Bedrock geology of the San Francisco Bay Area: A local sediment source for bay and coastal systems

• Franciscan oceanic bedrock dominantly underlies San Francisco Bay watersheds.
• Transform tectonism led to volcanism, small basin development and displacement.
• Fine-grained sedimentary rocks provide most modern sediment in local watersheds.

Local watersheds may contribute over half of the sediment load coming into San Francisco Bay today. The bedrock underlying these watersheds is the ultimate source for most of this sediment. This paper outlines the geologic history of this bedrock, which records the complex tectonic history of the San Francisco Bay Area over the last 200 Ma. The Jurassic to Eocene Franciscan Complex in the Bay Area is the most widespread bedrock. The local Franciscan can be broken into nine tectonic terranes that represent pieces of seafloor that were accreted to the North American margin in over a 100 Myr period of subduction. Each terrane has a unique age range, sequence of seafloor rocks, and metamorphic history. The Franciscan rocks were thrust eastward under the Great Valley Complex. The Great Valley Complex reflects a forearc basin comprised of Jurassic ocean crust—the Coast Range ophiolite—and overlying Jurassic and Cretaceous turbidite-dominated sedimentary rocks of the Great Valley Group. By the beginning of the Cenozoic Era, shallowing of the angle of subduction along the continental margin led to uplift and infilling of the forearc basin to shelf depth. Recorded at this time are sedimentary sequences governed by eustatic sealevel changes and evidence of the first unroofing of Franciscan rocks. Marine sedimentary rocks dominate the region through the Miocene and unconformities within the Early Tertiary sedimentary record reflect periods of tectonic activity and uplift. Middle to late Miocene sedimentary rocks are extensively deposited throughout the Bay Area. In the late Miocene, the Mendocino triple junction passed northward through the area and transform tectonism commenced. A slab window developed behind the triple junction resulted in volcanic activity that produced the Sonoma Volcanics in the North Bay and volcanics of the East Bay Hills. By Pliocene time, terrestrial sedimentary rocks were being deposited in many parts of the Bay Area. Faulting associated with the San Andreas system led to the development of small basins, the displacement of local bedrock blocks, and the transport of the Salinian Complex from the southern Sierra to the western margin of the Bay Area. The modern topography of the region started emerging by about 6 Ma to 4 Ma, when compression across the San Andreas system increased and uplift of the Coast Ranges began. The first estuarine influence in San Francisco Bay is recorded in ~ 600 ka rocks, about the same time as the modern Sacramento River system started flowing through the San Francisco estuary/valley.Today's local San Francisco Bay watershed is comprised of numerous small stream systems around the Bay with headwaters in the surrounding hills and mountains. Active tectonism produces relief in these watersheds of typically 500 m or more, with the ridgelines typically supported by more resistant bedrock units. Steep topography leads to rapid bedrock erosion, particularly of the finer-grained sedimentary rocks. Landslides provide a significant portion of the sediment carried by the streams and several of the bedrock units are more prone to landslides, particularly the Central and Novato Quarry terranes of the Franciscan Complex, and finer-grained units of both the Great Valley Group and Tertiary rocks. Many of the larger streams have dammed reservoirs that trap much of their sediment load before it can reach the bay.