PRELIMINARY ANALYSIS OF A POSTGLACIAL TEPHRA SECTION AT MOUNT CLEVELAND VOLCANO, CHUGINADAK ISLAND, ALEUTIAN ARC

Mount Cleveland volcano is a young, 1730-m-high, largely basaltic andesite to andesite composite volcano in the eastern Aleutian Islands. The volcano forms the western portion of Chuginadak Island and has been in intermittent eruption since 2001. Four monogenetic cinder cones stretch eastward from Cleveland to Tana volcano which dominates eastern Chuginadak Island. In 2014, as part of a multidisciplinary field project supported by the National Science Foundation, the Keck Geology Consortium, and the Alaska Volcano Observatory, we gathered tephrochronologic data from several locations around the volcano. This information will be combined with previous observations, geologic mapping, and geochronology to better understand the eruptive history and hazards of Cleveland and adjacent centers in the volcanic cluster known as the Islands of Four Mountains (IFM).

We examined a 6-m long section in a gulley at the base of Cleveland’s eastern flank in detail. Holocene Cleveland tephras comprise much of the section as well as ejecta from nearby mafic cinder cones and possibly from more distant silicic sources. The upper half of the section records more than 36 mostly dark-colored, fine-grained ashfalls interspersed with poorly developed soils, occasional lapilli-falls, and lahar deposits. The pumiceous tephra-fall deposit from Cleveland’s 2001 eruption is prominent at the top of the section. A 4-cm-thick, light-colored ashfall, informally named the “Neapolitan ash” occurs 3 m below the modern surface; soil beneath this ash returned an uncalibrated ¹⁴C date of 7,240±35. Preliminary observations of the extent, changing thickness, and character of this deposit suggest a very large eruption from a source to the east of Cleveland. About 5 m below the modern surface, a ~30 cm thick coarse lapilli scoria unit with clasts up to 7 cm in diameter, sits above a soil with an uncalibrated ¹⁴C date of 9,100±40. Clasts are basaltic and likely represent near-vent fall from the “Foxtrail cinder cone”, one of four mafic cones east of the Cleveland edifice. The base of the section contains a sequence of dacitic, lapilli (up to 3 cm diameter), plinian or subplinian pumice fall(s) overlying a soil with an uncalibrated ¹⁴C date of 9,570+/-40. Further work is needed to determine candidate sources for this deposit.