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Pacific Northwest Geology

Pacific Northwest Geology. Northwest Geology. Starting points We’re interpreting events & conditions in the past using available evidence – the rock record observable at the surface subsurface sampling (wells) & geophysics rocks in other places that are somehow relevant (age, etc.)

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Pacific Northwest Geology

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  1. Pacific Northwest Geology

  2. Northwest Geology • Starting points • We’re interpreting events & conditions in the past using available evidence – • the rock record observable at the surface • subsurface sampling (wells) & geophysics • rocks in other places that are somehow relevant (age, etc.) • models of how the Earth works (especially plate tectonics) • Þinterpreting the 3rd and 4th dimensions

  3. Northwest Geology • Starting points • Geologic nomenclature • names of formations, terranes, etc. derived from local geographic names in the areas where the features are first described and/or best observed • Use paleogeographic maps & diagrams • synthesize observations & interpretations • Proceed from oldest Þ youngest • and big/regional picture Þlocal

  4. Six major time periods • Early Earth (3.5–0.5 Ga) • we’re not in North America, Toto ... • Paleozoic & Mesozoic (500–50 Ma) • sweeping up the pieces • (accretionary tectonics) • Early Cenozoic (65–20 Ma) • Oregon & Washington are almost all here

  5. Six major time periods • Mid-Cenozoic (20–15 Ma) • the flood-basalt catastrophes • Later Cenozoic (15–2 Ma) • the “modern” landscape takes shape • Quaternary (2 Ma – present) • the PNW we recognize – eventually

  6. Six major time periods • Early Earth (3.5–0.5 Ga) • we’re not in North America, Toto ... • Paleozoic & Mesozoic (500–50 Ma) • sweeping up the pieces • (accretionary tectonics) • Early Cenozoic (65–20 Ma) • Oregon & Washington are almost all here

  7. Early Earth (3.5–0.5 Ga) • Oldest No American continental rocks well to the east • Old sedimentary rocks (Belt–Purcell) in MT–ID–BC–NE WA • Rifting of the continental margin ~800 Ma • As the Paleozoic began – most of the “Northwest” was under water

  8. Six major time periods • Early Earth (3.5–0.5 Ga) • we’re not in North America, Toto ... • Paleozoic & Mesozoic (500–50 Ma) • sweeping up the pieces • (accretionary tectonics) • Early Cenozoic (65–20 Ma) • Oregon & Washington are almost all here

  9. Paleozoic & Mesozoic (500–50 Ma) • As Paleozoic began – crustal fragments later accreted to No Am were elsewhere in the proto-Pacific basin • Beginning ~ 210 Ma – break-up of Pangaea Þsweeping up exotic terranes • Continued for > 150 Ma – attached most of the No Am western margin • Older terranes have the most Au, Ag, Cu, etc minerals

  10. Six major time periods • Early Earth (3.5–0.5 Ga) • we’re not in North America, Toto ... • Paleozoic & Mesozoic (500–50 Ma) • sweeping up the pieces • (accretionary tectonics) • Early Cenozoic (65–20 Ma) • Oregon & Washington are almost all here

  11. Early Cenozoic (65–20 Ma) • Subduction zone (trench) shifted west as terranes were accreted to No Am • Siletz–Crescent and Pacific Rim the last • marine shelf sedimentation • Blues, Klamaths, etc. rotating westward and clockwise • Volcanism in the Western Cascades and eastward (40–20 Ma)

  12. Six major time periods • Early Earth (3.5–0.5 Ga) • we’re not in North America, Toto ... • Paleozoic & Mesozoic (500–50 Ma) • sweeping up the pieces • (accretionary tectonics) • Early Cenozoic (65–20 Ma) • Oregon & Washington are almost all here

  13. Six major time periods • Mid-Cenozoic (20–15 Ma) • the flood-basalt catastrophes • Later Cenozoic (15–2 Ma) • the “modern” landscape takes shape • Quaternary (2 Ma – present) • the PNW we recognize – eventually

  14. Mid-Cenozoic (20–15 Ma) • Huge outpourings of flood basalts over a large area of the NW • Largest: Columbia River Basalt Group • > 200,000 km3 of lava • most erupted 17–14 Ma • later eruptions until ~ 6 Ma • (others: Steens, Chilcotin) • Mantle plume, asteroid impact, or ?

  15. Six major time periods • Mid-Cenozoic (20–15 Ma) • the flood-basalt catastrophes • Later Cenozoic (15–2 Ma) • the “modern” landscape takes shape • Quaternary (2 Ma – present) • the PNW we recognize – eventually

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