Provenance and Deposition of Glacial Lake Missoula Lacustrine and Flood Sediments Determined from Rock Magnetic Properties

Repeated outburst flooding from glacial Lake Missoula, Montana, affected large areas of Washington during Marine Oxygen Isotope Stage 2 (29–14 ka). We present the first high-resolution rock magnetic results from two sites that are critical to interpreting these outburst floods and that provide evidence of sediment provenance: glacial Lake Missoula, the source of the floods; and glacial Lake Columbia, where floodwaters interrupted sedimentation. Magnetic carriers in glacial Lake Missoula varves are dominated by hematite, whereas those in outburst flood sediments and glacial Lake Columbia sediments are mainly magnetite and titano-magnetite. Stratigraphic variation of magnetic parameters is consistent with changes in lithology. Importantly, magnetic properties highlight depositional processes in the flood sediments that are not evident in the field. In glacial Lake Columbia, hematite is present in fine silt and clay deposited near the end of each flood as fine sediment settled out of the water column. This signal is only present at the end of the floods because the hematite is concentrated in the finer-grained sediment transported from the floor of glacial Lake Missoula, the only possible source of hematite, ~ 240 km away.