Oakland Geology

For several miles starting near the top of Marlborough Terrace, Skyline Boulevard runs through a belt of blond chert that makes for striking roadsides and some of Oakland’s highest homes and best views. Near the south end of this stretch, north of Elverton Drive, the road runs east-west for about 700 feet. It’s just you, the view, and these rocks. In my book I describe them as “pale, flinty chert in layers a few inches thick alternating with thinner layers of soft brown shale. The layers, thousands of them, stand on end like a storm-swept forest of golden bamboo.”

Note that the nice wide verge is now blocked by a line of huge logs. These extend for miles along Skyline and Grizzly Peak Boulevard. There are still small places to park, though.

Here’s the location on the map. The land is an outlier of the Sibley Volcanic Regional Preserve, and a little trail runs along the ridgetop between the two boulevards.

The rock in this roadcut is chert: a hard, waxy-looking stone made from accumulations of microscopic diatom skeletons. It ranges from white to brown to black, simple to complex. It dates from the middle of the Miocene epoch, probably the Tortonian age around 12 million years ago.

The complexity extends to the larger, outcrop scale. Notice how the straight, even bedding on the left gives way to distorted layers on the right. The boundary between them is a fault. There are several more along the roadcut.

In addition to the faulting, this even-bedded, slowly accumulating sediment was periodically disrupted while it was still soft, presumbly by large undersea landslides triggered by earthquakes, just like today. Here’s a more extreme example; notice the wisp of chert entrained in the mixture on the left.

Here’s a foot-thick bed of clean white sandstone, preserving an ancient submarine landslide. By the way, it appears that nearly all of these rocks are overturned, their upper surfaces now facing to the right.

In the middle part of the roadcut, it really gets hairy. (This is the biggest image of the set.)

Other parts of the roadcut look tidy and regular, but erosion has cut into softer parts of the formation in the century since Skyline was built to attract developers.

Here’s a closeup of the groove in the middle. What caught my eye was the dark zone in it with orange veins, which shed pieces onto the roadside slope below.

The material (on the right) is a lightweight siltstone shot with films of black.

It’s likely that the black films are the remains of crude oil — not the only possible explanation, but it’s how to bet. Chert is a source rock for petroleum because the diatoms that compose the rock manufacture oil to help their opal shells float. As the diatom ooze becomes rock, the oil is released and migrates elsewhere until it’s trapped in an underground structure, eaten by microbes or oxidized in the atmosphere. Our Miocene chert is a close relative of the widespread Monterey Shale, which is responsible for California’s richest oil deposits. So the zone of orange veins might be a former escape avenue for the local oil — an injectite.

But above the roadcut, a little off the ridgetop trail, I encountered an intriguing alternative in this heavily etched material impregnated with silica.

Here’s a side view of another specimen showing the veins of hydrothermal quartz (chalcedony). The matrix between them appears to be the same stuff of the orange veins.

Now the formation of chert from plain diatom ooze, or diatomite, takes place at low temperatures and pressures. It’s very gentle. Hydrothermal silica is a sign of higher temperatures. Something more active than trickling crude produced this. Again, various explanations are possible, and these scattered blocks of rocks aren’t firmly connected to the putative injectite in the roadcut. I’ll just have to poke around here some more. The complexities multiply.

By the way, maybe some of you remember when this stretch of road was a cool, shady run through a high eucalyptus forest. That was how it was when I first explored the area. Google Earth stacks imagery of this area going back to aerial photos from 1939, and there was no eucalyptus here at all until the 1980s. Yet the trees were a hundred feet high in late 2003.

The Park District took them out around 2005, and in the fall of 2009 just a little fringe along Grizzly Peak was left.

Twelve years later, chaparral and oak-bay-madrone woods had made quite some headway.

But if you ask me, the site would be an excellent one to try restoring the traditional fire-groomed grassland that once covered all the hills north of the redwood groves. The Park District owns it, and California still has experts in the old ways.

This entry was posted on 1 August 2022 at 8:01 am and is filed under Oakland chert. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.