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.
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.
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?