MORE THAN YOU WANTED TO KNOW ABOUT DETRITAL ZIRCONS OF HELLS CANYON, IDAHO AND OREGON: REVISITING THE BUMPY BARCODE WITH LARGE-N U-PB GEOCHRONOLOGICAL METHODS

Detrital zircon (DZ) U-Pb age groups within the age spectra from modern river sands of the Snake River Basin (i.e., the “bumpy barcode” of Link et al., 2005, Sed. Geol) archive the geological history of a major portion of the Intermountain West, using small sample sizes (i.e., n < 100). Recent advances in laser ablation ICP-MS methodology affords rapid sample acquisition and cost-effective production of large-n (i.e., n > 200) DZ data sets and detection of subtle changes due to tributary inputs or historical floods.. We collected and analyzed 300 grains per sample from twelve samples of modern river sands and Pleistocene terraces of the mainstem Snake River and tributary streams in the Hells Canyon area, western Idaho. Mainstem samples were separated by ~150 miles and three major hydroelectric projects, while the tributary samples included three large rivers and three small tributaries. We applied Monte Carlo based mixture modeling of DZ age groupings to assess nuanced changes in downstream sediment source contribution. Results show (1) that mainstream zircon assemblages, both recycled from sedimentary rocks and primary from a magmatic source, are complicated but predictable, and represent the complex geology of the Snake River watershed. (2) These mainstem assemblages are consistent throughout the Hells Canyon area despite the distance and presence of hydroelectric dams. (3) The small changes in zircon distribution is well explained by influence of local tributaries with simple zircon age groupings. (4) There is a large change in the DZ ages of mainstem samples below the confluence with the Salmon River, a major undammed sediment source. (5) Simple forward models based on drainage area and basin geology do not predict the detrital zircon assemblages for these mainstem samples very well; however, when additional information on sediment production, routing, or zircon fertility are incorporated, these models improve. (6) There is a detectable difference between actively transporting mainstem sand, and sand from historic flood deposits at the same location. Future work will expand the use of large-n zircon samples in the surrounding areas to capture additional tributary basins, and further explore the discrepancies between actively and historically transported sand.