Willamette Valley & Puget Lowlands

      Extends from Cottage Grove To BC

      Often Linked to the Coast Range

      Created by the uplift of the coast range

      Not a true valley but a basin

      Basement rocks are the same as those of the coast range

 

      Oldest rocks created by accreted seamounts that have been rotated

      Subduction zone moves west

      Accreted rock block subsides, transgression occurs and a forearc basin forms

Paleocene to Eocene

      Lots of deposition primarily from Klamath mountains and Idaho Batholith

      Eastern edge of this Block/Basin feature becomes the Willamette Valley and the Puget lowland

Rock Units deposited during Eocene

      Southern WV:

            Flournoy Formation:  Sands and silts representing a delta deposit

            Tyee formation:  Delta and turbidite deposits

     

      Northern WV: 

            Yamhill muds:  Sand and ash with limestone

            Puget Lowlands:

                  Puget Group:  Delta deposits

                        Recognize Shoreline by deposits

      Nestucca Formation (McMinville and westward)

            Represents deep water deposits

      Spencer formation (Eugene Northward)

            Delta deposits

      Eugene formation (Eugene to Salem)

            Silts and sands with mollusks, crabs and sharks

            Warm semi-tropical water

      Fisher formation (covers Spencer Formation)

            Non-marine tuffs and conglomerates

            Contains fossils representing tropical climate

 

Oligiocene

      Localized uplift caused some withdrawl of sea

      Water only as far south as Salem

      See transgression and regression sequences in Silverton and Scott’s Mills

            Scott’s Mills Formation: 

                  Transgression then Regression

                  Shallow water, Swamps, Coal

                  Rocks tilted then heavily eroded before Miocene

Miocene

      Uplift of the Coast Range causes sea to withdrawl.

            Leaves behind swamps and ponds with gather clays and pollen.

            Shows what vegetation used to live in the WV during this time

                  Ginko, Metasequoia, etc…

      CRB’s Invaded valley

            Portland hills, Hills SW of Portland, and Silverton area

 

      Interlayered with CRB’s is Molalla formation

            Lahar deposits with good fossils

            Shows land based deposits (first ones after the withdrawal of the sea

 

Portland is low point in valley and rivers drain into it creating a lake

      Helvetia Formation (silts)

      Sandy River Mudstones (Clay rich river deposits)

      Troutdale formation (Columbia river deposits)

 

Pliocene

      Continued uplift of coast range and tilting of Willamette valley rocks

      Creates anticlines and synclines in valley

            Portland hills, Cooper Mountain etc…

      Also created faults (extension of Brothers fault zone)

            Separates WV and Coast range as separate provence

      Sea withdrawals to its current location

      Eruption of the Boring Lavas begin (~5 million to 0.8 million years ago)

            Boring Lavas

            Believed to be a possible extension from the Cascade base basalts (spreading due to arc system)

            Erupted a series of small cones from Boring to Woodburn

            Oldest cones are in Boring and youngest is La Butte (south of Wilsonville)

            Different compositions than CRBs

                  Larch Mtn, Mt Tabor, Mt. Scott, Mt. Sylvania, Kelley butte, Bob’s Mtn,         Battleground etc…

Southern Buttes

      NOT part of the Boring lavas

      Most are thought to be sills etc of Western Cascade volcanism

 

Pliestocene

      Here come the Glaciers….sort of

      Glacial outwash is deposited from the Cascade mountains from the southern valley to the northern valley

      Pushes Willamette river out of its original channel into a new channel

      Uplift of coast and lowering of sealevel due to ice formation creates extensive downcutting by streams

      Many terraces form

      Joined streams divert (Willamette and Tualatin)

            New channels form (Lake Labish)

 

Portland hills silt is deposited

      Loess deposits from northern glaciers

      25-100 ft thick

      70,000 years old

      Only currently found in Portland above 400 ft elevation

            Why?

 

Missoula floods

      Ice age floods that occurred between 12,000 to 15,000 years ago

      Originate in Montana and flowed down through Washington and the Gorge

      Filled Valley and created Lake Allison

      Left notable deposits

            Willamette silts

            Boulders

                  Erratics

            Old Flood channels