A tephrochronological framework for glacial lake outburst flood deposits in the Channeled Scablands, U.S. Pacific Northwest

The largest known freshwater floods in Earth’s history scoured the northwestern USA during the late Pleistocene, following the catastrophic drainage of glacial Lake Missoula. These glacial lake outburst floods, involving as much as 2500 km³ of water, covered over 7,500 km², leaving behind a dramatic landscape of erosional features and extensive sedimentary deposits. Concurrently, eruptions from Cascade volcanoes, primarily Mount St. Helens (MSH) and Glacier Peak, blanketed the region with volcanic ash. Remarkably, some of these tephra layers were preserved within and overlying the flood sediment sequences, providing a rare opportunity to investigate the timing and extent of glacial lake outburst floods.

We present new geochemical fingerprints of tephra layers using electron microprobe analysis, comparing them to reference samples from proximal deposits of MSH, and well-characterised deposits from Glacier Peak and Mount Mazama. Our new analyses of 37 tephra layers spread across 10 different sites within the U.S. Pacific Northwest, confirm previous results from fewer sites that these floods occurred during the eruptive phase known as MSH Set S. Notably, the ~16 ka So and Sg tephras are preserved as a distinct couplet within the flood sediments. In contrast, the ~13.5 ka Glacier Peak G and B tephras, which lie above the flood deposits, post-date the floods. These findings constrain these floods to have commenced prior to ~16 ka but ending before ~13.5 ka and highlight the value of geochemical tephra analysis across multiple flood sediment sequences for refining the chronology of major Pleistocene glacial lake outburst flood events.