NEW CONSTRAINTS ON THE SUMAS STADE OF THE CORDILLERAN ICE SHEET IN NW WASHINGTON

The Sumas Stade of the Cordilleran Ice Sheet (CIS) during the Fraser Glaciation has a complex and controversial history south of the Canadian border in northwest Washington State. In this study, we attempt to document the nature and timing of a previously undocumented moraine near Lake Padden, south Whatcom County, that appears to represent the southernmost extent of the CIS during the Sumas Stade. The moraine, which we term the “Padden moraine,” is mappable both in LiDAR data and in the field, and extends from the western end of Lake Padden northeast to the southwest shore of Lake Whatcom, with a correlative moraine continuing from the north shore of Lake Whatcom and across Toad Mountain to Stewart Mountain. The moraine is typically small (2-4 m high, 5-15 m wide) and typically consists of sandy-silty matrix surrounding cobble- to boulder-sized erratics. Erratics are generally more abundant on the proximal (north) side of the moraine, as are subglacial flutes imaged by LiDAR.

The position of the moraine at the north edge of Lake Padden is fortuituous because the lake would have existed while the moraine was forming, and thus it would have captured outwash rock flour during that time. We extracted three cores from the eastern basin of the lake, the latter two of which reached laminated inorganic (glacial) sediments. LP13-02 (262 cm length) shows an unconformable shallow contact at about 20 cm between overlying organic gyttja and deeper inorganic glacial The inorganic sediments record 242 cm of rhythmites, which may correspond with annual varves.. The upper 5.5 meters of the third core, LP13-03, comprises organic-rich gyttja in sharp contact with underlying massive to laminated inorganic silts similar to those in LP13-02. We interpret this sharp transmission as recording the initial retreat of the CIS from the Padden moraine. AMS radiocarbon analysis of bulk organic sediments directly above this contact indicate that the retreat began shortly before 12,450 ± 35 ¹⁴C yr B.P. (14,165-14,990 cal yr B.P. 2-σ) A bulk sample collected immediately below Mazama ash in the core supports the accuracy of these bulk sediment analyses. Together, these cores indicate that the Sumas Stade may have occurred significantly earlier than previously recognized. Ongoing core analyses should help further constrain the nature of this event.