HOLOCENE TEPHRA STRATIGRAPHY IN THE ISLANDS OF FOUR MOUNTAINS, ALEUTIAN ISLANDS, ALASKA: A PRELIMINARY REPORT

During July-August, 2014, an NSF funded, international team of geologists, archaeologists and paleoecologists conducted field surveys, excavations and collection in the little studied Islands of Four Mountains group. The overriding research focus was to study North Pacific human/ecosystem resilience against a backdrop of Holocene climatic and geological events. Four stratovolcanoes—Cleveland and Tana (both on Chuginadak Island), Carlisle and Herbert—are closely located, but exhibit distinct magmatic and eruption histories. Four small scoria cones are located between Cleveland and Tana volcanoes. To determine the Holocene eruption histories during the period of human colonization and habitation, we sampled tephra layers that are intercalated with aeolian deposits, peat layers, and human cultural deposits.

Cleveland volcano exhibits large-scale pyroclastic flows, lahar deposits, tephra falls and effusive lavas that are primarily basaltic andesite to andesite. Cleveland tephras intercalated with aeolian deposits are among the youngest layers. In contrast many older lavas from Tana volcano are more dacitic whereas the scoria cones erupted basalt to basaltic andesite compositions. At archaeological site CR-02, located on southeast Carlisle Island, a series of tephra layers (two layers of volcanic ash, a lapilli layer and a volcanic ash layer in ascending order) are intercalated with undisturbed cultural layers. This stratigraphic association suggests that prehistoric humans continued to inhabit, or immediately resettled, southeast Carlisle following several distinctive volcanic eruptions. We name this series of tephras the “CR-02 Tephra”. On east Carlisle and central Chuginadak we traced a clear distribution of tephra layers (Total thickness: 10-20cm; lapilli average grain size: 5-10mm), although the source volcano has yet to be identified. Tephra distribution and thickness suggests that Herbert volcano is a strong candidate as the source for these widely distributed and distinctive tephra layers.