Thermoluminescence dating of late Pleistocene loess and tephra from eastern Washington and southern Oregon and implications for the eruptive history of Mount St. Helens

Abstract

The first numeric dating of late Pleistocene loess from the Palouse region of eastern Washington is reported. This loess is interbedded with thin, distal tephra layers, some identified as Mount St. Helens products. Both the interbedded tephra layers and thick paleosols that have developed in the loess have previously been correlated across the region. A thicker correlative of one of these tephra layers (Mount St. Helens set C) was collected from Summer Lake, Oregon, for direct dating by thermoluminescence (TL). Polymineral, fine grain TL ages for loess from three chosen sections in Washington range from 17 to 83 ka. These imply ages for some tephra beds that are significantly older than implied by earlier radiocarbon (14C) dating. For example, TL ages of 17–21 ka for loess bracketing a tephra layer correlated to Mount St. Helens set S suggests a somewhat older age for this ash than the previously accepted 14C age of ∼15.5±0.5 ka cal yr (corrected to calendar years). TL ages of 46±6 (1σ) ka and 57±7 ka for loess above, and of 53 ± 7 ka and <75±6 ka below a tephra bed correlated to a Mount St. Helens set Cw layer, along with a TL age of 46±5 ka for fine‐silt‐sized glass from a Mount St. Helens set Cy bed in Oregon, revise upward from ∼38 ka cal yr (14C age is ∼36 ka B.P.) to ∼50 ka the age of this regionally important marker ash set. Several TL ages, taken together, imply a much longer eruptive history for Mount St. Helens than previously thought. A TL age of 83±8 ka just below a previously undated tephra layer from Mount St. Helens (EMSH ash) suggests that the eruptive history of the mountain extends back to at least 80 ka. Furthermore, age depth extrapolation through our sections to an ash (WA‐5C) probably correlated to Mount St. Helens suggests an eruptive history of at least 120 ka.