puget sound geology as quickly as possible

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puget sound geology (as quickly as possible)

• western WA geological history
• cascades, olympics
• formation of puget sound
• glaciers
• how 12 affect what we know as Puget Sound
• coastal river morphology
• human impacts
• dams, land use, levees

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the point

• basic geological context for PS oceanography
• coastal environment
• fluvial/river style
• sediment size discharge
• eg. hood canal is geologically different from
  southern basin
• steep short bedrock rivers vs. gradual longer
  alluvial rivers
• not to mention climate

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WA bedrock geology

• western WA is part of a large tectonic/volcanic
  system
• Mexico to Alaska

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plate tectonics 101

• earth is covered in multiple continental and
  oceanic plates
• plate boundaries
• divergent (rift)
• convergent (subduction zone)
• transform (strike-slip)
• subduction ? volcanoes

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when western WA started

• breakup of pangaea (ca. 200 My)
• switch from passive margin to subduction
• massive ocean (and its floor) had to shrink

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when western WA started

• accretion of volcanic arcs as Pangaea started to
  spread
• intermontane insular
• not all arc
• also chunks of oceanic/continental crust
  (terranes wrangellia, stikinia)
• followed by period of sustained subduction and
  volcanism
• coast ranges form one of the largest major
  igneous rock provinces in N. Amer.
• high mountains in B.C. (eg. Mt. Waddington)

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and then

• details debatable, but roughly
• complicated nature of subduction zone leads to
  fragmentation of Farallon plate (Kula)
• leads to divergent plate boundary and N movement
  of Kula
• a new arc (Challis), major lateral movement
  (Fraser fault) and thick oceanic crust formation
  (Crescent basalt)
• ends w/final subduction of Kula accretion of
  Crescent, forming the core of the Olympic Pen.

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and finally

• Major Farallon subduction reestablished
• Cascade volcanism (incl. ancient cousins of
  Baker, Ranier, Glacier)
• continued complications in subduction zone and
  increasingly small Farallon lead to more plate
  fracturing (Explorer, JDF, Gorda)
• Ultimately, as Farallon disappears under N.A.
• gradual extinction of Cascades as subduction
  ceases
• steeper JDF subduction ? compression
• uplift folding of Olympics Cascades

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final points

• Farallon is almost gone today
• JDF Cocos are remnants of Farallon
• no more subduction under California ? Sierra
  Nevada (extinct cascades)
• O.P. Cascades continue uplifting (still cascade
  volcanoes, too)
• Puget Sound is formed in low lying area between
  O.P. and Cascades
• not unlike Californias Great Valley, BUT

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Puget Sound glaciation

• we are far enough north to have had major
  glaciation!
• continental ice sheets advanced several times
  across the Puget Sound area
• most recent, Vashon (or Fraser, or Wisconsin)
  reached Seattle around 17,500 years ago
• 3000 ft. in seattle (6 space needles)

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what was left behind

• scoured landscape (in what orientation?)
• glacial deposits
• kitsap fm pre-Vashon lacustrine fluvial
• lawton clay lacustrine dark clay and silt
• esperance sand fluvial outwash sand
• vashon till mixed, unsorted boulders to clay

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sea level changes

• glaciers had 2 effects on relative sea level in
  Puget Sound area
• locally weight of ice sheet (6 space needles in
  seattle, 12 in bellingham)
• weighs down crust, which slowly (isostatically)
  rebounds after ice retreat
• globally large portion of Earths water tied up
  in ice sheets ? lower sea level
• ca. 130 m lower, globally (the cause of many of
  the estuaries you have been studying)

one local example the Elwha delta multiple spits
recording past, relatively lower sea level
positions
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puget sound coastlines

• morphology is generally a function of
• physiography high or low slope
• geology bedrock or glacial sedt
• s/l history relative position rate
• tidal range generally higher inland
• climate more precip. in SW
• wave climate fetch length
• general trends
• glacial sediments dominate in south
• bedrock important in San Juans northern Whidbey
  
• wave exposure more important near straits of JDF
• tidal range higher in south sound

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puget sound coastlines

• factors conspire to produce several primary
  coastal environments
• Important for our purpose here, as sediment
  availability transport (as well as coastline
  habitat stability) are related to morphology

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river deltas ? rivers

• The major source of sediment, by far, is the
  rivers
• rivers sourced in the cascades have generally
  higher discharges sediment loads (why?)

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• big drainage basins ? more water sediment
• more lowlands ? more glacial sediment to grab
• larger proportion of tidelands in these areas

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river deltas

• shape of delta relates to coastal processes
• classic delta deposits contains
• topsets marshes, channel(s)
• foresets rapid accumulation
• bottomsets thin plume turbidity current
  deposits

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deltas
nisqually
mississippi
hoh
elwha
skagit
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once the sediment reaches the sound

• three main ways to move sediment
• waves longshore transport, spit barrier
  formation (eg. Elwha, Ediz, Dungeness)
• tidal currents amplified where tidal range is
  large through narrows
• density currents turbidity currents, debris
  flows or slumps

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turbidity currents

• not the dominant transport mech. in PS, but
  certainly active

Mitchell (2005)
Prince Rupert Channel, Hay (1987)
Savoye, et al (2000)
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human impacts

• 3 important, short-term practices
• dams cut off large fraction of sediment supply
• de-snagging levee/dike building prevents
  floodplain deposition
• land use increase/decrease sediment load in
  rivers

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human impacts

• flood prevention
• involves keeping water (and therefore sediment)
  in channel
• prevents nourishment of delta topsets with
  sediment
• consolidation of sediments ? sinking of delta top
  ? relative rise in sea level or loss of habitat

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human impacts
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land use

• logging in Skokomish watershed has led to
  substantial aggradation of channel bed
• what does/will this mean for Hood Canal?

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so

• Puget Sound oceanography can be better understood
  in the context of the surrounding
  geology/geomorphology/anthropology!
• questions? now or email me sheets_at_u.washington.edu
  

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image sources

• Ron Blakeys NAU website
• http//jan.ucc.nau.edu/rcb7/globaltext.html
• Christopher Scoteses PALEOMAP project website
• http//www.scotese.com/
• Burke Museums Northwest Origins website by
• Catherine L. Townsend John T. Figge
• http//www.washington.edu/burkemuseum/geo_history_
  wa/
• Shipman, H., 2008, A geomorphic classification of
  Puget Sound nearshore landforms Puget Sound
  Nearshore Partnership Report 2008-1. Published by
  Seattle District USACE, Seattle, WA
• www.pugetsoundnearshore.org
• and references therein

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