Detrital zircon geochronology of Bonneville flood deposits along the Snake River, Idaho: implications for the sediment dispersal patterns of megafloods

James Frank Glover

The unique setting of the Bonneville Flood and the well-documented geochronology of the Snake River drainage in Idaho provide an opportunity to 1) characterize the detrital zircon signatures of Bonneville Flood deposits in context of depositional setting; 2) evaluate the evolution of these signatures in terms of sediment dispersal; and 3) assess the limitations of Bonneville Flood deposits and their detrital zircon signatures as a continental event bed. U-Pb detrital zircon age data are presented from 933 grains from six late Pleistocene Bonneville Flood and two late Pleistocene Missoula Flood deposits. The Bonneville Flood (ca 14.5 ka) lowered Pleistocene Lake Bonneville by 110 m and deposited megaflood features along Marsh Creek and the Snake River in Idaho. The Bonneville Flood was a single event, in contrast to the multiple catastrophic outbursts of glacial Lake Missoula (ca 15 ka-12 ka). The detrital zircon populations of the Bonneville and Missoula Flood deposits along the Snake River are linked to sources in the Snake River drainage system. The distributions of these populations are a proxy for sediment dispersal throughout the Bonneville Flood path. Detrital zircon populations that fingerprint a southern Idaho source for the Bonneville Flood are the Yellowstone-SRP hotspot (0-17 Ma), Eocene volcanics (35-60 Ma), Idaho batholith (70-110 Ma), accreted terranes (120-450 Ma), central Idaho thrust belt (330-450 Ma), central Idaho plutons (460-500 Ma), Neoproterozoic (500-800 Ma), and Grenville (900-1300 Ma) sources. Sedimentary scale and depositional setting control the detrital zircon population distributions within deposits, which reflect the remobilization of local sediment. Bonneville Flood slackwater deposits have uniform detrital zircon signatures whereas specific age populations within eddy deposits are unpredictable. The 0-17 Ma population is ubiquitous in southern Idaho Bonneville Flood deposits, but absent near Lewiston. The variations of the 40-60 Ma, 70-110 Ma, and 1600-1800 Ma populations at the Ten Mile and Tammany Creek localities suggest that the initial Bonneville Flood 'signature' was diluted as successive Missoula Floods reworked the deposit. Variations in detrital zircon populations as a function of hydraulic sorting, depositional setting, and sediment mixing complicate the ability to determine a unique Bonneville Flood detrital zircon event bed signature.

Detrital zircon geochronology of Bonneville flood deposits along the Snake River, Idaho: implications for the sediment dispersal patterns of megafloods

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