RECONSTRUCTION OF THE LATE PLEISTOCENE GLACIER SYSTEM ON STEENS MOUNTAIN, SOUTHEAST OREGON, WITH PALEOCLIMATIC IMPLICATIONS

Steens Mountain forms a spectacular, asymmetrically uplifted fault-block mountain range within the northernmost Basin and Range located in southeastern Oregon. The fault block is bounded on the east by a steep NE-SW trending escarpment, while the western slope dips much more gently and forms a high plateau dissected by several immense glacial troughs. The distribution and age of glacial drift on Steens Mountain suggests that an ice cap coalesced to flow down major valleys draining the plateau during more than one episode of glaciation.

Morphostratigraphic features, combined with the relative degree of surface and subsurface clast weathering and soil development, indicate a three-fold glacial sequence on Steens Mountain. The intermediate drift unit was interpreted to represent the last glacial maximum of the late Pleistocene. The surface topography of the ice cap associated with this glaciation was reconstructed by: 1) dividing the glacier system into independent lobes draining major valleys, 2) establishing ice-surface contours for each lobe by assuming convergent and divergent flow in accumulation and ablation areas, respectively, 3) using ice thickness and surface slope along the centerline of each lobe to calculate basal shear stresses along their respective lengths, and 4) modifying the initial ice-surface contours until computed shear stresses were near the accepted range of 50-150 kPa.

Using an accumulation-area ratio of 0.70±0.05 for the late Pleistocene ice cap, the mean equilibrium-line altitude (ELA) during this glaciation was determined to be 2284±39.5 m. Modern glacier mass balance is strongly correlated to the mean summer temperature and winter accumulation occurring at the ELA. Regressions of mean June-August temperature and April-1st snowpack were extrapolated to the reconstructed late Pleistocene ELA to obtain proxy mean summer temperature and winter accumulation values of 7.9±0.5 °C and 50.0±3.5 cm H₂O, respectively. These values were compared to the range of values occurring at modern mid-latitude glaciers. Assuming no change in precipitation from the present, an 8.0±1.3 °C mean summer temperature depression, associated with snowfall equivalent to 181±32 % of the modern value, was determined to have been necessary to sustain the ice cap on Steens Mountain during the late Pleistocene.