LATEST PLEISTOCENE (?) AND HOLOCENE CIRQUE GLACIATION IN THE ENCHANTMENT LAKES BASIN, NORTH CASCADES, WA

The Enchantment Lakes Basin in the Alpine Lakes Wilderness preserves two sets of moraines that record distinct post-Wisconsin maximum advances of cirque glaciers in the eastern North Cascades. Lake cores collected from five lakes adjacent to the moraines preserve detailed records of the timing and extent of the advances. The absence of a 450-year-old Mount St. Helens tephra (Wn) on the inner (Brynhild) moraines indicates that they are Little Ice Age equivalent (Waitt et al., 1982). The presence of the ~6800 ¹⁴C yr B.P. Mt. Mazama tephra on the outer (Brisingamen) moraines indicates that they are either early Holocene, as argued by Waitt et al. (1982), or latest Pleistocene. The distinction is important for assessing local effects, if any, of cold late-glacial/early Holocene climatic excursions indicated in ice cores (e.g., Alley et al., 1997).

Magnetic susceptibility and visual stratigraphy of multiple cores collected from lakes outside the older moraines preserve two tephra layers thought to be the Wn and Mazama tephras. In addition, two continuous lake cores that bottomed on rock contain three distinct low-organic clastic horizons that we interpret to be outwash. The uppermost outwash relates to the Brynhild advance, whereas the basal outwash relates to deglaciation of the basin after the Rat Creek advance (14-13,000 ³⁶Cl years B.P.; Swanson and Porter, 1999). The intermediate clastic horizon, which occurs below the Mazama tephra and above the basal outwash, appears to record the Brisingamen outwash. Preliminary sedimentation rates, estimated from post-Mazama sediments, suggest that the Brisingamen outwash predates the Mazama tephra by ~4000-5000 years.

Further detailed analyses of the sediment cores (magnetic susceptibility, organic content analysis (LOI), grain-size analysis, tephra analysis, paleomagnetic secular variation curve construction, and AMS radiocarbon dating of macrofossils) and moraines will enable us to date precisely each glacial event, and to establish the climatic significance of each. In addition, the paleomagnetic record provides a means to correlate the events regionally to other similar sequences.