland Park’s knockers include several different rock types, and the largest number of them are high-grade chert, or metachert. Unlike the red chert that’s typical of the Piedmont block (and San Francisco and the Marin Headlands’ great exposures), Knowland’s chert is green and hard and recrystallized. It has undergone deep burial, perhaps more than once, yet it appears to retain its original layering.

Here’s one of a number of very large boulders exposed down by Arroyo Viejo in the woods. (All images are large and should be clicked for the best detail.)


Most of the chert knockers aren’t this well organized. Here’s one that I showed to the group on the October 4 geology walk.


And here’s one I didn’t. The chert fabric is pretty much gone. Still other knockers are probably totaly chewed up, and without hammering them to get a fresh exposure, which is forbidden in the park (and against my practice anyway) it may be hard to identify.


This is the park’s most prominent chert knocker. Both the October 4 walkers and the September 20 mappers enjoyed the stone and the views.


Close up, the stone shows tantalizing details.


But elsewhere in the park I found this beautiful fragment that displays the clean color of the stone and the etched surface of its layers. I left it behind for you to find.


Perhaps some amateur with a hammer made this fragment. If people feel free to take pretty things away from the park, after a while there won’t be any more pretty things in it. On the other hand, maybe it was the graders who created this fragment as they maintained the roads in the park. Those are lucky breaks for the rest of us.


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