GLACIATION AND SEDIMENTARY ENVIRONMENTS IN THE PACIFIC NORTHWEST DURING MARINE ISOTOPE STAGE 3

During the lengthy span of marine isotope stage (MIS) 3 (ca. 59-24 ka ago), global ice volume was 50-80 % that of the last glacial maximum (MIS 2), implying the existence of large ice sheets over northwestern Europe and northeastern North America. World sea level at that time ranged between -90 and -50 m. Ice-free conditions during MIS 3 in central British Columbia and the Puget and Fraser lowlands point to a restricted Cordilleran Ice Sheet (or mountain ice cap). MIS 3 ELAs of most temperate-latitude alpine glaciers may have been depressed, on average, about half their full-glacial depression of ~900 m, equivalent to the ELA depression of late-glacial time (~13-11 ka ago). The Hoh glacier in the western Olympics is an exception, for Glenn Thackray has shown that it expanded ~42-35 and ~31-29 ka ago, terminating beyond its subsequent MIS 2 limit. Cascade glaciers, by contrast, apparently lack moraines of these ages; instead, during MIS 3 they may have fluctuated around limits similar to those of late MIS 2 time. Evidence for MIS 3 alpine glacier activity in the Cascades may exist in sediments beneath MIS 2 (Fraser) drift in the adjacent lowland belt.The most widespread and accessible deposits are the 'Olympia beds' and Quadra Formation of the Puget and Fraser lowlands, respectively. These units include coarse- to fine-grained alluvium, lahar deposits, tephra, peat, and other organic remains. Distal outwash of the largest glaciers likely was deposited close to sea level by major meltwater streams that crossed the lowlands. Glacier advances correlative with North Atlantic Heinrich events H1 and H2 are recorded by moraines of the southwestern Cordilleran Ice Sheet and nearby Cascade alpine glaciers. The two MIS 3 Hoh glacier advances in the Olympics apparently correlate with H3 (27 ka) and H4 (35.5 ka). Preliminary analysis of ¹⁴C-dated sediments in the Puget Lowland suggests that a record of these MIS 3 environmental changes may also exist in these strata.