Coast Range

      Formation:  55mybp (Eocene) to present

      Composed of undersea basalts, sedimentary deposits and accretionary wedge deposits

      Rotated and uplifted

      Uplift continues today

 

Features:  Terraces, Dunes, Beaches Headlands, Earthquakes, Drowned valleys. Tilted strata

      Changing of coastline with time

            Cretaceous (late Mesozoic):  Ocean extended near Idaho/Oregon/Washington border. 

            Eocene: shoreline moves west to edge of Western Cascades (east side of Willamette Valley and Puget Lowland)

            Early Oligocene:  Southern shoreline had moved west of Klamath Mtns but W.V and PLL were still under water.

            Miocene:  Coastline “straightens out”

            Pliocene close to what it is now.

     

HOW DO WE KNOW THIS?

      Transgressions and Regressions

            Ocean has moved on to and off of the continental shelf many times in history

            Transgression is the encroachment of the ocean on land

                  Causes:  Warm global climate, rapid seafloor spreading, or local phenomena

            Regressions is the retreat of the ocean from the land

                  Causes:  Ice ages and local phenomena

     

            Sedimentary Sequences

                  Near land:  Rocks and coarse material

                  Beach sand

                  Cont. Shelf:  Clay and silts

                  Ocean:  Limestone

            Transgression:  land seds covered by ocean seds

            Regression:  ocean seds covered by land seds.

                  By looking at the sequences of rocks and fossils we see in the valley and coast we can tell where the water was.

 

Formation of the Coast Range of WA and OR

      55 mybp (early Eocene)

            Series of Seamounts forming due to a hotspot straddling mid ocean ridge

            Hot spot possibly same one that formed Yellowstone

            See a progression of seamounts with the youngest at the ridge and the oldest in a line away from the ridge

 

      Mid to late Eocene:  Rotation of area

            Put chain parallel to coast line youngest is in middle and oldest are at ends

            Coastal rocks more rotate than east rocks

            Older rocks more rotated than younger rocks

            Look to paleomagentic data to determine this

            Over time volcanoes acreate

            These Eocene lavas form the base of the coast range.

            Basaltic platform subsides and creates a basin. 

            Basin collects LOTS of sediments from Klamaths, Western Cascades and rivers

 

            Basaltic platform is covered with varying sedimentary deposits

 

      Deltas, silts, and fan deposits:

            Roseburg, Looking glass, and Flournoy formations

                  Delta and turbiditite deposits

            Tyee formation

                  Deep water muds and fossils

            Nestucca and Bastendorff

                  Continental Slope deposits

            Keesey Formation

                  Regression rocks: Scotts mills

     

39-29 mybp (Eocene)

      Intrusion of dikes and sills invade sediments

            Forms:  Tillamook, Yachates, Cowlitz, Willapa Hills, Centraila, and Seattle Basalts

                  Marys Peak, Saddle Back Mountain, Laural Mountain, Fanno Ridge, Stott Mountains….

 

Early to Mid-Miocene (23-15 mybp)

      Continued uplift of coast range and Olympic mountains

      Basin forms on E. margin of uplifted area

 

Mid-Miocene

      Continental shelf deposition creates the Astoria formation

      GREAT FOSSILS

      CRB’s reach coast and create headlands from Seal rock to Tillamook Head

      Followed rivers and sank into soft coastal material—reversed dikes and sills

 

Late Miocene to Pliocene (15-2 mybp)

      More uplift with some faulting and deformation (folds coast range)

      Olympics lifted to current elevation and more depression occurs on east side of mountains

      Erosion of area accelerates so increases amount of material deposited on shelf and in basin                

By late Pliocene shoreline looks a lot like today’s shoreline

 

Olympic Mountains

      Accretionary wedge or Melange material

      Hurrican Ridge:  Pillow basalts, deep ocean clay and fan deposits

      Deposits are horseshoe shaped

      Main core is melange sediments

      Outer is submarine volcanic deposits

      Pushed under volcanic material is more melange sediments from other wedges

      Most headlands and terraces in Olympics are melange deposits

 

Pliestocene

      Olympic Peninsula and Puget lowland covered in ice

      Glacial outwash deposits are placed over older material

      Final glacial phase for this area is ~28,000 yrs ago.

      Ice extended ~150 miles into USA

      Ice blocked rivers and pushed them south

      Water flowed around end of sheet and through Chehalis river valley creating VERY large plain and lots of peat bogs

 

Puget Sound area covered by glaciers

      Under glacers rivers cut huge troughs

      Carry away 250 cubic miles of material

      Create 10 deep troughs

            Admiralty inlet

            Hood Canal

            Poscession Sound

            Puget Sound etc….

 

Other Coast Range features

      Dunes:

            Stable and unstable varieties

                  Foredunes (active)

                  Younger stabilized dunes

                  Older stabilized dunes

      Terraces

            Primarily created by sealevel changes

 

      Rocky shorelines vs. sandy beaches

      Earthquakes

      Drowned forests

      Terraces

            Differential uplift of coast range