FORWARD AND BACK, UP AND DOWN: RAPID GLACIAL AND SEA-LEVEL RESPONSES DURING THE FINAL ADVANCE OF THE CORDILLERAN ICE SHEET IN NORTHWESTERN WASHINGTON

Following its initial late-Pleistocene retreat into Canada, the Cordilleran Ice Sheet readvanced a final time into NW Washington during the Sumas stade, terminating in Bellingham Bay. Cross-cutting moraines and marine strandlines imaged in Lidar imagery indicate that, following its maximum extent ~14,500 cal yr B.P., the glacier experienced a complex sequence of glacier retreat and readvance coeval with a rapid rise and then fall of relative sea level (RSL). The timing of these remarkable events remains poorly understood.

A new core from Squalicum Lake provides crucial new constraints on these events. The lake (~5m deep) is sandwiched between two sets of Sumas moraines: an older distal moraine related to the maximum advance (~14,500 cal yr B.P.), and a set of younger proximal moraines that form the northern dam for the lake. Ages of the latter moraines are crucial because strandlines interleaved with them record RSL lowering by >60 m during their emplacement.

To constrain their timing, we collected a 5.4 m long sediment core from the center of Squalicum Lake, bottoming in inorganic blue-grey glacial gravel. The upper 4 m of the core is dominated by massive brown high-organic (30-40%) gyttja, except for a thin bed of Mazama ash (~7630 cal yr B.P.) at 338 cm depth. Below ~410 cm, sediments become progressively less organic, shifting to low-organic (<1.5%) grey silt/clay by 442 cm. A sharp contact with underlying high organic (26%) sediments occurs at 444 cm, below which organics again transition to low-organic (<1%) grey silt/clay by 466 cm depth. The grey inorganic sediments persist to the bottom of the core.

The basal sediments record emplacement of the moraines that dam the north end of the lake, including at least one major retreat and readvance represented by the interbedded organic sediments; AMS ¹⁴C analyses (in progress) will provide the first numerical constraints on the moraines and the dramatic isostatic RSL lowering that accompanied their emplacement.