SOIL PROFILE RELATIVE-AGE DATING AND WEST-TO-EAST DECREASE IN DUST INFLUX ACROSS THE CENTRAL WASATCH MOUNTAINS

Following methods of relative-age dating developed by Peter Birkeland and colleagues, in 1979-1983 we described 106 soil profiles in the eastern valleys of the central Wasatch Mountains and the western Unita Basin (~7000 km²) to estimate landform ages as part of seismic hazard studies for U.S. Bureau of Reclamation dams in the region. The variability in source rock lithologies, parent material texture, site surface stability, rainfall, and distance from major dust sources for our soils is greater than for many studied soil chronosequences in the western U.S. Fourteen profile development indices for soils on landforms of uncertain age were compared with indices for 16 soils on landforms whose ages we inferred from regional relative-age dating studies or nearby numerical ages. The indices most successful in placing landforms into five relative-age groups were rubification, non-arid total profile index, and g/m²/a of secondary clay. We estimate an average late Pleistocene rate of clay influx for the eastern Wasatch Mountains of 0.8 g/m²/a. But because clay accumulation is so variable in latest Pleistocene soils, using the average rate is problematic for estimating their ages. Results from 8 dust traps placed on plywood sheets on the ground for 1-2 years (1981-1983) show at least a 5-fold, west-to-east decrease (r² = 0.94) in the influx of silt and clay between Morgan Valley, near the crest of the Wasatch Range, and moraines and high fluvial surfaces in the northwestern Unita Basin. The Morgan Valley trap, 120 m above the western Bonneville shoreline in the valley, accumulated 57 g/m²/a of silt+clay. A trap along the Lake Fork River in the northwestern Unita Basin on Pinedale moraines, recently dated at 19 ka with ¹⁰Be surface exposure ages, yielded 4 g/ m²/a, similar to values obtained in recent studies of dust influx in the Unita Mountains and Wind River Mountains. About 16 km farther south in the central basin a trap on a fluvial surface incised 200 m showed 10 g/ m²/a of silt+clay. Discussion of the soil profile and other relative-age data can be found in unpublished reports and related publications, with additional unpublished profile and dust trap data at: https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/BAAYFC.