WEATHERING RINDS DEVELOPED ON CALCAREOUS SANDSTONE COBBLES IN MORAINES OF THE WESTERN PIONEER MOUNTAINS, IDAHO

Terminal moraine deposits on the west side of the Pioneer Mountains contain cobbles of gray calcareous fine sandstone (Wood River Formation) with conspicuous pale yellow (2.5Y 7/4-6/2) weathering rinds. Rinds on the upper surface of cobbles are 2 to 5 times thicker than those on the bottom surface. Rinds were measured on cobbles from the upper 0.3 m of soil pits at the crests of two left-lateral moraines. Moraines are near the terminus of the Trail Creek glacier at Wilson Creek, 12 miles NE of Ketchum. Elevation difference between the two moraine crests is 55 m. Both moraines project to a terminus of about 1935-m elevation, and are the lowest-elevation till in the Trail Creek canyon. The thickest 20% fraction of the lower-moraine rinds is 2 to 4 mm, whereas the thickest 20% fraction of the upper-moraine rinds is 8 to 12 mm. For reasons not understood, both have a similar mode of 2 to 4 mm, but are clearly distinguished by considering the thickest 20%. Soil pits dug in the upper moraine show at least Stage III+ soil carbonate development and Bk horizons from 0.4 m to deeper than 0.85 m below the surface. The lower moraine has a thin (6 cm) Bk horizon (Stage I+) 0.6 m deep. Rind and soil data indicate a significant age difference between these two terminal moraines. If the lower moraine is late Wisconsin (Pinedale) in age (16-23 ka) with 2 to 4 mm rinds, a log-log-plot extrapolation of 8 to12 mm rinds suggest an age older than 50 ka , and possibly correlative with Bull Lake moraines (95-130 ka) dated by Chadwick and others (1997, GSA Bull., 109, p. 1143).