Download PNW 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.)
• 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
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Use paleogeographic maps & diagrams
• synthesize observations & interpretations
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Proceed from oldest  youngest

and big/regional picture  local
Six major time periods
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Early Earth (3.5–0.5 Ga)
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Paleozoic & Mesozoic (500–50 Ma)
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

we’re not in North America, Toto ...
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)


Later Cenozoic (15–2 Ma)


the flood-basalt catastrophes
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)


Paleozoic & Mesozoic (500–50 Ma)



we’re not in North America, Toto ...
sweeping up the pieces
(accretionary tectonics)
Early Cenozoic (65–20 Ma)

Oregon & 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)


Paleozoic & Mesozoic (500–50 Ma)



we’re not in North America, Toto ...
sweeping up the pieces
(accretionary tectonics)
Early Cenozoic (65–20 Ma)

Oregon & 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)


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
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)


Paleozoic & Mesozoic (500–50 Ma)



we’re not in North America, Toto ...
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)


Later Cenozoic (15–2 Ma)


the flood-basalt catastrophes
the “modern” landscape takes shape
Quaternary (2 Ma – present)

the 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)


Later Cenozoic (15–2 Ma)


the flood-basalt catastrophes
the “modern” landscape takes shape
Quaternary (2 Ma – present)

the 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)


Later Cenozoic (15–2 Ma)


the flood-basalt catastrophes
the “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 –