COMPARISON OF UNCONSOLIDATED DEPOSITS ON AREAS OF ANOMALOUS ALTITUDE AND FLATNESS IN AND AROUND THE SOUTHERN WIND RIVER BASIN, FREMONT CO., WYOMING

Despite its close proximity to some of the most studied geology in Wyoming, the age of the surficial deposits on Table Mountain, south of Lander, remain unclear. Several sources (Keefer, 1965; Whitcomb and Lowry, 1968; Love et al., 1979; Love and Christiansen, 1985; and Mears, et al.,1986) give ages ranging from Eocene to Pleistocene. Recent Be-10 ages from boulders on the surface, near the southwest edge, range from 783 to 135 ka (Dahms, 2004). As implied by its name, Table Mountain, elevation 7000 ft, is quite flat. The surface exists primarily as loose gravel (grus) embedded with large boulders, the majority of which are granitic, all superimposed upon uplifted, tilted Paleozoic and Mesozoic layers.

The lithology of the majority of the boulders on most of the surface is coarse-grained leucocratic granite with considerable pegmatite and aplite. This is consistent with the Bears Ears pluton ~10 miles to the southwest. Other boulders consist of medium-grained granodiorite (Louis Lake batholith), micritic limestone (Madison Formation), banded sandstone (Flathead Formation) and several other rock types representative of the surrounding geology. We believe that the granitic boulders were deposited by glacial action because of the following: 1) the enormous size (up to 4 m³) are not likely to have been transported by fluvial or debris flows at this distance from the mountain front and 2) the boulders have the typical faceted surface indicative of glacial movement.

There are several other anomalous flat (although not as extensive) surfaces on the southern edge of the Wind River basin under investigation and at least one of these (Red Butte, ~5 miles northwest of Table Mountain) is also topped with enormous granitic boulders. The implication of this boulder distribution is that early Pleistocene pre-canyon glaciation events in the Wind River Mountains extended into the basin. This returns us to the early ideas of pre-canyon glacial events presented by Blackwelder, 1915, and Love, 1979. By evaluating landforms of similar altitude and topography around the Wind River Basin and flanking the eastern edge of the Wind River Mountains, we hope to determine an accurate age of the surface deposits and to gain a deeper understanding of how exactly these anomalous surfaces came to look the way they do today.