Abstract:
A sequence of glaciation based on relative dating parameters was established
in each of nine mountain ranges located along a northwest to southeast transect
through the northern Great Basin. Each sequence consists of two or three drift units.
Degree of weathering suggests that the younger drift unit in a two-fold sequence and
the intermediate drift unit in a three-fold sequence represents deposition during the
late Pleistocene. The late Pleistocene equilibrium-line altitude (ELA) in each range
was determined by reconstructing the maximum ice extent associated with these drift
units and using the accumulation area ratio technique. Paleo-ELAs increase from
about 2100 m in northeastern Oregon to approximately 3200 m in central Utah.
Paleoclimatic conditions along the study transect were estimated by comparing
modern climatic conditions at the reconstructed late Pleistocene ELAs with climatic
conditions occurring at the ELAs of modem mid-latitude glaciers. Assuming no
change in winter accumulation or in the seasonal distribution of precipitation from
the present, a mean summer temperature depression ranging from about 9.0 °C at the
northern end of the transect to about 4.0 °C at the southern end would have been
necessary to sustain glaciers in these ranges during the late Pleistocene. To simulate
possible changes in late Pleistocene precipitation patterns, a change in winter
accumulation between 0.5 and 2.0 times the modem value resulted in a respective
increase or decrease in temperature depression by approximately 2.0 °C. These
results suggest that increases in precipitation during the late Pleistocene alone could
not have sustained glaciers in the mountain ranges of the study transect, except
possibly at its southern end, in the central Great Basin. An increase in precipitation
in this area during the late Pleistocene agrees with the interpretation of other
paleoclimatic records and paleoclimatic model simulations for the Great Basin.
