A New Interpretation for Latest Pleistocene Ice Dynamics of the Puget Lobe, Northwest Washington

Geomorphic constraints provided by new LiDAR data indicate a complex interplay between eustatic sea level rise, isostatic rebound, and glacier fluctuations at the end of the Fraser Glaciation in northern Puget Sound. The stratigraphy of late-glacial drift and non-glacial sediments in the type deposits of the Everson interval near Bellingham, WA, have long been interpreted as recording a sequence of deglaciation, marine transgression to ~115 m above modern sea level (asl), regression to near-modern sea level, a second transgression to ~200 m asl, and final regression, followed by glacier readvance during the Sumas stade. The short duration of this sequence (~1000 ¹⁴C yr) and the lack of a plausible mechanism for such rapid, large-magnitude changes in local sea level make this sequence difficult to explain.

Well-defined geomorphic relationships expressed in the LiDAR data offer an attractive alternative interpretation. A distinct pattern of cross-cutting moraines, till-plains, and marine strandlines indicate that 1) there was only a single transgression in the region to ~100 m asl following initial retreat of the Puget Lobe (indicated by shells in the basal Bellingham drift), 2) this transgression was driven by rapid eustatic sea level rise that temporarily outpaced the local isostatic rebound, 3) the transgression was closely followed by (and possibly caused) a late-glacial readvance that culminated in Bellingham Bay, and 4) isostatic rebound then surpassed eustatic rise, coincident with final glacier retreat. Deposits previously mapped as Sumas terminal moraines instead appear to be recessional moraines from this more-extensive “Bellingham Bay” advance. Also, geomorphic relationships indicate that the Bellingham drift transitions from glacial marine at its base to till at its top, coincident with the regression. The close timing between transgression and advance, and regression and retreat, suggests a dynamic cause (i.e., reduced basal shear stress from marine inundation) for the “Bellingham Bay” advance.