TSUNAMI GENERATION BY PYROCLASTIC FLOW DURING THE 3500-YEAR BP CALDERA-FORMING ERUPTION OF ANIAKCHAK-VOLCANO, ALASKA

A discontinuous pumiceous sand, a few centimeters to tens of centimete rs thick, is located up to 15 m above mean high tide within Holocene p eat along the northern Bristol Bay coastline of Alaska. The bed consis ts of fine-to-coarse, poorly to moderately well-sorted, pumice-bearing sand near the top of a 2-m-thick peat sequence. The sand bed contains rip-up clasts of peat and tephra and is unique in the peat sequence. Major element compositions of juvenile glass from the deposit and radi ocarbon dating of enclosing peat support correlation of the pumiceous sand with the caldera-forming eruption of Aniakchak Volcano. The distr ibution of the sand and its sedimentary characteristics are consistent with emplacement by tsunami. The pumiceous sand most likely represent s redeposition by tsunami of climactic fallout tephra and beach sand d uring the approximately 3.5 ka Aniakchak caldera-forming eruption on t he Alaska Peninsula. We propose that a tsunami was generated by the su dden entrance of a rapidly moving, voluminous pyroclastic flow from An iakchak into Bristol Bay. A seismic trigger for the tsunami is unlikel y, because tectonic structures suitable for tsunami generation are pre sent only south of the Alaska Peninsula. The pumiceous sand in coastal peat of northern Bristol Bay is the first documented geologic evidenc e of a tsunami initiated by a volcanic eruption in Alaska.