TEPHROCHRONOLOGY OF LATE PLEISTOCENE AND HOLOCENE SAND DUNE DEPOSITS, HANFORD REACH NATIONAL MONUMENT, WA

A 25 m-thick succession of Late Pleistocene and Holocene sand dune deposits exposed at the Hanford Reach National Monument, WA, contains four compositionally distinctive tephras generated by eruptions of Mount St. Helens (MSH) and Glacier Peak. Located on the central Columbia Plateau along the Columbia River, this eolian succession caps gravelly sand- and silt-dominated glacial outburst flood sediment from which it was largely derived. Tephras are concentrated in beds, laminations, and lenses. From oldest to youngest the tephras originated from the Late Pleistocene MSH Sg, MSH So, and Glacier Peak G eruptions and from the Holocene MSH D (AD 1980) eruption. The Late Pleistocene tephras are <8 cm thick, laterally discontinuous and occur in the lower 2.5 m of the succession. The Holocene tephra is 0.5 cm thick, laterally continuous and exposed 12 m above the base of the succession. The only other sand dune-hosted Late Pleistocene tephra documented on the Columbia Plateau is a Glacier Peak G tephra preserved on the northern plateau; MSH set S tephras on the plateau have been recognized previously only in loess, lake, and outburst flood slackwater deposits. Mazama tephra, commonly preserved in sand dune deposits across the plateau, is absent from this succession. MSH set S tephras at this site are preserved in high-angle, planar cross-stratified, fine-medium sand deposited by an actively migrating sand dune. Glacier Peak G tephra is preserved in massive to low-angle planar cross-stratified, fine-medium sand deposited by a semi-stabilized sand dune; this and overlying dune deposits contain locally abundant cicada nymph burrows. Primary and secondary sedimentary structures suggest that the climate during and following Glacier Peak G tephra accumulation was relatively warm, semi-arid, and marked by dune stabilization and soil formation. The absence of Mazama tephra is attributed to eolian-terrain and climatic factors that promoted net sand dune deflation. Sand dune activity since the MSH D eruption has been markedly aggradational due to abundant source sediment, persistent winds, and relatively extensive vegetation. The preservation of the MSH set S tephras in loose, easily remobilized dune sand indicates that late-stage glacial outburst flood waters at this site did not actively rework sand dunes above ~210 m asl.