Lecture 12: Santa Barbara Geology - Notes

Santa Barbara Geology

Tectonic Evolution of California

  • The lecture discusses the tectonic evolution of California and Santa Barbara.
  • Santa Barbara's geology and topography are key focus points.
  • The plate tectonic history explains large-scale features of California:
    • Cascade volcanoes.
    • Sierra Nevada.
    • Central Valley.
    • Coast Ranges.
    • Transverse Ranges.
    • Channel Islands.
    • Salton Trough.

Plate Boundaries and the Farallon Plate

  • Current plate boundaries include the Juan de Fuca Plate to the north, the Pacific Plate/North American Plate boundary in the Santa Barbara area, and the Cocos Plate in Central America.
  • 56 million years ago, the Farallon Plate was present near the western US but has mostly subducted.
  • Remnants of the Farallon Plate are now the Cocos, Rivera, and Juan de Fuca plates.
  • The Farallon Plate hasn't been destroyed; it's still under North America.

Convergent Plate Boundary

  • During the Mesozoic and early Cenozoic, the western margin of North America was a convergent plate boundary.
  • Features of an ocean-continent convergent plate margin:
    • Volcanic arc.
    • Forearc basin.
    • Accretionary prism.

California Context

  • The ancestral Sierra Nevada represents the volcanic arc.
  • The future Central Valley represents the forearc basin.
  • The accretionary wedge forms the future Coast Ranges, known as the Franciscan assemblage.
  • Cascade volcanoes are associated with the subduction of the Juan de Fuca plate.

Topography of California

  • California's topography includes:
    • Cascade Volcanoes
    • Sierra Nevada (result of uplift and erosion, exposing plutons).
    • Central Valley (former forearc basin).
    • Coast Ranges (accretionary wedge).
    • Transverse Ranges.
    • Channel Islands.
    • Salton Trough.

Geology of California

  • Franciscan Complex = accretionary wedge.
  • Great Valley Group = Forearc sediments.
  • Sierra Nevada = Jurassic to Cretaceous magmatic arc.

Metamorphic Rocks in the Franciscan Assemblage

  • Blueschist facies metamorphic rocks are expected in the Franciscan Assemblage.
  • The Big Sur Coast and Santa Ynez Valley contain Franciscan Complex rocks, including blueschist facies, serpentinite, and jade.

Formation of Transverse Ranges and Channel Islands

  • Paleomagnetic data indicates clockwise rotation beginning around 16 Ma, which helps explain the formation of the Transverse Ranges and Channel Islands.

Tectonic Evolution of Santa Barbara

  • As the boundary transitioned from subduction to transform, slivers, including the microplate Santa Barbara is on, rotated.
  • Distinctive conglomerate with volcanic cobbles is found near San Diego and on the Channel Islands, suggesting they were once closer.
  • The bend in the San Andreas Fault causes compression, resulting in the E-W trending mountains in Santa Barbara and Los Angeles, as well as the Salton Trough.

Bends in Transform Faults

  • Bends in transform faults cause:
    • Localized compression, forming mountains.
    • Localized extension, forming features like the Salton Sea.
  • The plate boundary at the bend is compressional.

Santa Barbara Channel Oil Reservoirs

  • Oil collects in folds offshore, explaining the presence of oil rigs.

Santa Barbara Oil Reservoirs

  • Folds extend across the Santa Barbara Channel, creating oil reserves arrayed in lines.
  • Oil rigs are positioned accordingly.
  • Historical oil rigs were more common in areas like Summerland in the early 1900s.

Oil Seeps and Source Rocks

  • Oil can seep out of the seafloor or cracks on land.
  • The Monterey Formation is the source rock for this oil and tar.

Stratigraphic Column for the Goleta - Santa Barbara Area

The field trip will emphasize the Juncal through Sespe formations.
The Franciscan and Jalama formations might be visible from the Santa Ynez Mountains.

  • Deposition of Juncal through Coldwater Formations occurred from 66-37 Ma.

Compression in Santa Barbara

  • Compression results in uplift of wave-cut terraces and the Santa Ynez Mountains.
  • Marine terraces result from a combination of uplift and wave erosion.
  • During winter, storm waves cut into cliffs, generating flat surfaces that become future terraces.
  • Santa Barbara mesas are uplifting at approximately 1-2 meters per thousand years or 1-2 mm per year.