RE-EVALUATING THE TEPHROCHRONOLOGY OF THE PALOUSE LOESS, WASHINGTON STATE, USING SINGLE-SHARD MAJOR- AND TRACE-ELEMENT ANALYSES

Tephra derived from Mount St Helens (MSH) have played an important role in developing a chronostratigraphic framework for the Palouse Loess region, Washington State, USA. New analyses of both proximal and distal tephra units from Washington State using EPMA major-element and LA-ICP-MS trace-element geochemistry have been undertaken. These analyses reveal that MSH tephra commonly employed to constrain the timing of loess deposition, including tephra from MSH eruptive sets S (~16 ka) and M (~22 ka), cannot be differentiated using major-element chemistry alone. Further, some distal tephra in the Palouse Loess which were previously assigned as MSH Set S on the basis of major-element geochemistry or stratigraphic position are now suggested to be other tephra, including MSH Set M, based on their trace element compositions.

Additional support for this re-evaluation of these Palouse Loess tephra units has been provided by newly generated luminescence ages for the loess that brackets the tephra units. This combination of major- plus trace-element geochemistry and luminescence chronology provides a coherent picture of the tephrochronology of the Palouse sites examined. More broadly, this work demonstrates the role of trace-element geochemistry in discriminating between tephra, and highlights the importance of combining both detailed geochemistry and geochronology in areas where tephra are geochemically indistinct.