DETRITAL ZIRCON GEOCHRONOLOGY OF BONNEVILLE FLOOD DEPOSITS ALONG THE SNAKE RIVER, IDAHO: IMPLICATIONS FOR THE SEDIMENT DISPERSAL PATTERNS OF MEGAFLOODS

The Bonneville Flood occurred ca 14.5 ka lowering Pleistocene pluvial Lake Bonneville by approximately 110 m and leaving extensive flood features along Marsh Creek, the Portneuf River, and the Snake River in Idaho. Lake overflow and subsequent flooding resulted from down-cutting through alluvial fan sediment that had formed the lowest threshold of the Bonneville Basin in southeastern Idaho near Red Rock Pass. Extensive megaflood erosional and depositional features are well-preserved throughout the Idaho portion of the flood path, and in particular, slackwater sediments deposited in ponded reaches offer a rare opportunity to characterize the detrital zircon geochronology of the Bonneville Flood. The Bonneville megaflood contrasts with multiple glacial outburst floods from Lake Missoula, Montana between ca. 17 and 14 ka that generated the Channeled Scabland, WA. Detrital zircon geochronology has resolved some uncertainties about the provenance and chronostratigraphy of Missoula flood sediment, but the multiple, complex nature of these floods limits the use of detrital zircon as a tracer in characterizing the sedimentological evolution of individual megafloods. The singular nature of the Bonneville Flood provides an ideal setting to test the use of detrital zircon as a sedimentological tracer for determining sediment transport, dispersal, and deposition for a single megaflood over the reach and regional scale.

The previously well-documented detrital zircon age populations of the Snake River drainage in south-central and southeastern Idaho provide excellent constraints for using detrital zircon to trace the degree of sediment mixing throughout the course of the Bonneville Flood. Comparison of detrital zircon spectra along this flood path has the potential to reveal whether the flood derived the bulk of its sediment locally or transported a regionally mixed population of sediment with only minor local input. Preliminary detrital zircon data from Missoula flood deposits at Lewiston, Idaho has revealed a strong Bonneville Flood/Snake River influence, indicating that this signature can be traced into distal reaches of the Bonneville Flood path. Detrital zircon analyses are currently underway to further characterize the detrital zircon spectra of Bonneville Flood deposits along the Snake River in Idaho.