A GEOPHYSICAL STUDY OF AN ANOMALOUSLY FLAT LANDFORM ALONG THE EASTERN FLANK OF THE WIND RIVER MOUNTAINS

Despite its close proximity to some of the most studied geology in Wyoming, the age and structure of the surficial deposits on Table Mountain, southwest of Lander, remain unclear. Previous mapping gives three different ages, ranging from Eocene to Pleistocene, and Be-10 ages from boulders on the surface, near the southwest edge, reveal ages ranging from 783 to 135 ka (Dahms, 2004). Table Mountain, at an of elevation 7000 ft, stands out from the surrounding landscape. 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 composition of the boulders points to a Bears Ears pluton origin, more than 20 miles to the Southwest, and faceted sides imply glacial activity. These previous studies suggest that early Pleistocene glacial events in the Wind River Mountains extended into the basin, beyond the extent of glacial canyons and known glacial maximums, prior to incision of river canyons into the older strata along the flanks of the range.

This study focuses on the stratigraphy of the surface deposits and its contact with the underlying bedrock. Three resistivity surveys were conducted on 275 meter lines using a 5 meter interval with the AGI SuperSting RBIP provided by the Wy-CEHG. Line length and interval were chosen to obtain as deep a survey as possible. Survey sites were chosen based on their flat topography, and proximity to the middle of the mountain surface, where deep erosion is limited. Two surveys were run perpendicular near the center of the mountain surface, and another survey line ran near the Eastern flank of the mountain surface.

Results indicate layers of higher resistivity about thirty meters thick above a layer with lower resistivity values. East-West oriented survey lines reveal a fairly uniform, smooth contact between the layers. Perpendicular survey lines running North and South show a slight incline from South to North, about 5 m rise over 100 m.

The relatively smooth contact between unconsolidated surface deposits and the less resistive bedrock is consistent with glacial action. The relative uniformity of the top layer suggests only one event of deposition of this material. Future use of complementary geophysical methods should show the finer scale structure of the top layer.