Oakland Geology

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.

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.

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.

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.

This entry was posted on 29 March 2010 at 9:30 pm and is filed under Oakland sandstone/shale, Sausal Creek watershed. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.