LATE QUATERNARY GLACIATION OF THE UPPER DESCHUTES RIVER BASIN I; METOLIUS RIVER WATERSHED, CASCADE RANGE, OREGON

The Oregon High Cascades form an elongated north-south trending platform comprised of Pleistocene stratovolcanoes and more youthful volcanic vents and lava flows. The range amply displays classical features of glaciation representing multiple episodes of activity; its proximity to the Pacific Ocean, orientation with respect to westerly air flow, and elevation provided a nexus for glacial conditions. Scott (1977) defined the late Quaternary glacial chronology of the central Oregon Cascade Range; interpreting the glacial deposits of the upper Metolius River drainage within the Deschutes River basin east of the Cascade Crest to represent three glaciations: the Abbott Butte, Jack Creek, and Cabot Creek, from oldest to youngest. We used field data from morphostratigraphy, clast weathering, and soil development and digital mapping techniques to reassess Scott’s glacial chronology, refine the maximum extent of the Jack Creek and Cabot Creek glaciations in the Metolius watershed, and extend the chronology to adjacent drainages. We interpret the Jack Creek and Cabot Creek glaciations to coincide with late middle and late Pleistocene oxygen-isotope stage 6 and stage 2 (the LGM), respectively. Features of Jack Creek and Cabot Creek glaciation are so well preserved in the Metolius watershed as to allow reconstructions of their former ice surfaces and equilibrium-line altitudes (ELAs). We partitioned former ice masses into lobes draining major tributaries of the Metolius. Ice surface contours were established for each lobe from ice-marginal features of erosion and deposition and by assuming convergent and divergent flow in accumulation and ablation areas, and modified until basal shear stress computations from ice thickness and surface slope fell within the accepted range of 50 to 150 kPa. Using an accumulation-area ratio of 0.70+/-0.05 for valley glaciers emanating from alpine ice caps, we determined regional weighted-mean paleo-ELAs to be about 4608 ft (1404 m) and 4722 ft (1439 m) for the Jack Creek and Cabot Creek glaciations, respectively. Former ELAs of both glacial periods were locally lowest in the southern portion of the watershed where the modern Cascade Crest is lowest between Santiam and McKenzie Pass, suggesting paleo-precipitation patterns and rainshadow affects were similar to the present-day.