Archive for the ‘Leona volcanics’ Category

The great Tunnel Road cut

2 September 2019

The land on the south side of Hiller Highlands is far from its native state: it’s been extensively quarried for many years, and what’s left is a rocky, weed-choked waste. But the roadcut is also a geological treasure.

It’s one thing to look at a hillside and determine what it’s made of, another to study it carefully enough to determine what formation it belongs to. These are worthy accomplishments to be sure, but a more precious one is to find and study a place where different rock units come in contact.

Here’s where I bring out one of my favorite quotes from the history of geology, in the early 1800s when people were beginning to work out what the rocks were telling them. A party of geologists including the eminent Sir James Hall took the Rev. William Richardson, a notorious opponent of the newfangled Scottish school (now textbook orthodoxy), to the Salisbury Crags in the heart of Edinburgh. There they showed him a contact between traprock (a basalt lava flow) and sandstone, pointing in particular to bits of sandstone that were enclosed within the traprock. This contact was proof positive that the basalt was (1) a formerly molten rock that (2) had intruded into the sandstone long after the sandstone had formed:

“When Sir James had finished his lecture, the Doctor did not attempt to explain the facts before him on any principle of his own; nor did he recur to the shallow evasion of regarding the enclosed sandstone as contemporaneous with the trap; but he burst out into the strongest expressions of contemptuous surprise, that a theory of the earth should be founded on such small and trivial appearances! He had been accustomed, he said, to look at nature in her grandest aspects, and to trace her hand in the gigantic cliffs of the Irish coast; and he could not conceive how opinions thus formed could be shaken by such minute irregularities as those which had been shown to him.”

Contacts among Oakland’s rock units are hard to find because our rocks are poorly exposed to begin with. And even when you do find contacts, they may not preserve the small and trivial appearances that might tell you the most. But the Tunnel Road cut exposes a large area of rock, as seen in this aerial view from Google Maps. The slope is interrupted by several cutbacks that serve to stop runaway boulders and allow access for maintenance — most recently by the herds of goats that helpfully cleared away most of the French broom — and visitors like me last week.

This big cut exposes a significant contact right along the road, just west of the Gateway Emergency Preparedness Exhibit Center (under the word “Hiller”). The geologic map shows the spot as the contact between the Leona volcanics, in pink, and a teeny splinter of olive-green Knoxville Formation directly above the “o” in “Substation.”

The contact today is somewhat obscured by vegetation, so let me show it to you first as it appeared from Tunnel Road in December 2007, when the state last cleaned it up.

From lower left to upper right, the rocks gradually give way from highly altered volcanic rocks, containing some shaly beds, to dark-brown shale. Some of the experts heartily disagree on what exactly is happening here, but it’s widely taken to represent the very top of the Leona volcanics and the very base of the Knoxville Formation, lowest member of the highly sedimentary Great Valley Group.

Here’s how it looked in July 2019. You can tell in both photos that the shale is crumbling down the slope almost as fast as it’s exposed whereas the volcanics stand sturdier.

Along with a few other localities scattered around California, here’s proof that the two rock groups started out as peaceful neighbors, the shale laid gently down upon the volcanics under the Late Jurassic or Early Cretaceous sea. Although later tectonic events wrenched and stretched and broke all of these rocks as California gradually became its present self, this spot remained untouched for geologists to argue over. As an example, I captured Cliff Hopson pressing a point to the late Eldridge Moores at this very outcrop in 2005, no doubt discussing some minute irregularity.

It was a pleasure to stop here in July with John Wakabayashi, leader of that 2005 field trip. He noted how important it is to revisit outcrops: “When you come out, you notice things you didn’t notice before.” And he pointed out features of the volcanics I hadn’t picked up on my own. For instance, the locality is unusual in featuring fairly fresh volcanic glass, which can yield more faithful geochemical data than the altered rocks around it.

He also found something he hadn’t seen before: carbonate veins with shapes that reminded us of soft-sediment deformation. This suggested to him that they may have been original constituents of the rocks and not later alterations, in which case they might preserve microfossils.

Familiar features of the Leona volcanics are well displayed here, including its lumpy and fractured texture, a reminder that the unit is mostly not lava flows but ash beds and landslide deposits, fused and altered by hydrothermal springs.

Slickensides — polished fracture surfaces — testify to much later activity related to the Hayward fault and the rise of the Coast Range.

And I always take pleasure in spotting the green devitrified ash whose color is attributed to celadonite, possibly with other green secondary minerals like prehnite, chlorite and epidote.

Familiar places can still reveal new things, if you keep your eye on seemingly small and trivial appearances.

Oakland and the Coast Range ophiolite

24 June 2019

A commenter asked, in connection with a recent post, if I’d written anything about Oakland’s ophiolite. The answer is, not specifically until now. The Coast Range ophiolite (OH-feel-ite) is a string of mostly disconnected outcrops of unusual rocks that extends north almost to Redding and south almost to Point Conception, rather like the way my writing about it runs through the ragged string of posts on this blog between late 2007 and today. In Oakland, the serpentinite patch is part of it, the San Leandro gabbro is part of it, and the Leona volcanics are part of it (see posts about that part here and here).

Here’s a recent simplified geologic map from a 2005 paper by ophiolite savant John Shervais (doi: 10.1130/B25443.1, available here) showing the most important bits of the ophiolite in black. It’s kind of a privilege to have a piece of it in our city.

The details in California are intricate and I’m about ankle-deep into them at the moment, so I’ll be pretty general here. Ophiolites were first recognized as a suite of related rock types over 200 years ago, at the dawn of scientific geology, in the Alps. Alexandre Brongniart gave them the name, which means “snake rock” in scientific Greek, because serpentinite (“snake rock” in scientific Latin) is so typical of them. About 50 years ago, at the dawn of plate tectonics, they were recognized as pieces of oceanic crust that somehow have ended up on land during the elephantine dance of the tectonic plates.

The oceanic crust of most ophiolites is not as well organized as the standard oceanic crust formed at mid-ocean ridges. It forms in the roiling setting near subduction zones, where subducting plates sink straight down and draw the other side toward them. (This situation, called slab rollback, is the opposite of what we’re taught in popular accounts that talk about subduction in terms of colliding continents and mountain-building.) As the plate on the other side is stretched thin, new magma forms beneath it, rises and freezes into fresh oceanic-style crust (the ophiolite). It’s because most ophiolites form near land next to subduction zones (the “suprasubduction-zone setting” in academic lingo), not way out to sea at mid-ocean ridges, that we find scraps of them plastered onto the continents in a couple hundred places around the world. There are some other tectonic schemes that make ophiolites, but this is the typical one.

Ophiolites consist of rocks that correspond to the major layers of oceanic plates, which are a deep base of peridotite, a middle layer of gabbro, an upper layer of basalt and a cap of mixed seafloor stuff: red clay, seamounts, volcanic chains and the odd limestone basin here and there. And most of these can be found in Oakland. The peridotite, when seawater reacts with it, turns quickly into serpentinite (but you can see rare remnants in places).

The gabbro, a coarse-grained rock of the same composition as basalt lava that has cooled slowly enough for visible mineral crystals to grow, underlies much of San Leandro and the deep-East Oakland hills.

And the Leona volcanics is a big pile of volcanic ash, shot through with dikes of basalt and now strongly altered, that sat on top.

The Coast Range ophiolite is highly disrupted now. It’s been caught up in millions of years of squeezing, stretching and kneading North America’s western edge — and that was before the San Andreas fault system arose and smeared everything sideways.

A handful of intrepid specialists continue the work that Brongniart started, reassembling and correlating and extrapolating and collecting ever more data. Ophiolites are important in the bigger scheme because some of them are the only pieces of seafloor rock that are older than the present ocean floor (which barely covers the Cretaceous period, back to about 140 million years). But if ophiolites are born poorly organized, unlike proper deep seafloor, attempting a perfect restoration may be an delusion, a will-o’-the-wisp. Our insights may always be a string of fragments, and we may have to let mystery be, but we have to try.

I also wrote this introductory piece about ophiolites for another website, once upon a time.

Return to Sugarloaf Hill

13 May 2019

It’s been almost four years since my last visit, and no locality, even the wildest, ever stays the same. Sugarloaf Hill, that iconic bump in the ridges of East Oakland, is one of the city’s wildest places. It helps being part of the Leona Canyon Open Space Reserve, an odd holding of the East Bay Regional Park District away from the usual watershed lands and coastal strips.

Sugarloaf Hill is the highest point underlain by the Leona volcanics. The drainage is sharp enough to discourage trees, and the EBRPD considers it a good example of grassland that still includes a lot of native species. Last week the peak, like most of the hills, was nearing the end of the green season and starting to turn summer gold.

The loose stones on the peak have been moved around since my last visit. Then, they were arranged in a rectangle, like the outline of a small building. Now they’re piled in a cairn that displays them nicely. The same energetic person or people who did that also brought up a chair, which I found very welcome after scrambling around the steep slopes.

This hilltop deserves a real bench, and a decent path to reach it. The existing trail is steep enough to be tricky footing, and the poison oak keeps edging closer on all sides.

On this visit I made a concerted attempt to find another trail to the top, both from the bottom up and from the top down. And there are some faint paths on the lower slopes. One of them led me past this old city benchmark, undoubtedly recorded on some obscure list but not relevant for quite a while.

This wild place did not start that way. Its wildness is not a primordial state or a static climax; it’s a temporary illusion created by depopulation — in Oakland’s case, the depopulation of genocide, followed by its softer sibling gentrification — leading to “parkification” or managed neglect. Untended, the hilltop will become impenetrable chaparral, the most dangerously fire-prone habitat we have.

For centuries, perhaps millennia, this hill was maintained as grassland by its native caretakers. They did controlled burns to do that, and the deer and the antelope helped keep it grazed. When the Franciscan priests of New Spain captured and enslaved the natives, the abandoned land made its way into the hands of the Realty Syndicate. Cattle grazing kept it in a simulacrum of the aboriginal flower fields.

In the 1970s the developers of Caballo Hills sought to divide this rangeland into premium country estates: nine large parcels of 40 to 50 acres. Someone would surely have stuck a private castle up here. The city of Oakland just wanted to start harvesting property taxes instead of a few steers. Instead, after neighborhood opposition, the developers deeded it to the EBRPD and went on to subdivide the ridgetop of Campus Drive into one-acre lots.

Nowadays what threatens the meadows of Sugarloaf Hill is the relentless growth of brush and chaparral. As decades pass, the ground cover rises, alien broom sprouts without hindrance, poison oak burgeons. Footpaths devolve into deer trails or disappear altogether. Eventually the most intrepid hikers give up, until a well-funded crew can reclaim the way. The EBRPD is committed to monitor the plants and animals in the park, so it’s up to that agency.

A rugged jeep trail used to be here, running up from the north end and circling the peak.

Bits of it are still accessible, but most is heavily overgrown. If EBRPD restores the road, the land would be ready for controlled burns again. The hill is a perfect site — isolated on all sides, yet accessible. The park’s planning document envisages controlled burns here, along with fuel reduction and similar half-measures.

Sugarloaf Hill could be a showcase for this deeply traditional land-management technique. For Merritt College students who already study the park, the rejuvenated hill would enhance their educational resource. It would be kept prime habitat for the Alameda whipsnake and other precarious species. And the views would remain fantastic in all directions.

Next, the park district could advance another item in its planning document: bringing back the historic York Trail. The old right-of-way, still visible in Google Maps, runs along the north side of Sugarloaf Hill, then up to Skyline Boulevard near Brandy Rock Way.

It would open a much-needed connection to Anthony Chabot Regional Park over the Parkridge land bridge.

Geology of King Estate Open Space

4 March 2019

After tramping all over Oakland, I still find its landscape full of uncertainty and mystery. The alluring hills of King Estate Open Space Park have brought me here time and again, sometimes to lead walks, sometimes to just stop and smell the flowers. Last month I came back yet again, this time to look harder at it.

The best resource on the park’s history and vision is on the Oak Knoll Neighborhood Improvement Association site at oknia.org. The aspirational Site Plan has the following concept for the park: “The winds sweep my imagination across the horizon. We move over the hills exploring the wilds and oaks embrace us. Here, in this place for everyday we cultivate community.”

King Estate Park is a grassy ridge at the south end of the Millsmont-Eastmont hills, an apparent pressure ridge that stretches along the Hayward fault from Mills College to the Oakland Zoo area, between Seminary Creek and Arroyo Viejo. What drew me here as a geologist was the geologic map (USGS MF-2342) that depicts the area as a peculiar ancient gravel (Qpoaf on the map), the largest piece of this material in Oakland.

The attractive thing is that according to the map key, these deposits “locally contain freshwater mollusks and extinct Pleistocene vertebrate fossils.” The odd thing is that they don’t match another criterion: that this dense gravel “can be related to modern stream courses.”

I explored the portion of the park north of Fontaine Street. Here’s the street map, marked with the three locations I’ll be showing photos of.

And here’s the aerial view, from Google Earth.

And just for fun, here’s the digital elevation model, with buildings and trees removed.

OK! Location 1 is on the steep western slope at the north end of the park, which I climbed. Halfway up is a sizable area of rock rubble consisting entirely of Leona volcanics, the same bedrock shown in pink labeled “Jsv” on the geologic map. The near-outcrop is in the lower left corner of this shot.

And the rock looks like this.

Down at location 2, there’s a spiral labyrinth that people have made in the last few years; I don’t remember seeing it before. But on the assumption that it’s made of stones from the immediate surroundings, I infer that it’s Leona volcanics over here too.

The Leona is generally light-colored with some greenish bits and some red-orange coatings where it’s weathered, hard to describe in detail but distinctive once you’re familiar with it. Once a range of underwater volcanoes that subsequently underwent a lot of alteration, it offers up a variety of intriguing bits that lacking a petrographic lab I can only scratch my head at.

Anyway I’m looking all over, and every bit of gravel on this hill is Leona volcanics. Now a river gravel, which is what the map describes and what I expected (indeed, what I actually perceived on previous visits!), consists of rounded clasts and a variety of rock types from the stream’s catchment. Other gravels in this town are just that way, but not this. The whole time I’m there I’m muttering to myself, “this stuff is colluvium” — raw rock rubble, mixed with soil, that hasn’t moved from its birthplace except maybe in landslides.

In location 3, we have proper bedrock. It show up where the soil has been scraped bare . . .

. . . and farther down the slope in genuine outcrops, which I always cherish.

So in sum, the whole north half of the park, far as I can tell, is either bedrock or colluvium of the same stuff: Leona volcanics. How did it get to be mapped as Pleistocene river gravel? The MF-2342 geologic map was published in 2000. There are two previous serious maps of this area. Dorothy Radbruch mapped it in the 1960s for map GQ-769, and there the area looks like this.

She called it “Qg”, “gravel, sand and clay” and noted that it contains pebbles of Leona Rhyolite (what I call Leona volcanics). She also said this: “Contains molluscs of probable early Pleistocene (Irvingtonian) age.” They were at the locality marked by the triangle with “22133” next to it; that number refers to a “report filed at Washington, D.C.” which is probably gathering dust deep in a back room. She also referred to reports on two boreholes, numbered 95 and 96 on the map, which recorded various kinds of gravel down as deep as 45 feet. That could’ve been deeply weathered Leona as easily as anything else.

But you know what? I’m going to go with Andrew Lawson’s original map of the area from 1914 (Folio 193), in which everything is just straight Leona.

Even though he thought the Leona was very young (hence the name “Tln” meaning Tertiary Leona), he could tell what the ground was saying. At least, he and I agree. I’m sure, though, that he scratched his head as much as Dorothy Radbruch and I did. And they must have enjoyed the view as much as me.

I’ll just have to poke around here some more.