MOUNT-CHURCHILL, ALASKA - SOURCE OF THE LATE HOLOCENE WHITE RIVER ASH

Although the White River Ash was first recognized by F. Schwatka in 18 83, the source of this widespread bilobate Plinian deposit in northwes tern Canada and adjoining eastern Alaska has not been definitively ide ntified. Recent investigations disclose that Mount Churchill, in the S t. Elias Mountains of south-central Alaska, contains a summit crater w hose rim is chiefly composed of ash, coarse pumice, and exotic rock fr agments. The pumice clasts decrease in size and the exotic rock fragme nts disappear with increasing distance from the summit area. Pumice fr om the Mount Churchill summit, and from the White River Ash deposits u p to 25 km from the summit area, are mineralogically and chemically si milar. Microprobe analyses of the summit pumice glass are virtually id entical to those of more distant proximal White River Ash glass. Fe-Ti oxide compositions in the Mount Churchill proximal tephra indicate th at all tephra sampled during this study is from the younger, eastern l obe of the White River Ash. These field and laboratory data demonstrat e that Mount Churchill is a Holocene volcano and the source of the eas tern lobe of the White River Ash. Glass compositions and distribution patterns, however, indicate both lobes of the ash have a common source .