Kaguyak to Katmai: Post-Glacial Tephras in Katmai National Park, Alaska

At least 15 explosive eruptions from the Katmai volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mt Martin and from Shelikof Strait to Bristol Bay ( ∼8500 km²). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing "soils" have established a time framework for entire soil-tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics (grainsize, thickness, color and mineralogy) have been combined with microprobe analyses of glass and Fe-Ti oxide minerals to correlate ash layers with their source vents including Mounts Martin, Mageik and Katmai, as well as Snowy Mountain and Kaguyak volcano. Microprobe analyses (25-70 analyses per glass or oxide sample) show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent and distinctively low in TiO₂ relative to the rest of the Katmai group that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra yet recognized is associated with emplacement of the Snowy Mtn summit dome (<250 C¹⁴ years). East Mageik erupted at least twice (2 and 4 ka), Mt Martin's blocky lava coulees were emplaced ∼6 ka, and Mt Katmai has three times produced very large explosive events (Novarupta 1912; plinian to sub-plinian "Lethe Assemblage" 12-16 ka; and a plinian rhyodacite 23 ka). Kaguyak's caldera-forming event (5.8 ka) generated a distinctive bright orange fine ash that is distributed widely throughout the Katmai district. Radiocarbon dating of loess, soil and peat enclosing this tephra are in good agreement despite their varying organic contents.