A reconnaissance of the major Holocene tephra deposits in the upper Cook Inlet region, Alaska

Abstract

The upper Cook Inlet region of southcentral Alaska would be significantly impacted by a major tephrafall, owing to a widespread population and heavily travelled transportation corridors. To evaluate the likelihood of such an occurrence, the tephra deposits of the region have been inventoried. Approximately 90 deposits of Holocene age are sufficiently thick to have been preserved for sampling; the frequency of such major tephrafalls ranges from 1 every 200 years near sources on the west side of upper Cook Inlet, to 1 every 1000 years on the more populated east side. The volcanoes located on the west side of upper Cook Inlet are, from north to south, Hayes, Spurr, Redoubt, and Iliamna. Hayes volcano produced the most extensive set of 6 to perhaps 8 tephra layers in the region about 3650 yr B.P. and produced one other, less extensive tephra layer during Holocene time. Spurr and Redoubt volcanoes have produced, respectively, approximately 35 and 30 Holocene layers which were dispersed eastward toward population centers. No Holocene tephra layers of Iliamna have been recognized with certainty; consequently, several tephra layers which originated to the south of the region must have a source at Augustine Volcano, or some more distant volcano. Tephra layers of Hayes volcano are calc-alkaline dacites. Most of the Spurr deposits are tholeiitic, basaltic andesites whereas those of Redoubt Volcano are calc-alkaline andesites and dacites.