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. In particular, previous studies on the geomorphology of the Snake River Plain of Idaho propose 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. This suggests that cosmogenic dating tools can provide greater understanding of how the Snake River Plain was formed.

This study uses GIS analysis to determine if any of the canyons could plausibly have been formed by ephemeral stream flow prior to emplacement of late Pleistocene basalt flows. Additionally 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, Boulder Ridge, and Devils Corral. These cosmogenic nuclide analyses will allow a comparison of the timing of canyon formation with the known age of the Bonneville flood and may also provide direct age constraints on previous flood events. In particular, we consider previous flood events from the Big Lost Rigver and the Yellowstone Palateau. It is anticipated that there will be a range of dates from these canyons and that the hypothesis for mega-floods prior to the Bonneville will be supported. If ³He dates from olivine in the samples indicate exposure dates prior to the Bonneville, this contribute to the scientific community's the North American paleoclimate and encourage the employment of cosmogenic dating techniques other areas of geological interest.