Pacific Northwest Geology. Northwest Geology Starting points We’re interpreting events &...

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 –