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.

Extending the Late Holocene White River Ash Distribution, Northwestern Canada

“…2) were first noted by Bostock (1952), and likely have Mt. Churchill, Alaska, as their source (Richter et al, 1995). The smaller lobe trends northwards along an axis just west of the Alaska-Yukon boundary and is found as far north as the Yukon River.…”

“…Published maps of the White River ash distribution suggest that the ashfall did not extend as far as the Mackenzie Valley (Bostock, 1952;Hughes et al, 1972;Lerbekmo et al, 1975;Clague et al, 1995), with only the map of Richter et al (1995) suggesting that the easterly extent might be much greater. However, several researchers have observed the ash in peatlands and lakes at various locations in the Mackenzie Valley (Rowe et al, 1974;Zoltai and Tarnocai, 1975;Rostad et al, 1976;Slater, 1985;Robinson and Moore, 1999;Robinson, unpubl.…”

Extending the Late Holocene Tephrochronology of the Central Kenai Peninsula, Alaska

“…To determine the source of the tephras, we compared the glass geochemical data to a large data set compiled from previously published studies of tephra in southern Alaska (Downes, 1985;Riehle, 1985;Riehle et al, 1987Riehle et al, , 1990Riehle et al, , 1992Riehle et al, , 1999 (selected data); Begét et al, 1991Begét et al, , 1992Richter et al, 1995;Begét and Motyka, 1998;Child et al, 1998). Previous studies have used different electron microprobe centres and adopted a variety of analytical conditions, and these differences may complicate data comparisons.…”

Initiation of the Wrangell Arc: A Record of Tectonic Changes in an Arc-Transform Junction Revealed by New Geochemistry and Geochronology of the ~29–18 Ma Sonya Creek Volcanic Field, Alaska

“…Late Miocene-1500 ka WA volcanism is dominantly characterized by lavas that erupted from shield volcanoes, cinder cones, silicic domes, and small plugs. The adakitic White River Ash is a product of the Holocene Plinian eruption (volcanic explosivity index of 6) from Mount Churchill, a large stratovolcano (Lerbekmo and Campbell, 1969;McGimsey et al, 1992;Richter et al, 1995;Lerbekmo, 2008;Preece et al, 2014). Shallow intrusive and hypabyssal rocks that are coeval with WA volcanic rocks are likely the sub-volcanic roots of eroded volcanic centers (MacKevett, 1978;Trop et al, 2012).…”