HOLOCENE TEPHROCHRONOLOGY OF SANAK ISLAND, ALASKA

An important record of large Holocene eruptions from volcanoes located along the western Alaska Peninsula and eastern Aleutian Island Arc is well preserved in the lacustrine sediments of Sanak Island, AK. Sediment cores obtained from five kettle ponds on Sanak Island provide a nearly continuous, high-resolution sedimentation record since deglaciation, approximately 11,900 +/- 60 yrBP. Over 20 distal tephra layers were identified and sampled from each core, resulting in over 100 tephra samples collected and analyzed. These samples were characterized and correlated based on their physical descriptions, stratigraphic relationships within the cores, major element glass shard geochemistries using electron microprobe analysis, glass shard micromorphology using scanning electron microscopy, and by the petrographic assessment of mineral assemblages. In addition, ten radiocarbon dates provide age constraints on eruptions, including ages for two unusually thick deposits extrapolated to about 6800 and 9000 C14yrs BP recording large caldera-forming eruptions, the latter being the cataclysmic eruption that created Fisher Caldera, the largest of 12 Holocene calderas in Alaska. The Sanak Island site provides important information on tephra distribution in a previously unstudied region, adjacent to the most active portion of the Aleutian volcanic arc. It has helped us modify the previously determined tephra distributions, identify previously unknown eruptions, and recalculate eruption frequencies and magnitudes from eastern Aleutian volcanoes, thus allowing us to better assess the potential tephra hazard to aviation, infrastructure and local residents.