COSMOGENIC 3He DATING OF OUTBURST FLOODS ON THE SNAKE RIVER PLAIN, IDAHO

The central Snake River Plain contains a rich array of mega-flood features such as dry cataracts, large boulder lag deposits, and amphitheater-headed canyons. Previous studies on the geomorphology of the Snake River Plain of Idaho have proposed several possible formation mechanisms for the amphitheater-headed canyons including old river systems, spring sapping, and mega-flood events. One traditionally cited hypothesis is that these features were carved by the Bonneville outburst flood About 17.5 ka. However, recent studies using cosmogenic ³He dating suggests that earlier mega-floods may have played an important role in carving the canyons. The purpose of this study is to use cosmogenic ³He dating of mega-flood deposited boulders to determine the exact timing of past flood events in an effort to better understand the formation of these canyons.

Olivine bearing basalt samples were collected from mega-flood features for cosmogenic ³He dating from sites at the Eden pot holes, Box Canyon, Blue Lakes Canyon, Boulder Ridge, and Devils Corral. Cosmogenic exposure ages for ³He dates from olivine in the Blue Lakes samples support the hypothesis for mega-floods prior to the Bonneville. Six of the eight samples from that sight have a grand mean exposure age of 20.7 ka ± 1.1 ka. These ages are significantly higher than the mean ages of 17.8 ka ± 1.0 ka for three boulders deposited by the Bonneville flood near Pocatello, Idaho (Cerling and Craig, 1994), but match very well with a mean age of 19.7 ka ± 1.1 ka reported by Cerling et al. (1994) for two flood-deposited boulders in the Big Lost River area. Based on this agreement, we propose that floodwaters from the Big Lost River flood may have joined the Snake River near Twin Falls, contributing to the formation of Blue Lakes Canyon. Our own ages from flood-deposited boulders in the Big Lost River area are scattered, however, three of the eight samples have ages between 24 and 25.4 ka. Because these boulders are sourced from a relatively low cliff wall (~4-6 m), the possibility of inheritance is likely, and may explain the slightly older ages. It is anticipated that there will be a range of dates obtained from the remaining canyons that may also support a complex history of pre-Bonneville flooding.