The Cordilleran Ice Sheet

The Cordilleran Ice SheetBy Derek Booth, Kathy Goetz Troost, John

Clague, & Richard Waitt

From Developments in Quaternary Science Vol. 1, 2004

Vic Prest’s (Canadian Geological Survey) reconstruction of the Cordilleran Ice Sheet has stood the test of time.

Even his age constraints on the timing of deglaciation has fared well with additional radiocarbon dating.

Fig.1 The Cordilleran Ice Sheet

We now recognize that the Puget Lowland was occupied with glacial ice many

times in the past and as recent as ~15,000 years ago.

The Alaskan tidewater glacier provides a nice analog to the glacial setting

of the Puget Lowland during deglaciation ~15,000 years ago.

Glacial features such as kettle and kames, glacial marine deltas and ice-streamlined topography are seen on the 10 m DEM image of central and south Whidbey Island.

Penn Cove

North

Post-glacial

marine delta

Ice-flow

direction

Kettle & Kame

The Vashon Till is well-exposed near the top section of many bluffs lying adjacent to the Puget Sound.

Mt. Erie, Fidalgo Island is a classic stoss and lee landform. Ice flow was from your right to left (north to south) on the image.

Glacial grooves and striations are preserved on quartzite bedrock overrun by the Puget ice sheet.

Diverse lithology of glacial erratics and till boulders reflects the diverse bedrock lithology that was eroded and deposited by the Puget Lobe.

Coupeville erratic (Whidbey Island) was plucked from the lee side of Mt.Erie lying ~20 miles north of Coupeville.

Wedgewood erratic (near NE75th St. and 28th Ave) is one of the larger glacial erratics in the Seattle area. It is a greenstone and may have also been excavated from Fidalgo Island.

The Saratoga erratic (south Whidbey Island) is one of my favorites. It is over 30 feet tall and big enough to eat lunch on it. It was also excavated from Mt. Erie, Fidalgo Island.

The large Columbia basalt haystack erratics were excavated by the Okanogon Lobe from the Columbia Gorge and deposited north of Withrow, Washington.

How would you explain the presence of this large Whidbey Island erratic lying 250 feet above sea level without a context of “glacial theory?”

““Field investigations Field investigations Puget Lowland show Puget Lowland show that ice sheets have that ice sheets have advanced south into advanced south into the lowlands of the lowlands of western Washington western Washington at least SIX times” at least SIX times” (18)(18)

BUT…..BUT…..

The MARINE The MARINE OXYGEN-ISOTOPE OXYGEN-ISOTOPE RECORD suggests RECORD suggests that there were that there were MANY more; possibly MANY more; possibly 20-30 glacial 20-30 glacial advances in the last advances in the last 2.5 million years.2.5 million years.

Why don’t we see evidence for so many glacial advances?

• Obliterative Overlap (recurring glaciations “erase” records of previous glaciations)

• Tectonic deformation: “the bedrock and overlying Quaternary sediment in the Puget Lowland have been deformed…other faults cross the glaciated lowlands both N and S of the Seattle fault, with likely displacements of meters to tens of meters, thereby complicating interpretation of the Quaternary stratigraphic record” (20)

• Lack of significant advance into this region

Fig. 2 Marine Oxygen Isotope Curve (MIS)Fig. 2 Marine Oxygen Isotope Curve (MIS)P

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Blu

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Blu

ff

Salm

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Fig. 5 - Locations of Fig. 5 - Locations of recognized pre-recognized pre-Fraser Fraser stratigraphic stratigraphic units:units:

1.1. Possession DriftPossession Drift

2.2. Double Bluff DriftDouble Bluff Drift

3.3. Salmon Springs DriftSalmon Springs Drift

4.4. Orting DriftOrting Drift

5.5. Stuck DriftStuck Drift

1122

33

4455

Fig. 7a

Possession Drift (Pre-

Fraser) stratigraphic section from Point Wilson, Whidbey Island

Fig. 7bFig. 7b

Pre Fraser Pre Fraser lithologies of Double lithologies of Double Bluff, Whidbey Bluff, Whidbey Island (Double Bluff Island (Double Bluff Drift)Drift)

Fig. 7cFig. 7c

Pre Fraser Pre Fraser stratigraphic section stratigraphic section from the Puyallup from the Puyallup River Valley (Stuck River Valley (Stuck Drift)Drift)

Much of the sediment record exposed in unconsolidated bluffs lying adjacent to the Puget Sound are glacial in origin. Non-glacial sediment units, mainly alluvium are present in the lower exposed sections. This image was taken from the Double Bluff, Whidbey Island.

Eustatic sea level rise.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Isostatic effects and relative sea level.

Marine limit (m)

Whidbey Island

• Missoula Floods and CIS