W. Locke - Ice Cap Modeling - Wind River Range

Modeling Glaciation of Wyoming's Wind River Range
Glacier Modeling Because glacier ice is a relatively well characterized material, it behaves in a generally predictable way. As recognized by Nye (1952, Journal of Glaciology, 2, 82-93 and 103-107) and discussed by others in the intervening years, this predictable rheology can be used to reconstruct former glaciers. Alternatively, if former glacier extents can be reconstructed on the basis of field evidence, flow models can be used to interpret variability in the factors which control ice flow, especially effective basal shear strength. The discussions below involve the application of iterative models to former ice cover on various mountain masses of the western United States and their interpretation. Wind River Range In 1996, undergraduate students in my Geomorphology class used spreadsheet modeling techniques to reconstruct the major Pleistocene glaciers draining the 200-km long Wind River Range of NW central Wyoming (Figure 1: contour interval 1000'/305 m). Significant towns in the area include Pinedale (west) and Dubois and Lander (east). This range is fringed by massive terminal moraines (dark green) and occupies a storied position in Rocky Mountain Quaternary stratigraphy as the home of the type localities of both the Bull Lake (east) and Pinedale (west) glaciations. The massif is one of the largest in the intermountain West, rising well above 10,000' (3050 m; bold contour) across most of its length, and above 12,000' (3650 m) along the Continental Divide in the northern 50 km of the range. Yet, to my knowledge, no published work examines the vertical extent and flow patterns of the lastglacial ice cover.
Like Yellowstone, the Wind River Range demonstrates major topographic, geologic, and glacial asymmetry (Figure 2). The east side of the range is bounded by basin fill (TQ) of the Wind River Basin over the complete Mesozoic and Paleozoic section (PzMz). This east-dipping sedimentary sequence rims the uplift with classic flatirons of Paleozoic sandstones and especially carbonates, leading up to mesas cut onto the underlying Archean basement rocks at over 4000 m (Downs Mountain, Horse Ridge), separated by deep glacial canyons. In contrast, the west side of the range is defined by near-vertical faulting juxtaposing basin-fill sediments with Archean basement. Topographically the range steps from the Green River Basin up to a west-tilting, lake-dotted plateau at about 3000 m, then steps again at cirques and breached divides to the continental divide. The modeling does not address the origin of the plateau surface, which is heavily ice-scoured and shows prominent range-parallel structural weaknesses. Although west-flowing major streams drain from the range through spectacular canyons, they originate in scoured, structurally-controlled, "deranged" drainage on the plateau. The other striking topographic characteristic of the range is the "hook" of the upper Green River as it flows north from its sources within the northern Wind River Range, then west and SW. As the largest discrete drainage basin within the range, the upper Green River was a major outlet for Pleistocene ice draining from the high northern range. Using spreadsheet modeling techniques applied to each of the major drainages around the range, we defined the approximate vertical extent of the Pleistocene Wind River ice cap. Note that, because Bull Lake ice extents were only a few percent greater than Pinedale ice extents and because ice surface slope is more usually constrained by valley slope than by horizontal extent, the reconstruction here (Figure 3) is approximately correct for all glaciations affecting similar underlying topography.
The west-side glaciers are well-modeled to moraines, nunataks, and breached divides using an effective basal shear stress throughout of about 100 kPa (1 bar). Nearly all of the west-side glaciers were outlets from a continuous ice cap which ran almost the entire length of the range. At the north, these outlets were topographically confined, with intervening nunataks. In the south, the entire west-sloping plateau was covered by as little as 100 m of ice, with only rare nunataks on the plateau itself. Many outlet lobes, some quite small, drooled off the faulted plateau edge to pool as piedmont lobes on the basin floor below. The high peaks of the continental divide also stood as nunataks above the icecap, which breached the divide (apparently flowing eastward) in numerous places. Reconstructions of the east-side glaciers require effective basal shear stresses ranging from about 20 kPa (0.2 bar) in the extending, till- and Cretaceous-floored termini to as much as 800 kPa (8 bar) in the compressive, well-drained (karstic?) bedrock canyons (Figure 4), in order to fit to morainal ramparts in strike valleys at up to 3000 m in elevation. Despite the high apparent effective basal shear stresses most of the east-side glaciers remained well below the high plateau surfaces as discrete valley glaciers. A few of the glaciers merged with others at divide breaches either north-south (along the range) or east-west (across it). Most of the N-S divides align with range-parallel valleys and lake basins, strongly suggestive of structural controls. There was a striking asymmetry of unglaciated terrain, with most flat-lying areas above 10,000' (3000 m) on the west flank of the range covered by ice, whereas mesas at that elevation on the east side supported either no ice or thin, protective, probably cold-based ice. Summits under 11,000' (3350 m) along the eastern margin of the range were unglaciated, unless affected by ice draining from even higher areas to the west. This asymmetry is supportive of a dominant west-to-east air and moisture flow during peak Pleistocene glaciation as it is today. The significantly lower equilibrium line altitudes at the northern end of the range are suggestive of major moisture influx from the NW (the Snake River Plain) rather than W or SW.

Glacier Modeling

Wind River Range