NEW CONSTRAINTS ON THE EVOLUTION OF THE SUMAS STADE OF THE CORDILLERAN ICE SHEET, NORTHWESTERN WASHINGTON

The timing and nature of the Sumas Stade, the final advance of the Cordillera Ice Sheet (CIS) into western Washington at the termination of the Fraser Glaciation, has been a contentious topic for decades. New ¹⁴C age constraints from lake cores indicate that the event culminated shortly before 12,450 ± 35 ¹⁴C yr B.P. (14,165-14,990 cal yr B.P. 2-σ), substantially earlier than previously recognized. Recent bare-earth LiDAR data indicate that the CIS subsequently developed a complex record of retreat, readvance, and marine incursion and regression during the next millenium.

Sediments from Lake Padden, southwest Whatcom County, record inorganic deposition associated with emplacement of the small but distinct Padden (outermost Sumas) moraine as well as a sharp transition to organic non-glacial sedimentation associated with initial retreat of the ice from the moraine. AMS radiocarbon analyses of bulk sediments indicate that this transition occurred ~14,500 cal yr B.P., significantly earlier than previously appreciated; other dates in the cores indicate that the dating is robust.

After formation of this small distinct moraine, the CIS retreated and readvanced to progressively smaller extents, forming a series of moraines and till-plains that dominate western Whatcom County northward to the International Border. Furthermore, marine strandlines imaged in the LiDAR data are intimately linked with these deposits, indicating complex interactions between sea level rise and fall and glacier advance and retreat. Existing dating indicates that the major marine transgression coincided with Meltwater Pulse 1A. Ongoing lake coring of other lakes and bogs adjacent to several of the key landforms should help further constrain the timing of many of the more prominent deposits related to the Sumas Stade, and help link these events to those documented in southwestern B.C.