Rock surface luminescence dating of gravel determines the age of a glacial outburst megaflood, Glacial Lake Missoula, Montana, USA

Abstract Giant gravel bars are important archives of megafloods; however, determining their depositional ages requires reliable geochronometric methods. Five bars, reaching heights 150–170 m, formed in the bedrock-lined Alberton Gorge along Clark Fork River, Montana (USA), during draining Glacial Lake Missoula (GLM). We report first numerical for megaflood deposits GLM basin by successfully applying novel optically stimulated luminescence (OSL) rock surface dating technique to date cobbles collected from three locations one bar’s transport direction. Depth-dependent infrared and post-infrared pulsed OSL signals showed that exteriors only 3 out 38 cobble samples were well bleached exposure daylight before burial, hence suitable dating. The provided 16.5 ± 0.9, 18.5 1.4, 21.7 1.1 ka, all which indistinguishable average cosmogenic nuclide age 18.2 1.5 ka (n = 4) a large Channeled Scabland, eastern Washington State. two younger ages, 17.5 1.0 is our best estimate deposit age. interpret older be was reworked flood. Our results show these techniques have great potential providing chronologies paleofloods other high-energy events.