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) 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

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 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) Huge outpourings of flood basalts over a large area of the NW Largest: Columbia River Basalt Group > 200,000 km 3 of lava most erupted 17–14 Ma later eruptions until ~ 6 Ma (others: Steens, Chilcotin) Mantle plume, asteroid impact, or ?

Six major time periods

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

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 –