Glaciofluvial Infilling and Scour of the Puget Lowland By: Alexander Pacubas *Presentation based on paper by Derek B. Booth (1994)

Pleistocene/Fraser Glaciation Advancement of the Puget Lobe of the Cordilleran Ice Sheet During The Vashon Stade The Puget Lowland has well-preserved records of glacial sediment deposition Below an altitude of 1000m, almost the entire land surface of the Lowland was shaped by glacier processes

Vashon Stade The most recent glacier advance period or the last Glacial Maximum (15ka) Much of the sediment deposition in the Lowland is from glacial advance The Advancing Ice blocked the Strait of Juan de Fuca

The Blocking the Strait Aggradation farther South of the Strait of Juan de Fuca would have been limited Much of the sediment supply would have been emptied there Once blocked, sediment build up became easier in the Puget Lowland

“The Great Lowland Fill” Sediment accumulated in Ice Dammed Lakes Two main members: 1. The Lawton Clay Member 2. Esperance Sand Member Outwash would have prograded as deltas significant sediment build-up during the glacial advance created an extensive outwash plain AKA “The Great Lowland Fill”

Continued Advance The advancing ice overridden the outwash sediment and deposited basal till From dating the basal till, the outwash of the advancing ice-sheet must have finished prior to 14.5 ka The Advance sediment consists of the clays, outwash and overlying till

Puget Lowland Topography Elevations of most of the topographic features in the Puget Lowland is determined by thickness of the advance outwash It is possible to reconstruct the surface of the outwash sediment

Basin fill represents the lower topography in the foreground (120-180 m above sea level). The Olympic Mountains are seen the background. Image taken from Whidbey Island, WA looking west.

Reconstructing the Proglacial Fill Plotting location and altitudes of hilltops underlain by glacial sediment -> Develop Smooth topographic map Typical elevations of 120- 150m Outwash deposit averaged to 100m thick

Drainage out the South end of the Puget Lowland into Chehalis River Blocked drainage out of the Strait *The slope of the drainage is not corrected for isostatic rebound. Depression occurred after outwash deposition.

Notable Morphologies The Eastern and Western Margins are concave Topographic Relief on the Eastern Side due to drainage from high elevation Cascade Range

Notable Morphologies The Southwest Corner has a broad southeast sloping plain of till-mantled outwash The Maximum elevation (+180m) extends along and beyond the Hood Canal trough in the Southwest These two characteristics infer a strong erosional process

Trough Formation Sub-parallel alignment Extend to depths up to 400m below the glaciated uplands Thought to have been formed by ice tongues or the subaerial channel erosion. Both reasoning are IMPOSSIBLE.

Trough Formation Ice tongues would have no bed contact since proglacial lakes would have floated them Subaerial channels could not erode the deepest trough, which run 300m below the basin outlet SUBGLACIAL MELTWATER IS THE ANSWER *The troughs here match dimensions and studies of troughs in New York, Ontario, Nova Scotia, and Germany.

Trough Formation Erosion into the Lowland fill occurred: After advance and outwash deposition Before subaerial exposure of the glacier bed by ice recession Most of the troughs are several times deeper than the original fill Most of the eroded sediment was pre-Vashon age -Much of the troughs are mantled with basal till

Erosion Volume The difference in altitude between model surface and modern surface Calculated the volume of sediment eroded to be about 1000 km3 or 100 m of ice-occupied area *Trough Erosion accounts for well over three-fourths of this volume

Erosion Rate The magnitude of erosion must be very large for such deep erosion Glacial meltwater is notorious for having high sediment content 0.0026-0.0049 m3/m3 (sediment/water concentration) Median suspended load of 5000 mg/L Avg. water discharge 3000 m3/yr Transport rate of 0.2 km3/yr *These are estimates

Erosion Power To erode 1000 km3 at a rate of 0.2 km3/yr would take 5,000 years! This estimate is about twice the available time Sediment concentrations must be double what is estimated here! Sediment may have came from catastrophic discharges from elsewhere

Conclusion Although direct ice interaction may have stronger geomorphic impact, much of the formation of the Puget Lowland was caused by fluvial processes.

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Image Source: https://en.wikipedia.org/wiki/Glacier