GLACIERS, ISOSTASY, AND EUSTASY IN THE FRASER LOWLAND: A NEW INTERPRETATION OF LATE PLEISTOCENE GLACIATION ACROSS THE INTERNATIONAL BOUNDARY

High-resolution lidar data and new stratigraphic, lake-sediment, and radiocarbon constraints help resolve a long-standing dispute regarding the timing and nature of the Everson interstade and the Sumas stade, the last major events of the late Pleistocene Cordilleran Ice Sheet in the Fraser Lowland. The new data indicate that: 1) an early, maximum Sumas advance occurred roughly 14,500 cal yr BP, extending into the Salish Sea near Bellingham, Washington; 2) ice retreated north of the International Boundary long enough for forests to establish in deglaciated lowland sites; 3) a rapid, short-lived local relative sea-level rise (RSL) of ca. 20-30 m, possibly related to Meltwater Pulse 1a or the collapse of a transient glacioisostatic forebulge, inundated the US portion of the lowlands up to ca. 130 m above modern local sea level; 4) directly following this peak in RSL at ca. 14,000 cal yr BP, ice readvanced across the border to nearly the same extent as the early Sumas. Distinct cross-cutting marine strandlines (erosional and depositional remains of emerged marine shorelines), subaerial moraines, and till plains imaged in lidar data indicate that following the maximum of the second Sumas advance, local RSL progressively lowered as the glacier fluctuated and gradually thinned. By ca. 13,000 cal yr BP, ice had retreated north of the border and local RSL had fallen to within ca. 4 m of modern.

Apparent aeolian dust recorded in sediments in Squalicum Lake potentially record the timing of a third and final Sumas readvance between 13,000-11,150 cal yr BP, which left a moraine a few kilometers south of the International Boundary near the town of Sumas, Washington, and another in Chilliwack River valley, British Columbia, before final collapse of the ice sheet. Together, our results indicate that the concept of a distinct Everson interstade and Sumas stade should be abandoned in favor of a more nuanced “Sumas episode” that encompasses the sequence of events recorded in the Fraser Lowland.