Late Quaternary Glaciation of the Mckenzie RIVER Basin, Cascade RANGE, Oregon; With Paleoclimatic Implications

The spectacular Three Sisters volcanic platform in the Oregon Cascade Range is comprised of Pleistocene strato-volcanoes, youthful volcanic vents and flows, and a plethora of glacial features. Three watersheds drain this area, the Deschutes River to the northeast, and the Middle Fork of the Willamette and McKenzie Rivers to the west. In the McKenzie River basin, tributaries form textbook glacial troughs that coalesce to form the trunk stream; glacial drift is extensive and well preserved. Morphostratigraphy, clast weathering, and soil development indicate at least two glaciations in the McKenzie, its watershed having been occupied by the western portion of an ice cap that originated on the Three Sisters volcanic platform. We interpret the younger glaciation to represent the late Pleistocene Last Glacial Maximum (LGM). The surface of the LGM ice cap was reconstructed by dividing the glacier system into lobes draining tributaries of the McKenzie, establishing ice surface contours for each lobe by assuming convergent and divergent flow in accumulation and ablation areas, and modifying ice surface contours until basal shear stresses computed from ice thickness and surface slope along each lobe's axial profile were within the accepted range. Using an accumulation-area ratio of 0.70±0.05 for the ice cap, we determined the weighted-mean equilibrium-line altitude (ELA) during LGM glaciation to be about 4627ft (1410m). Regressions of modern mean June-August temperature and April 1st snowpack were extrapolated to the LGM's ELA to obtain proxy mean summer temperature and winter accumulation values of about 12.2 ^oC and 772.2mm H₂O. We compared these values with those occurring at modern mid-latitude glaciers, and assuming no change in precipitation from the present, a 10.5 ^oC mean summer temperature depression was determined to have been necessary to sustain the ice cap in the McKenzie River basin during the late Pleistocene LGM.