Two bits of gabbro

20 February 2017

I’ve noted that while the San Leandro Gabbro has a presence in easternmost Oakland, it’s hard to find. The geologic map shows what seems like a lot of it, marked “Jgb” for Jurassic gabbro.

gabbrobitsgeomap

But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But once last year and once the year before, I found some. The two spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, and the southern one is in Sheffield Village at the north end of Middleton Street where it meets Marlow Drive.

The northern site, Seneca Reservoir, was once the upper pit of the old Catucci quarry. (The lower pit was repurposed as the site of Bishop O’Dowd High School.) Not much of it is accessible, but here and there you can spot pieces of the quarry waste. It looks like nothing else in town and everything like San Leandro’s namesake stone.

gabbrobit-seneca

I can’t say the same for the Middleton Street exposure, but on the positive side it’s real easy to visit. Oddly, the last time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

gabbrobit-middleton1

This site too is very near a strand of the Hayward fault, so it’s been rattled and squeezed for quite some time. It has a battered appearance, even a little fried.

gabbrobit-middleton2

And no matter how close you get, it doesn’t show much detail. As a whole, though, it has the typical color of the gabbro: light gray with a slight blue-green tinge. This resulted from petrochemical disruption at the time of its eruption, some 165 million years ago, when a pulse of younger magma sent up fluids that changed its black pyroxene minerals into green amphibole and some other greenish minerals — an obscure process known as uralitization.

gabbrobit-middleton3

I mentioned the Hayward fault being next to the reservoir. We probably would think twice before building that there today.

The Idaho connection

13 February 2017

I’ve been getting into the weeds as I work on my book manuscript about Oakland’s geology (tentative title, Deeper Oakland). Where did Oakland’s rocks come from? Specifically, how did they get from where they formed to where they are? This problem is particularly vexing for the older rocks with Mesozoic ages. The western edge of North America has been built, unbuilt, shifted, rebuilt and disassembled for hundreds of millions of years.

Generally the pieces have been carried northward by the vagaries of plate tectonics. Rocks that were once Californian now sit as far north as Alaska, and likewise rocks that live here now come from as far south as Mexico.

The rocks in lower Shepherd Canyon (the Shephard Creek and Redwood Canyon Formations) belong to the huge set of sandstones and related rocks underlying the Central Valley — the Great Valley Group — but are separated from them. How they broke off and how they wandered to where they sit today are, as we say, poorly constrained. One clue may be within the sand itself.

Last year a paper in Geology laid out an intricate case that the sediment making up one part of the Great Valley Group was eroded from an ancient set of rocks in Idaho, the Lemhi subbasin of the Belt Supergroup. Papers dealing with the assembly of the North American Cordillera usually have gnarly figures, because the story is so complex, and this one, the product of an all-California team of geologists led by Stanford’s Trevor Dumitru, was no exception.

idaho-gvg-connection-figure

The whole scenario is based on microscopic grains of zircon, which lurk in many sandstones because the mineral is extremely durable. Luckily, we can determine the age of zircon grains because they’re superbly suited for the gold standard of dating techniques, the uranium-lead method. Think of them like pennies with dates on them.

So there’s a big body of rock in the Lemhi subbasin full of a unique combination of zircon ages, including a bunch around 1380 million years old. At one point during Cretaceous time, around 80 million years ago, a mountain-building episode pushed these rocks into a knot of high peaks, which eroded into sand that was carried by rivers in at least four directions. Dumitru and his coauthors duly gave these hypothetical ancient rivers names, because that’s one of the perks of doing historical geology.

Sandstones containing Lemhi zircons, with their telltale 1380 Ma peak, are found in Wyoming, Oregon, Washington and California. In the Bay Area, the paper identifies them in sandstone from Del Puerto Canyon, west of Patterson in the Central Valley. Apparently a huge pulse of Lemhi sand poured down the “Kione River” and filled the whole sedimentary basin for a while. (Basins keep sinking as they fill, because the sediment load depresses the crust.) That sandstone is mapped as the Kione Formation, a portion of the Great Valley Group that’s been considered mysterious because the sand clearly didn’t come from its usual source, the nearby (ancestral) Sierra Nevada and Klamath Range.

The point of all this is that the Oakland rocks I’m talking about date from this same period! If only we could get a zircon researcher to check them out, we might learn a little something. I mean, Dumitru dated rocks from Albany Hill, Stinson Beach, Bolinas Ridge and Sutro Baths (localities H, G, F and E in the inset map) among other places. He dated rocks from the Novato Quarry terrane of the Franciscan complex, the same unit our own Franciscan rocks belong to. He dated rocks from the Sierra de Salinas Schist, down near Monterey.

sierrasalinasschist

I love it; it has its own cool story. So why not visit Oakland too? I guess rocks on this side of the Hayward fault aren’t as important for the bigger story. But you never know until you find out, right?

By the way, I will be speaking at the upcoming East Bay Nerd Nite, Monday the 27th; watch its Facebook page or website for details. The topic is, “Are Rocks People?”

A march for science

6 February 2017

On Earth Day this year, April 22, an unknown but large number of scientists will be gathering together, in Washington and other cities, in a March for Science. I’ll be joining them somewhere in or near Oakland. Not only is science central to my being, it’s also central to our civilization.

As the March for Science site puts it, “At its core, science is a tool for seeking answers. It can and should influence policy and guide our long-term decision-making.”

While Earth Day lately has devolved into a day for innocuous tasks, this year it’s the best possible occasion for this march, because Earth science is the central science for the vital tasks ahead — breaking free of the destabilizing carbon economy, fostering a civilization that’s as sustainable as a forest, protecting and repairing the natural systems that provide our resources. Those tasks will require people who are experts at approaching the unknown, ninjas of curiosity.

curiousbarite

Curiosity is an undervalued skill. You won’t see it in job requirements. Yet without our intense curiosity, we would still be bands of hairless apes huddled in the African savanna, if not entirely extinct. And when new questions need answers, no one is better equipped to find them than scientists.

Consider what happened during the terrible Deepwater Horizon oil-well blowout in 2010. When standard procedures failed, a panel of scientists was called in to seek answers. When the Challenger spacecraft exploded in 1986, a panel of scientists was called in to seek answers.

These were not experts in drilling or spacefaring technologies; they were experts in handling the unknown. We support jobs for such people not just to keep them busy with their pet problems — what many call “pure research” or “curiosity-driven research” as a put-down — but to ensure a supply of curiosity ninjas. Everyone understands the need for top skills in the performing arts, athletics, law and war. It’s the same with science.

curiosity

I don’t know exactly what is driving so many powerful people to fear and downplay and deride and defund scientific research, but I know they need to be opposed and replaced by people who prize science. We have tremendous questions about our future on Earth. Who will seek their answers?