LATE QUATERNARY GLACIATION OF THE UPPER DESCHUTES RIVER BASIN II; WHYCHUS CREEK WATERSHED, CASCADE RANGE, OREGON

The spectacular Three Sisters volcanic platform in the central Oregon Cascade Range is a dynamic landscape comprised of four major Pleistocene strato-volcanoes, a plethora of glacial features, and multiple youthful volcanic vents and flows. The “L” shaped highland is anchored at its long and short ends by the older North Sister and Broken Top Volcanoes, primarily constructed during the late middle Pleistocene, and likely extinct by about 100,000 years ago. Middle and South Sister Volcanoes lie near the bend in the “L” and are considerably younger; concurrent late Pleistocene eruptions beginning about 50,000 years ago. Whychus Creek, a major tributary of the Deschutes River east of the Cascade Crest, drains from this highland area and exhibits an extensive, well-preserved cover of glacial drift. Morphostratigraphy, clast weathering, and soil development indicate only one period of late Pleistocene glacial activity that we correlate with the Cabot Creek glaciation of Scott (1977) and oxygen-isotope stage 2 (the LGM). The surface of this former alpine glacial system was reconstructed by dividing it into several lobes, 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 were within the accepted range of 50 to 150 kPa. Using an accumulation-area ratio of 0.70+/-0.05 for the Whychus Creek ice cap, we determined the weighted-mean equilibrium-line altitude (ELA) during Cabot Creek glaciation to be about 5594 ft (1705 m), much higher than its counterpart in the upper Metolius River watershed just to north. Whychus Creek lies solely in the rainshadow of the Three Sisters volcanic platform and a higher paleo-ELA is to be expected assuming former precipitation patterns similar to present-day. The Metolius drainage also contains deposits of Scott’s older Jack Creek and Abbott Butte glaciations and presumably these units lie buried beneath a blanket of more extensive Cabot Creek material in the Whychus Creek area. A more extensive, younger Cabot Creek glaciation can likely be attributed to the formation of a considerably more extensive volcanic highland source-area during the interglacial period between Jack Creek and Cabot Creek glaciations.