Archive for March, 2010

Shephard Creek Formation

29 March 2010

Moving up the stratigraphic section along Shepherd Canyon Drive, as seen in the Railroad Walk, we go from the Joaquin Miller Formation to the Oakland Conglomerate. (Because I’ve already posted three pages on the Oakland Conglomerate, I’ve put links to them instead on the Joaquin Miller Formation post.) Then comes the Shephard Creek Formation, separated from the underlying Oakland Conglomerate by a fault. It appears in this sloppy outcrop at the very beginning of the Railroad Walk under a tree.

shephard creek formation

Because the walk arcs into and then out of the underlying Oakland Conglomerate, you see the Shephard Creek Formation again at the far end of the big cut in this better outcrop. But beyond that the rock is hard to find.

shephard creek formation

It’s a unit of mostly soft, fine-grained rock, mudstone and shale. Shale is more or less pure clay, and mudstone is shale with some fine grit (silt) in it. The unit also has some siltstone and a few thin beds of coarser wacke. What stands out about it is that, as the mapper says, it’s “distinctly bedded.” The whole thing is of Campanian age, meaning the stretch of the Late Cretaceous Epoch between 83 and 71 million years ago, presumably early in that time. (The age is named for the well-studied rock sequence around Naples, Italy, in the Campania region.) It’s not really as blue as this hand specimen—blame the light from the sky for that—but it does get dark where thin coatings of iron minerals accumulate.

shephard creek formation

I would say that this rock does not support the steepest slopes. The canyon is a little wider here, along Shepherd Canyon Park, than it is upstream where the rocks change to the Redwood Canyon Formation and the hills grow higher.

Living on Bay mud

27 March 2010

The Tidewater district is the nose of land west of the freeway at the end of the channel between Oakland and Alameda:

tidewater liquefaction map

Right now it’s totally industrial, but landowners there want to open it up to residential uses, like the cozy parts of Alameda right across the channel. (Easy to catch up with the news by googling “oakland tidewater industrial“.)

I’m showing this image to point out a geological aspect to the zoning proposal. The map is a portion of the state’s official map of landslide and liquefaction potential, and it’s clear that this low-lying piece of mostly filled swampland is one of Oakland’s worst places in a large earthquake. Just saying.


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