What makes the Bay?

san francisco bay

We all admire the Bay when we visit high places, whether it’s here at Dunsmuir Ridge (click for a 2X version) or elsewhere in the Oakland Hills. But why is there such a big basin here?

One ready answer comes when you contemplate the major faults, the San Andreas and the Hayward, that bound the bay on the west and east sides respectively. They aren’t quite parallel, but fan slightly apart as you progress to the north. As the crust moves along these faults, the part between would sink, just like what happens at the smaller scale when a sag basin forms. Maybe it does. Why, then, are the Oakland Hills still rising from compression across the Hayward fault? Well, maybe things shift direction slightly from time to time and the sense of stress across the fault changes. So today we have compression while other times we have extension. But maybe the analogy of sag basins is the wrong one.

Geologists have the same question when we consider the Great Valley, that vast trough between the Sierra and the Coast Range. Most sedimentary basins are depressed by their load of sediments, but not to the point of lying at sea level like the Great Valley. The current working hypothesis about the Great Valley is that it’s a captured slab of oceanic lithosphere, a dense bottom layer, with thick sediments on top. It naturally, persistently rides lower than the continental rocks around it. Might the San Francisco Bay basin be a chunk of the same slab? That’s one hypothesis; we don’t really know.

The roots of the Bay are pretty deep for active-source seismic imaging, the kind of technique used at the Garrido property in Antioch to study a crime scene. The Bay is also pretty small for the passive-source seismic studies used to study the crust and mantle at the regional and continental scale. If you gave me a few million dollars, I could make a stab at an investigation, but no one can spare that for an idle question. So we’ll keep wondering as we stand on the high places.

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4 Responses to “What makes the Bay?”

  1. Eric H. Phillips Says:

    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!

  2. Andrew Says:

    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.

  3. Eric H. Phillips Says:

    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.

  4. Eric H. Phillips Says:

    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.

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