G207 PNW Geology

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G207 PNW Geology

• 12/5/07
• Guest Lecturer
• Eriks Puris

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Topics

• Glacial Floods
• PNW Earthquakes

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Sculpting the Columbia Basin
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09_18a.jpg
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Region Covered by Basalt Flows
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Deposits after the Basalt

• Silt
• Glacial Till

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Palouse Silt Wind-blown dunes cover basalt.
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Palouse Silts in the Columbia Basin - once more
extensive?
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Glacial TillUnsorted deposits left by melting
ice sheetGlacial Maximum Advance,15,000 years ago
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J Harlan Bretz 1882 - 1981
Biology
Geology!!!
Parfit, M., 1995, Smithsonian, v. 26, n. 1, p.
48-59
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Bretz Takes to the Field

• 1913 1922
• Described, documented, mapped the distinctive
  erosional features of Eastern Washington
  landscape.
• Everywhere he found evidence that enormous
  quantities of water had sculpted the landscape in
  very short amount of time and not too long ago!

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What Did Bretz See in Eastern Washington?
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Scablands
2,999 mi2 of raw, peeled ground containing black
basalts, broad expanses of gravel, and dry stream
channels.
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Satellite Image of theChanneled Scablands
Telford-Crab Creek
Dry channels cross divides 300 feet above present
streams. Complexly braided pattern
Cheney-Palouse
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A Modern Day Braided Stream
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StreamlinedIslands of Palouse Siltwithin Dry
ChannelsIslands rimmed by high water marks
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Coulees - long, broad, deep gorges with steep
vertical sides.
The channels look like they carried huge amounts
of water at one time, but now they were dry.
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Upper Grand Coulee
25 miles long, 1 to 6 miles wide, lined by steep
walls of basalt 800 ft high from floor!
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Lower Grand Coulee at Dry Falls
A cataract but no river!
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A Modern Day Recessional Cataract
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Dry Falls is 350 ft. deep and 3 mi. wide. (5 x
the width of Niagara!)
Plunge Pools
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Plunge Pools and Pothole Lakes
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Kolks!! Extremely strong vertical vortices that
develop in deep flows of very fast water!
These under water tornados can pluck bedrock
creating potholes.
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Giant Ripples and Gravel Bars!
Laying in all the expected places, but often, no
water nearby!
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Modern Ripples Along a Shore
Typical water currents pile the sand into small
dunes, elongate ridges, a few inches high
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Ripple Marks along the Columbia (Fig. 12.40)
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The giant ripples of Eastern Washington are
gravel! They stand as high as 35 ft with crests
250 feet apart!
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Erraticsmisfit rocks, littering areas from
Spokane to Columbia River Gorge
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Erratic a 130 ton angular, block - rafted into
place by icebergs stuck in shallowing waters
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Bretz and His Humongous Flood

• Presented finding to geologists (1923).
• After laying out all the evidence, he made a bold
  conclusion An enormous flood has eroded the
  scablands in a very brief time, perhaps a matter
  of days!!

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Sequence of Flood Events (drawings/supplemental
information from Baker, 1978)
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Flood waters overtop a divide, encountering soft
Palouse silts. High velocity waters expose
basalt entablature, grooving it and leaving
isolated remnants of Palouse silt
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• Kolks pluck the basalt column layers, forming
  potholes.

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Enlargement and coalescence of potholes, creating
the basin and butte topography and erosion of
inner canyons
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The Heretic in the Scablands

• Bretzs conclusion and the thinking that led to
  it were adamantly and vigorously denounced.
• Colleagues were shocked, appalled, outraged.
• It was heresy!

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Scientific Geology

• Began in 1790s - Uniformitarianism
• Rocks and landscapes are the result of slow
  processes and weak forces operating over long
  periods of time.
• Invoking a catastrophic explanation for a
  geologic event geology in the dark ages bad
  science.

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The Debate Begins

• What Bretz had
• A lot of good, solid evidence.
• What Bretz lacked
• A source of water!

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Alternatives for the Channeled Scablands Pop Up
in Droves

• Alden, 1927, (Head of USGS) collapsed lava
  tubes and normal stream erosion including minor
  floods.
• O. E. Meinzer, 1927 enlarged Columbia River
  carves Grand Coulee and Dry Falls.
• J. Gilluly, 1929 - ongoing erosion of
  present-sized streams.
• Etc., Etc., Etc.
• None explain observations many never came to
  Eastern Washington!

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A Glacial Lake in Montana?
T.C. Chamberlain, USGS, 1886, observed shorelines
of a ancient lake in the Mission Valley of
Montana.
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Glacial Lake Missoula
Follows ancient shorelines around the slopes of
the Clark Fork River Drainage
J. T. Pardee (1871-1960)
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Wheres the Dam?(Clark Fork River enters Lake
Pend Oreille)Parfit, M., 1995, Smithsonian, v.
26, n. 1
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Ice in the Purcell Valley
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Pardee reconstructs Glacial Lake Missoula (1910)
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Glacial Lake Missoula, the inland sea
Covered an area of 2,900 mi.2 Highest shoreline
elevation 4,250 ft. Volume of water 520
mi.3 Maximum depth 2,000 ft. at ice dam 900
ft. in Missoula Valley
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A Breakthrough?

• Pardee returns to Glacial Lake Missoula (1942)
• Finds evidence for catastrophic draining of the
  lake!
• Giant ripples marks, gravel bars, pothole lakes,
  erratics and scouring of the valley walls and
  floor of the lake.

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Scouring of valley floor and walls to bare
bedrock stripped of soils
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Estimated flow of water out of lake

• Discharge 600 million cfs!!!
• Mississippi River (1993) 1 million cfs
• River up to 1500 ft deep ran with 10x the water
  of all rivers combined.
• Velocity 58 mph
• Discharge could empty lake in 2 days!

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Lake got deep enough to float its ice dam it
fails and breaks up!
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Fig (12.37)
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End of Controversy?

• Pardee hands Bretz the source of water for his
  tremendous flood.
• Widespread acceptance in late 50s, a few still
  resist the idea today.
• Growing body of evidence for repeated floods as
  many as 80-100!

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Deposits of Lake Missoula

• Glacial varves
• Light layers summer
• Dark layers winter
• Each pair records one years deposition.
• By counting varves, we can determine how many
  years the lake existed.

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Lake Deposits, Missoula
36 sequences of varves separated by river
deposits!
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Chambers and Alt, 1971

• 36 sequences of varves (lake) separated by
  complexly bedded river deposits.
• Lake had drained a minimum of 36 times!
• Bottom varved sequence 58 years
• Each successive sequence less years.
• Top sequence 9 years.

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Waning of Ice Sheet?
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Water reaches Wallula Gap and backs up to form
temporary lakes.
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Flood deposits in Lake Lewis
Touchet Formation Each layer of pebbles and silts
represent one flood event. Graded bedding.
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Waitt, (1977, 1980)

• Layers record at least 41 events, maybe many
  more.
• Thicker layers are at the base of stack.
• Volcanic Ash layers provide some dates!

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The Story Hasnt Ended Yet

• Investigations into details of flooding still
  ongoing.
• Before his death, J Harlan Bretz received
  geologys highest award the Penrose Medal.

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Earthquakes in the Pacific Northwest

• Tectonic Setting
• 3 Sources
• Subducting slab
• Subduction zone interface
• Crustal Earthquakes

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Cascadia subduction zone
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• Previous
• PNW
• Earthquakes

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The PNW has 3 sources of Earthquakes
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A 3-D View

• http//spike.geophys.washington.edu/SEIS/EQ_Specia
  l/WEBDIR_01022818543p/hypos.html

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The 3 types of earthquakes in the PNW

• Deep (intraplate) earthquakes
• (45-60 Km in the Juan de Fuca subducting plate)
• 2. Subduction zone (interface) earthquakes
• (at the contact between the 2 plates)
• 3.  Crustal earthquakes
• (in the North American plate)

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Deep earthquakes in the subducting slab

• Where
• Why?
• Historic examples
• 1949 M 7.1 Olympia
• 1965 M 6.5 Sea-Tac
• 2001 M 6.8 Nisqually
• How strong?
• How frequent?
• Shaking effects?
• Area affected?
• Potential damage?

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Where? Why?
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Location of epicenters for the 1949 and 1965
earthquakes
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Isoseismic map for the 1949 Olympia earthquake
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Puyallup high school damage from the 1949
earthquake
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Isoseismic map for the 1965 Sea-Tac earthquake
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Damage to a masonry building from the 1965
earthquake
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Epicenter
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Summary Deep earthquakes in the subducting slab

• Where? at depths of 40-60 km in the Juan de Fuca
  subducting plate
• Why? Phase changes in the subducting slab
• Historic examples
• 1949 M 7.1 Olympia
• 1965 M 6.5 Sea-Tac
• 2001 M 6.8 Nisqualy
• How strong? 6-7 M (Moderate strength)
• How frequent? one every 20-30 years on average
• Shaking effects? 15-30 seconds
• Area affected? Localized in area above subducting
  slab (i.e. Puget Sound region)
• Potential damage? Significant to extensive

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Subduction zone interface earthquakes

• Where?
• Why?
• What evidence?
• How frequent?
• How strong?
• Shaking effects?
• Potential damage?

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Where?
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Examples of interface subduction zone earthquakes
Alaska, 1964 M 9.2
Chile, 1960 M 9.5
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What causes the subduction zone interface
earthquakes?
Locking and Bulging GPS ½ in/year
subsidence
uplift
Rebound
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What happens when the plates shift
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Has it Happened Here?

• No Historical Subduction Interface Earthquakes

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Evidence for Prehistorical Earthquakes

• Ghost Forests
• Peat Layers
• Tsunami Sands
• Tsunami in Japan (January 29, 1700)

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Ghost Forest
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Coastal evidence of subduction zone earthquakes
And deposition by tsunami
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Top forest killed by inundation of sea-water
after tectonic subsidence during 1964 Alaska
earthquakeBottom forest killed by
inundation of sea-water after tectonic subsidence
300 years ago on the Cohalis river SW Washington
coastal
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Stumps of trees killed in 1700 A.D. (at low tide
along the west shore of Willapa Bay)
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Peat Layer topped by Tsunami Sand-Oregon
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Brian Atwaters area of study
Chehalis River
Willapa River
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Orphan Tsunami in Japan Jan 29, 1700
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How frequent? One every 300-500 years. Last one
300 y. a.
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How strong?

• historical record of similar earthquakes (i.e.
  Chile and Alaska),
• tsunami wave heights (13 ft. Japan)
• evidence of earthquake and vertical displacement
  along the entire PNW coast from the prehistoric
  events
• suggest events of
• M 8-9
• These are the BIG ones!!!

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Summary Subduction zone interface earthquakes

• Where? at the interface of the Juan de Fuca and
  North American plate
• Why? locking-bulging, slipping-rebounding
  mechanism
• How frequent? every 300-500 years (We may be due
  for one)
• How strong? 8-9 M (This is the BIG one!)
• Area potentially affected? the entire PNW
• Shaking effects? 1-3 minutes
• Potential damage? extensive to catastrophic

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Crustal Earthquakes

• Where?
• Why?
• An example the Seattle fault
• What evidence?
• How strong?
• Other faults?
• The SHIPS experiments

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Where? Why?
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There are many local faults in the Puget Sound
region
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An Example The Seattle Fault
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Cross section view of the Seattle fault
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The Seattle fault line (zone!)
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LIDAR image showing surface evidence of the
Seattle fault (Bainbridge Island)
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• Is the Seattle fault active?
• Is there evidence of earthquakes along the
  Seattle fault?

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Possible evidence of very recent seismic activity
along the Seattle fault the Point Robinson
Earthquake of 1995
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Lines of evidence for a prehistoric earthquake
along the Seattle fault 1100 y.a.

• Uplifted wave-cut platforms
• Massive landslides (earthquake-triggered)
• Tsunami deposits and submerged areas

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Formation of an uplifted wave-cut platform
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Restoration point, Bainbridge Island
The date of shells found on the bench is 1100
years!
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Location of uplifted areas
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Location of drowned forest in Lake Sammamish
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A landslide (slump block) was found under water
The radiocarbon date on the trees is 1100 years!
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Location of pre-historic landslides in the Puget
Sound region
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West Point, Discovery Park (Seattle)Tsunami-lai
d sand was found here
The radiocarbon date on tree logs in tsunami-laid
sand is 1100 years.!
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Location of tsunami deposits in submerged areas
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How strong was this earthquake?

• Based on vertical displacement
• measurements probably around
• M 7-8

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Tectonic Setting Compression
http//geology.wr.usgs.gov/wgmt/pacnw/rescasp1.htm
l
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Summary Crustal Earthquakes

• Where? Shallow in the North American plate
  (includes the Puget Sound region)
• Why? regional compressional stress applied on
  pre-existing crustal faults
• How strong? up to 8 M
• How frequent? uncertain
• Damage? localized but extensive

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Summary of crustal earthquakes
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Conclusion

• There are 3 distinct types of earthquakes in the
  PNW, each with its own characteristics and
  hazards
• We are due for at least two of them
• If you live here for long, you will be in a major
  Earthquake

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There are some things you can count on

• It rains here
• We have great coffee
• We will have a major earthquake
• PREPARE BECAUSE YOU CARE!

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