MAXIMUM EXTENT AND RETREAT OF THE PUGET LOBE OF THE CORDILLERAN ICE SHEET, NORTHWEST WASHINGTON

Richly-detailed LiDAR topography and multibeam bathymetry facilitate a new reconstruction of the maximum extent of the Puget Lobe of the Cordilleran Ice Sheet during the last glaciation. The reconstruction depicts significantly less ice than in earlier reconstructions: ~1,170 m thickness at Port Townsend, cf. 1,265 m (Thorson, 1980 QR) and ~2,250 m (James et al., 2009 JGR). Ice flow along the western margin of the Cascade Range was complex: the Skagit basin was an ice source at glacial maximum and afterwards, whereas valleys farther south were ice sinks. Ice flowed south and east towards a sump at Darrington that was drained by sub-ice flow of meltwater along the range front. Farther south, calculated ice-sheet hydrology predicts numerous ice-marginal lakes that probably were refugia for fish stocks. Uncertainties in the reconstruction stem from incomplete understanding of glacio-isostatic adjustment, incomplete bathymetric and LiDAR data for the eastern Strait of Juan de Fuca and the Olympic and Cascade mountain fronts, incomplete evidence for the ice limit along the mountain fronts, and perhaps from the assumption that glacial landforms primarily record flow at maximum ice extent.

13,410, 13,410, and 13,510 ¹⁴C ybp radiocarbon dates from wood and plant fragments in pre-ice sediments in the southern Puget Lowland (Borden and Troost, 2001 WDGER; Polenz et al., 2011 WDGER) and older dates of 13,700 and 13,600 ¹⁴C ybp from post-ice gyttja near Kingston, 110 km north of the ice limit (Anundsen et al., 1994 QR), (1) indicate the Puget Lobe reached maximum extent after 13,400 ¹⁴C ybp, (2) require incorporation of old carbon in post-ice gyttja, and (3) suggest that ice retreat from the southern Lowland was extremely rapid. Widespread dead-ice features (kame-kettle topography, eskers, and—especially—washboard moraine) demonstrate that shortly after reaching its maximum extent the Puget Lobe stopped moving and melted in place. I speculate that stagnation resulted from beheading of the Puget Lobe by collapse of the Juan de Fuca Lobe to the northwest. After collapse of the Juan de Fuca Lobe and stagnation of the Puget Lobe, the ice margin stabilized to form the terminal moraine that extends from Coupeville on central Whidbey Island northwest to Victoria, BC.