Home | Session 5 | Bay Area Geography Pg 1, 2

Bay Area Geography and Geology

Presented by Richard Sedlock
San Jose State University and BAESI

3. Paleogeography of the Bay Area since 15 Ma

In 1995, Anna Buising and Jim Walker (B&W) reconstructed what the East Bay looked like over the last 15 million years. To do this, they restored slip on faults, removed tectonic distortions like squeezing or stretching, and identified the parent-rock source of sandstones and conglomerates.

15 Ma (B&W Figure 3)

• Most of Bay Area was below sea level.
• The Pacific shoreline trended WNW and lay well to the northeast of the modern bay.
• Rivers draining the Sierran region brought sediment rich in pieces of volcanic rocks — that once must have lain atop the Sierran granites we currently see. contained much of the volcanoes eroded from atop the Sierran granites.

10 Ma (B&W Figure 4)

• An oceanic embayment invaded the East Bay, and accumulated shallow-marine and estuarine sediment (small dot pattern).
• The Bay Block must have been topographically high, for it contributed Franciscan sediment eastward a low area in the "East Bay Hills Block" (EBHB).

5 Ma (B&W Figure 6)

• The Livermore basin (LB) became (and is) a low-lying area surrounded by hills.
• The Bay Block persisted as a topographic high.

Why did areas rise and fall?
Uplift and subsidence could be due to two causes:

1. bends in right-lateral strike-slip faults
   Mt. Diablo, Santa Cruz Mtns and terraces, San Pablo Bay
   
   
   
   
2. transpression (margin-wide) (Coast Ranges)
   
   
   

Why did the Bay Block change from a topographic high to SF Bay/Santa Clara Valley?

No one knows for sure! Several models have been suggested, and are summarized in the rather technical Figure 5 from Sedlock (1995).

 

 

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updated March 4, 2002

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