Comments on: What makes the Bay?

By: Eric H. Phillips

Eric H. Phillips — Thu, 07 Jan 2010 04:23:27 +0000

I had one additional thought that I suppose could be relative to indicating the absence of lower Tertiary beds between the San Andreas and Hayward faults in San Francisco Bay. When the huge excavation was dug for the cable anchors midway between Yerba Buena Island and San Francisco, in the late 1930s, it was reported that that the excavation was almost entirely in Franciscan rock containing considerable serpintine. There was a controversy between the Cal and Stanford Geology Departments as to the holding strength of the Franciscan at this location. So far the anchor has held. This at least bears slightly on the present discussion, showing that Franciscan rocks are very shallow in this portion of the bay and little room is left for an even shallower Lower Tertiary sequence. I have no evidence as to the age of the rocks above this shallow Franciscan, but would presume they were very young sediments.

By: Eric H. Phillips

Eric H. Phillips — Wed, 06 Jan 2010 20:00:03 +0000

Andrew, you’re following me just fine . You just want the evidence and there is not a lot. If there were you’d be reading about this in a text book. Check out the online USGS Map San Francisco Bay 100th Anniversery 1906 Earthquake. In the northwest corner of the map, north of San Pablo Bay and up the coast to Stewarts Point you see some Lower Tertiary west of the San Andreas fault and east of the extension of the Hayward fault. Between the faults you see Pliocene, Miocene and younger rocks in depositional contact with Franciscan. Lower Tertiary is absent between the faults. It’s the same pattern south of San Jose, but here a short slumped segment, near Gilroy, along the San Andreas fault that complicates the interpretation. Franciscan has been pushed eastward, beneath the granite Salinian Block. For much of its trace, the San Andreas fault has adjusted to the east edge of this huge granite block, riding on the Franciscan. The “mushy” Franciscan rose to the east in the Vaca Mountains, Mount Diablo and many other places in the Coast Range. It rose in Early and perhaps Middle Miocene as California underwent considerable extension. This is extremely well documented in Southern California. Blocks of Cretaceous, Paleocene, Eocene and Oligocene were rifted, “rafted” and rotated all over the place as detached blocks, moving on a Franciscan surface. I think this happened in Northern California, too. Note, there is one printing error on the USGS Map. At the north edge of the Salinian Block Kgr, at Point San Pedro, some Tertiary beds are in depositional contact on the granite. The dark brown outcrop is Paleocene, not Pliocene. The outcrop shows the correct color for Paleocene, but has the letter symbol of Tps, for Pliocene. I’ve seen Earl Brabb’s mapping and Paleocene is correct, although I wouldn’t argue if someone else wanted to call it Eocene.

By: Andrew

Andrew — Wed, 06 Jan 2010 05:58:16 +0000

I’m not following you very well. Do you have a source for the subsurface stratigraphy of the Bay block? I’m having trouble seeing how a “new fresh surface” arises, and how you rule out pre-Miocene sediment everywhere in the Bay block. And even so, how is it that the Bay rides low? That’s my basic question.

By: Eric H. Phillips

Eric H. Phillips — Wed, 06 Jan 2010 00:41:21 +0000

What makes San Francisco Bay? Here’s one interpretation you might think about. At the end of the Oligocene the San Andreas and the Hayward faults were a single fault which thet still are to the south. In the Early Miocene they spread apart, making a “V” in the Bay Area. Granite, and depositional Paleocene, Eocene and Oligocene are now west of the San Andreas. Depositional Great Valley Cretaceous, Paleocene, Eocene and Oligocene are now east of the Hayward fault. Between the faults, in San Francisco Bay, rubble and chunks of these sediments rest in non-depositional contact upon the Franciscan Complex. Between these faults only the younger Miocene, Pliocene, Pleistocene and Recent sediments are in depositional sequence with the Franciscan basement. That Franciscan surface was a new fresh surface exposed in the Early Miocene for the first time and was then able to recieve sedimentation. Cretaceous, Paleocene, Eocene and Oligocene sediments were never deposited within the “V” between the San Andreas and Hayward faults!