Late Quaternary History of the Uinta Mountains, northeastern Utah

Jeffrey S. Munroe, Ph.D. University of Wisconsin-Madison, Dept. of Geology & Geophysics, 2001

Active (?) rock glacier near the head of Painter Basin, 2001

Abstract

Nineteen discrete valley glaciers covering approximately 940 km² were present in the northern Uinta Mountains during the peak of the Smiths Fork Glaciation (marine oxygen isotope Stage 2). Basal organic sediment retrieved from Hacking Lake indicates that deglaciation from the Smiths Fork maximum was at least locally complete by 15 – 14 ka B.P. Average reconstructed ablation-zone basal driving stresses under the Smiths Fork-age glaciers range from 14 to 72 kPa. The lower values indicate that meltwater must have been present at the glacier beds, enhancing basal sliding and regelation. Annual ice fluxes through the equilibrium lines of three reconstructed glaciers were estimated, assuming that basal sliding was at a minimum where basal shear stress reached maximum values. Ablation and accumulation gradients calculated from the annual ice fluxes range from 1.4 to 2.8 mm/m, indicating that glaciers in the northern Uintas at the last glacial maximum were sustained more by low summer temperatures than extreme winter snow accumulation. This conclusion is supported by comparison of modern conditions at the northern Uinta reconstructed equilibrium line altitudes (ELAs) with modern conditions at worldwide glacier equilibrium lines, which suggests that mean summer temperatures during the Smiths Fork Glaciation were approximately 5.5ºC lower than modern (3.2 to 5.7ºC). Mean winter precipitation at the ELAs ranged from 943 to 3038 mm, compared with the modern values of 354 to 590 mm. Primary moisture transport was from west to east along the location of the modern winter boundary and snowfall at the western end of the range was enhanced by moisture derived from Pluvial Lake Bonneville. Glaciers at the eastern end of the range received additional snowfall at the onset of the seasonal monsoon because mean temperatures would have been below 0ºC in May and June.

The elevation of the modern equilibrium line in the northern Uintas averages 3970 m asl, approximately 100 m below the highest peaks in the center of the range. Yet glaciers are absent in the range because no cirque floors intersect the ELA. Instead, the land area (~6 km²) above the ELA is distributed in roughly 20 steeply sloping uplands that offer no suitable locations for snow accumulation. Previous estimates of 1000 to 1475 m of equilibrium line depression during the last glacial maximum are too large; last glacial maximum ELA depression in the northern Uintas was closer to 900 m.

The cirque floor moraines, which are present in 12 of 19 north slope valleys, were deposited shortly before 10 ka B.P, based on radiocarbon dates from the upper Henrys Fork, and above Allsop Lake. These moraines may represent the final pulse of a latest Pleistocene glacial episode that began during the Younger Dryas and persisted until shortly before 10 ka B.P.

Pollen spectra from the upper Henrys Fork suggest that an open Picea parkland was established at timberline by 9.5 ka B.P., accompanied by mean annual and July temperatures ~1.0ºC greater than modern. Wet meadows dominated by Cyperaceae and Salix, analogous to those found along the modern drainages, were also present in the early Holocene. Increasing warmth into the middle Holocene allowed Pinus to grow up into the parkland by 7.5 ka B.P. A period of maximum warmth was reached shortly after 5.5 ka B.P., and conditions were near modern by 3.8 ka B.P.

High-altitude moraines at Deadhorse Lake indicate an active cirque glacier system that postdates the cirque floor moraines. Numerical age estimates obtained through application of lichen growth-rate curves indicate that these moraines predate the Little Ice Age and were formed during a localized pulse of glaciation in the middle Neoglacial, ~1650 to 1300 B.P.