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

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