Mount Churchill, Alaska: source of the late Holocene White River Ash

Abstract

Although the White River Ash was first recognized by F. Schwatka in 1883, the source of this widespread bilobate Plinian deposit in northwestern Canada and adjoining eastern Alaska has not been definitively identified. Recent investigations disclose that Mount Churchill, in the St. Elias Mountains of south-central Alaska, contains a summit crater whose rim is chiefly composed of ash, coarse pumice, and exotic rock fragments. The pumice clasts decrease in size and the exotic rock fragments disappear with increasing distance from the summit area. Pumice from the Mount Churchill summit, and from the White River Ash deposits up to 25 km from the summit area, are mineralogically and chemically similar. Microprobe analyses of the summit pumice glass are virtually identical to those of more distant proximal White River Ash glass. Fe–Ti oxide compositions in the Mount Churchill proximal tephra indicate that all tephra sampled during this study is from the younger, eastern lobe of the White River Ash. These field and laboratory data demonstrate that Mount Churchill is a Holocene volcano and the source of the eastern lobe of the White River Ash. Glass compositions and distribution patterns, however, indicate both lobes of the ash have a common source.