Archive for May, 2011

Fault gauge, 39th Avenue

27 May 2011

South of the LDS Temple, the Hayward fault runs down a well-defined gulch, past the stream offset at Rettig canyon, and becomes more subtle as it traverses Redwood Heights. It’s mapped going down Jordan Road and then Victor Avenue. Here’s where Victor butts against 39th Avenue.

hayward fault

It only takes a moment with a concrete saw to set an unobtrusive fiduciary mark like this. It will document offset along the fault, whether it’s aseismic creep or a more massive wrench when the fault lets loose.

Caldecott Tunnel and the Orinda Formation [updated]

17 May 2011

caldecott tunnel

Those of us in the geology community have been eager to hear more from the Caldecott Tunnel’s fourth bore project. The bore is cutting through the Berkeley Hills in a perfect transect, allowing geologists to sample every meter of the rocks along the way. This weekend, I heard some news in a presentation at the CalPaleo 2011 meeting.

The east end was where the boring began. The Orinda Formation is beautifully exposed there—most of it, anyway—on both sides of Route 24.

orinda formation

Much of it is coarse conglomerate of late Miocene age, about 10 million years old, that’s thought to represent landslides into a freshwater basin in the middle of volcanic terrain.

orinda formation

That part, naturally, has no fossils because the environment was too rugged for even large bones to survive. But there’s other stuff in the Orinda, like lava flows at the top of the unit belonging to the Moraga Formation. This is a view of the underside of the lava, where it flowed onto the moist sediments one day long ago.

moraga lava

And there’s a good amount of fine-grained sedimentary rock, suitable for preserving fossils, as you move down section toward the hills. Caltrans has a contract with PaleoResource, a well-regarded firm, to monitor the work and recover fossils that turn up. In the initial digs, the scientists found and cataloged thousands of items, including lots of fish fossils, clams and crabs, birds and the first leaf fossils ever found in the Orinda. The press tends to zero in on large mammals, though, so we’ve heard about bones from wolverines, horses, rhinos, camels, pronghorns and even the obscure oreodont.

However, PaleoResource scientists have told me that Caltrans has not allowed the paleontologists into the tunnel proper, in violation of the contract and the agency’s own policies. That’s the kind of thing that makes me awaken at night and grind my teeth. Not only are we not learning about the lower Orinda Formation, we’re not studying the transition into the underlying Claremont Formation and the Sobrante Formation on the west side.

I try to take the long view. The tunnel is being dug in three passes, and conceivably the rocks can be sampled later. But PaleoResource officials have told me their contract runs out this summer, before the next phase of digging.

So I try to take a longer view. Once upon a time, nobody cared about paleontology. Heck, they didn’t care about archaeology. Today, turning up a human bone will stop a job in its tracks. Fossils aren’t that disruptive; they can be salvaged and documented in a day or two. Many agencies and jobs go well, the paleo people and the construction people interacting well and yielding good science. Other jobs are jobs from hell, but the long arc of history is curving toward respect for science.

Update: I’ve put up some photos of new fossils from the dig on KQED Quest Science Blogs.

Rettig canyon

5 May 2011

I paid a visit yesterday to part of the Hayward fault in Oakland, but while there I felt the pull of a neighborhood treasure I call Rettig canyon. The name is from Rettig Avenue, which traverses it. Here’s the topography, from Google Maps. This is just south of the LDS temple.

rettig map

The fault runs from top middle down Jordan Road and exits where Victor Avenue leaves the map. The hills to the west are the southernmost part of the Piedmont block of Franciscan rocks. Rettig canyon cuts right through the hills thanks to Peralta Creek, which comes here from Butters Canyon on its way to the bay through Peralta Hacienda and Foothill Meadows Park.

Normally when you see a stream cutting through a bedrock ridge, you explain it as either stream capture or a water gap. That is, either the stream eroded its way headward through the ridge or was running that way already when the ridge rose underneath it. Given the intense tectonic activity here, I’m inclined to call it a water gap, as I do Dimond Canyon (with the addition of tectonic stream capture).

I saw some possible evidence of this in the streambed. But first, a look at the scene.

rettig road

Rettig Road is a single lane through the canyon and is coned off as a landslide zone. It’s been that way for at least six years; I hope a local will say more in the comments. The canyon is steep, dark and thickly wooded. You can scarcely see the stream, but you can hear the water everywhere.

rettig canyon

But there is a place to scramble down to the streambed. It’s well populated with rocks that appear to be local Franciscan melange, pretty jagged and hence not transported far.

rettig streambed

I was looking for bedrock and found some candidates like this scaly schist. I didn’t have my hammer and was reluctant to disturb the scene anyway, so I can’t say much about it. It might be serpentinite.

rettig schist

This is the outcrop that excited me, showing what looks like a thrust contact.

rettig contact

Ignore the green patches; that’s just algae. The rock on the left is fairly soft and foliated parallel to the contact. I picked out a small piece and can’t say much about it, but in the hand lens it looks like a highly altered talcy kind of stone. At the base is a good centimeter-think layer of nice gray clay, then we hit clean tightly packed sediment with highly tilted bedding; indeed it’s tilted steeper than the contact above it. So my best guess is that it may be the contact between the Franciscan and much younger Pleistocene sediments. Due to squeezing along the Hayward fault, the older rock has been thrusted up and over the sediment. This isn’t unheard-of, but I haven’t seen it documented around here so I could easily be wrong. But that would explain the rising ridge, the topography of Jordan Road (which sits in a long trough here that may well be a sag basin) and the course of the stream.

I couldn’t resist bringing home a pocket-sized cobble of beautiful actinolite schist.


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