RE-EVALUATION OF THE SIGNIFICANCE OF THE GLACIAL STRATIGRAPHY OF SW BC

The Fraser Lowland and surrounding watersheds in southwestern British Columbia (BC) have been the focus of seven decades of Quaternary stratigraphic study that have resulted in a refinement of late Pleistocene events at a much higher resolution than in other areas of BC. Specifically, the MIS 2 Fraser Glaciation has been divided into the Coquitlam (21.5 – 18.5 ka) and Vashon (17.5 ka) advances with at least one late glacial readvance, the Sumas (ca. 12 ka). The Coquitlam and Vashon stades were thought to record glacier invasion of the Fraser Lowland from the Coast Mountains to the north. Between deposits of the Coquitlam and Vashon stades are organic-rich sediments of the Port Moody interstade (18.5-17.5 ka). Paleoenvironmental studies indicate it represents a subalpine forest. The Coquitlam Stade has been correlated to the Evans Creek Stade, an alpine glacier advance in the North Cascades Range in Washington State.

Recent stratigraphic work in Coast Mountain watersheds north of the Fraser Lowland, drill core data in the Metro Vancouver area, and lidar imagery have led us to question the climatic significance of these stades and interstades. Specifically, work in Chehalis Valley in the eastern Fraser Lowland indicates that it was not directly affected by Coquitlam ice; at ca. 20 ka, a glacier in the eastern Fraser Lowland blocked the mouth of Chehalis Valley, impounding a lake. Importantly, this blockage occurred at least 1.5 ka later than the Coquitlam advance in western Fraser lowland. The organic-rich sediments associated with the Chehalis flooding event are identical to those of the later Port Moody Interstade, indicating that glaciers were advancing into an established subalpine forest in both areas. Drill holes in the western Fraser Lowland confirm the presence of the Coquitlam and Vashon stades, separated by Port Moody, but also indicate the presence of marine sediments, implying significant glacio-isostatic depression. Another study concludes that at least 85 m of glacio-isostatic depression of the westernmost Fraser Lowland had been achieved by 26 ka. We speculate that Coquitlam glacier retreat from the western Fraser Lowland was caused by glacio-isostatic depression in tandem with eustatic sea rise at ca. 18.5 ka. In this scenario, the Port Moody interstade records establishment of subalpine fir forest in recently deglaciated areas, driven more by glacio-isostatic adjustments of the crust than climate.