Abstract
ABSTRACT Recently obtained radiocarbon ages from the southern Puget Lowland and reevaluation of limiting ages from the Olympic Peninsula in the light of new light detection and ranging (LiDAR) data suggest that the Juan de Fuca and Puget lobes of the Cordilleran ice sheet reached their maximum extents after 16,000 calibrated yr B.P. Source areas for both lobes fed through a common conduit, likely requiring that downstream responses to changes in either source area were similar. Dates for ice-sheet retreat are sparse and contradictory, but they suggest that retreat was rapid. Depositional and geomorphic evidence shows that retreat of the Juan de Fuca lobe predated retreat of the Puget lobe. No recessional end moraines have been identified in the Puget Lowland, in contrast to numerous recessional end moraines constructed by the Okanogan lobe east of the Cascade Range, and in contrast to later ice-sheet retreat in western Whatcom County north of the Puget Lowland. These observations lead to the hypothesis that collapse of the Juan de Fuca lobe, hastened by the instability of a marine-based ice sheet, steepened the ice-sheet surface over the eastern Strait of Juan de Fuca and diverted ice flow upstream of the Puget lobe to the west. Starved of ice, the Puget lobe retreated quickly.
Highlights
During the last glaciation, the Cordilleran ice sheet advanced south from British Columbia into northwest Washington (Fig. 1)
The ice sheet split at the northeast margin of the Olympic Mountains, where one arm flowed west to form the Juan de Fuca lobe, which extended onto the continental shelf, and the other arm flowed south to form the Puget lobe, which reached its terminus near Olympia, Washington
Porter and Swanson (1998) elegantly summarized radiocarbon constraints on the age of the Puget lobe. They concluded that (1) the ice front crossed the 49th parallel at ca. 18,800 cal. yr B.P. and advanced south at ~135 m/yr, (2) the maximum extent was at ca. 16,950–16,850 cal. yr B.P., and (3) retreat was about twice as rapid as advance
Summary
During the last glaciation (marine isotope stage [MIS] 2), the Cordilleran ice sheet advanced south from British Columbia into northwest Washington (Fig. 1). This report (1) summarizes the sequence of events during Vashon-age deglaciation of northwest Washington; (2) reviews new light detection and ranging (LiDAR) topography and published radiocarbon data, updating Porter and Swanson’s summary, to conclude that the maximum extents of the Juan de Fuca and Puget lobes were younger than previously thought, after 16,000 cal. The Puget lobe was fed by accumulation east of Vancouver (southern Coast Mountains, interior of British Columbia, and North Cascades) As noted above, this apportionment of source areas could have been fluid: Variations in ice supply or ice sinks (e.g., increased snowfall in the western Coast Mountains, increased ablation at the snout of the Juan de Fuca lobe) would have been compensated by a shift in flow direction over the San Juan Islands. W side Capitol Hill underneath Lawton Clay, location assigned by R.H
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