The geochronometry of Idaho and the bumpy barcode revisited: Snake River detrital zircons in Hells Canyon and beyond

Abstract

Detrital zircons in modern stream sands and terraces of the Snake River drainage upstream of and within Hells Canyon, western Idaho, U.S.A, effectively reproduce the geochronometry of Idaho magmatic rocks, both from Laurentia in eastern Idaho, and from the accreted terranes west of the Salmon River Suture. We used large-n (300 grain) analysis of detrital zircon ages and Monte Carlo-based mixture modeling to estimate the percent contributions from the upper Snake River and several tributaries. The large number of grains analyzed allows more precise statistical modeling than previous studies, which used 60 or 100 grains.Immediately upstream of the Hells Canyon Complex of dams modeling suggests that the Snake River at Farewell Bend contains detrital zircon contributions of Payette River 50 % and Boise River 10 % (mainly from the Atlanta lobe, Idaho batholith). The upper Snake River has 30 % contribution and contains a complex detrital zircon signature including both primary zircons derived from magmatic sources and recycled zircons derived from sedimentary rocks with an ultimate magmatic source. Thus, 60 % of the detrital zircons come from the 98–73 Ma southern Idaho batholith, with less than a third coming from the upper Snake River drainage, despite it representing 60 % of the drainage area. The Owyhee, Malheur, and Weiser rivers contribute <5 % each.Within Hells Canyon, modeling suggests that the main Snake River contributes by far the largest component (85 %); side canyons contribute <5 % of the zircon load. Mixing models for Snake River stream sediment downstream of Lewiston, below Lower Granite Dam (sampled from sands deposited before installation of the dam), suggest 53 % contribution from the main Snake River above the Salmon River, 34 % from the Clearwater River, 7 % from the Salmon River, and 6 % from the Grande Ronde River.Comparing relative zircon inputs from the modern Snake and Salmon rivers to a pre-dam river terrace just downstream from their confluence suggests that the Snake River in Hells Canyon today contributes only 4 % of the detrital zircons whereas before installation of the Hells Canyon Dams it contributed 29 %. We interpret this reduction in detrital zircon supply to be caused by sediment trapping above the dams.One influence on the detrital zircon contribution of various streams is peak flow volume. Zircon fertility of specific source rocks is the main influence on detrital zircon percentages within individual drainages. Mesoproterozoic, Cretaceous and Eocene granitic rocks produce euhedral magmatic detrital zircon grains that dominate several drainages. Mesozoic metavolcanic and Eocene Challis Volcanic Group rocks yield fewer and often broken detrital zircon grains. Large outcrop areas of Neogene Columbia River and Snake River Plain basalts provide almost none. Recycling of older magmatic detrital zircon through Proterozoic and Paleozoic sandstones is a signature of the eastern and central Idaho thrust belt.