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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.)• models of how the Earth works (especially
plate tectonics)• interpreting the 3rd and 4th dimensions
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 youngest
and big/regional picture local
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
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
Six major time periods
Early Earth (3.5–0.5 Ga) we’re not in North America, Toto ...
Paleozoic & Mesozoic (500–50 Ma)Paleozoic & Mesozoic (500–50 Ma) sweeping up the piecessweeping up the pieces ((accretionary tectonicsaccretionary tectonics))
Early Cenozoic (65–20 Ma)Early Cenozoic (65–20 Ma) Oregon & Washington are almost all hereOregon & Washington are almost all here
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
Six major time periods
Early Earth (3.5–0.5 Ga)Early Earth (3.5–0.5 Ga) we’re not in North America, Toto ...we’re not in North America, Toto ...
Paleozoic & Mesozoic (500–50 Ma) sweeping up the pieces (accretionary tectonics)
Early Cenozoic (65–20 Ma)Early Cenozoic (65–20 Ma) Oregon & Washington are almost all hereOregon & Washington are almost all here
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 sweeping 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
Six major time periods
Early Earth (3.5–0.5 Ga)Early Earth (3.5–0.5 Ga) we’re not in North America, Toto ...we’re not in North America, Toto ...
Paleozoic & Mesozoic (500–50 Ma)Paleozoic & Mesozoic (500–50 Ma) sweeping up the piecessweeping up the pieces ((accretionary tectonicsaccretionary tectonics))
Early Cenozoic (65–20 Ma) Oregon & Washington are almost all
here
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)
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
Six major time periods
Mid-Cenozoic (20–15 Ma) the flood-basalt catastrophes
Later Cenozoic (15–2 Ma)Later Cenozoic (15–2 Ma) the “modern” landscape takes shapethe “modern” landscape takes shape
Quaternary (2 Ma – present)Quaternary (2 Ma – present) the PNW we recognize – eventuallythe PNW we recognize – eventually
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 ?
Six major time periods
Mid-Cenozoic (20–15 Ma)Mid-Cenozoic (20–15 Ma) the flood-basalt catastrophesthe flood-basalt catastrophes
Later Cenozoic (15–2 Ma) the “modern” landscape takes shape
Quaternary (2 Ma – present)Quaternary (2 Ma – present) the PNW we recognize – eventuallythe PNW we recognize – eventually
Later Cenozoic (15–2 Ma)
No Am overrode part of oceanic plate beginning of transform boundary
Basin and Range extension Yellowstone hot spot & Snake River Plain Olympic subduction complex and Coast
Range uplift High Cascades uplift & volcanism All these continue to the present
Six major time periods
Mid-Cenozoic (20–15 Ma)Mid-Cenozoic (20–15 Ma) the flood-basalt catastrophesthe flood-basalt catastrophes
Later Cenozoic (15–2 Ma)Later Cenozoic (15–2 Ma) the “modern” landscape takes shapethe “modern” landscape takes shape
Quaternary (2 Ma – present) the PNW we recognize – eventually
Quaternary (2 Ma – now)
Cordilleran ice sheet into No WA–ID–MT Alpine glaciers in the Olympics,
Cascades, Rockies, and other mountain ranges
Sea-level fluctuations (minus 120 m) Pluvial lakes in the Basin and Range Glaciers changes in drainage patterns
Puget lowland and Columbia basin Missoula floods
Portland region –
