Cascade Mountains
Extends from BC to CA
Northern Cascades from Snoqualmie pass to Mt Garibaldi
Southern Cascades from Snoqualmie pass to Mt. Lassen
Structural difference between two regions
Northern: Accreted terranes
Southern: volcanic terranes
Cascades (S. End)
Started developing ~40 mybp
Uplift and faulting causes deformation
Created originally by subduction of Farallon Plate and then later by subduction of the Juan de Fuca plate
2 primary stages of development
Western Cascades (40-7 mybp)
High Cascades (7mybp to present)
Western Cascades
Eocene:
Shoreline through the Willamette Valley
Subduction of Farallon Plate begins
Small volcanoes in Eastern OR produce ash and andesite
Very broad deposits suggests low subduction angle and rapid subduction
Oligocene:
Coastline in Willamette Valley with lots of volcanic sediments accumulating on shelf
Preserved many plants and created coal deposits
Alluvial deposits : Colestin formation is base of Western Cascade deposits.
Miocene:
Folding and Faulting of area
CRB’s deposited during this time
Other volcanic activity during this time
Strawberry mountain volcanics
Steen’s mountain basalt
Violent eruptions from Western Cascades
Due to faulting in this area, deposited material sank and didn’t grow up
Prior to this time Western Cascade material was widespread. As time progressed volcanic center narrowed…Why?
Rotation of NW
Changing of subduction angle
Produced a variation of rock ages for Western Cascade rocks.
West side of range: ~40 mybp
East side of range: ~10 mybp
Western Cascades created LOTS of material. Much more than the Cascades do today…Why?
Change subduction rate 3”/ yr vs ½”/ yr
More folding/faulting occurred ~5 mybp which created Coberg hills and McKenzie-Horse Creek fault.
These two faults mark the boundaries of the Western Cascade range.
Units of the Western Cascades
Ohanapecosch formation (Eocene)
Mostly andesite
10,000’ thick near Mt. Rainier
Upper sections weathered to clay
Exposed on WA side of Columbia river gorge
Skamania Volcanics (Eocene to Miocene)
Basalt flows and andesite tuffs
Exposed below the Yakima CRB’s
Possibly western extension of Clarno formation
Eagle Creek Formation (early Miocene)
Lahar deposits with some silts and sandstones
Derived possibly from Skamania volcanics
Lots of clays from weathered pumice
250’ thick on OR side of Col. River
1000’ thick on Wa side
Contains many fossilized logs and plant matter
Very eroded by the time the CRB’s came along.
These units are the cause of many landslide deposts in N. OR and S. WA.
Washington side of gorge has more slides due to the tilting of the strata
Exposures of Western Cascade volcanics due to slides
Hood river, Kelso, Skamania Lodge, Bridge of the Gods…..
High Cascades
2 stages of development for the High Cascades.
7-1 mybp and < 1mybp
Miocene tilting of Western Cascade block created Blue River Ramp
Responsible for rain shadow in NW
Most of High Cascades built on basaltic material from other eruptions and Cascade eruptions (early on)
Grabens form during end of Miocene and average drop is 2000’
Removal of magma from below continues to drop grabens
Grabens most pronounced from Hood to Sisters
Plio-cascades: Most deposits buried beneath current Cascades
Started Basaltic and evolved to andesitic
Plio-Cascade (ancestral Cascade deposits)
The Dalles group
Deschutes formation
Chenoweth formation
Tygh Valley formtion
Rhododendron formation
W of Cascades and N. of Santiam Pass
All above are andesite and dacite flows, lahars, tuffs etc…
Today’s Cascades
All Cascade peaks we see are less than 1 mys.
Formation began with a graben and basaltic lava flows
85% of Cascades volume is basalts
Buried ancestrial or plio-cascades
Transition of basalt to andesite over time
Plutons also present but very deep Possibly due to subduction angle
Current mountains related to subduction of Juan de Fuca Plate
Melting at ~60-75 miles depth
Plate divided into three tongues
OR and BC angle ~65o
WA less steep
Non-linear orientation of volcanoes due to this division
Recent volcanics are andesitic
Eruptions?
Most deposits are less than 690,000
Based on the magnetic signature of the lava flows
Many peaks show glaciation
Mt Bachelor: 6,600 -10,000 years
Mt. St. Helens: mostly <4,000 years
Mt Hood: eruptions from 30,000 to 200 yrs
Mt Rainier: Many eruptions in past 500,000 yrs.
Mt Mazama
Known as Crater Lake
Mazama built during Pleistocene
Glaciated throughout it’s history
Elevation ~12,000 ft
Covered older volcano (the Phantom cone)
Collapsed Caldera eruption ~6900 yrs ago
Lava flows on caldera floor created shield volcano on floor (1200 feet)
Final eruption—Wizard Island (800-900 yr)
Other Cascades and Hazards
Mt Rainier
Mt Hood
Mt St. Helens
Sisters
Lassen
Shasta