ABSTRACT: Evidence for winter eruption of the White River Ash (eastern lobe), Yukon Territory, Canada.

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Abstract

Rhyodacitic tephra of the bilobate White River Ash deposit covers more than 340,000km2 of the Yukon and Northwest Territories, Canada, and extends into the adjacent eastern region of Alaska, U.S.A. (Richter et al, 1995). The White River Ash was deposited during two volcanic eruptions, the first of which produced the northern ash lobe ca.1887 years B.P. (Lerbekmo et al., 1975); the eastern ash lobe formed ca.1140 years B.P. (Clague et al., 1995). The source of the two eruptions has been identified as Mount Churchill in the Wrangell Mountains, Alaska (McGimsey et al., 1990). The bilobate nature of the White River Ash deposit indicates that both lobes were deposited from high eruption columns under the influence of strong unidirectional winds (c.f. Fisher and Schmincke, 1984). However, the eastern lobe contains agreater volume of tephra (up to 30km3 ) than the northern lobe (< 20km3 ), indicating a greater intensity for the latest eruption (Downes, 1985). Based on patterns of seasonal wind distribution, Workman (1979) suggested that the eastern lobe eruption occurred during the winter, and the northern lobe during the summer. Other workers have also speculatedon winter emplacement for the eastern lobe (Hanson, 1965; Donaldson et al., 1995). This paper provides new evidence for winter emplacement of the eastern lobe of White River Ash, based on sedimentological observations of stratigraphic sections in the banks of Bock's Creek and Donjek River, at sites adjacent to the Alaska Highway. Hanson (1965) based his model for winter eruption on observations of uneroded ash beds on slopes as steep as 40o. To account for such stability, Hanson suggested that the ash may have been deposited while it was snowing, with the ash providing nucleation centres for condensing water vapor, thus enhancing compactionfollowed by freezing. This would allow preservation under the subsequently accumulated snow load, whereas if unfrozen, much of the the ash would be washed off steep slopes by rain, and/or redistributed by wind (Workman, 1979). Hanson (1965) also suggested that preservation of these deposits is best explained by eruption during the early winter, which would allow accumulation of a substantial protective snowfall cover, thus leading to maximum preservation during the spring thaw. New evidence indicating that the eastern lobe of White River Ash was emplaced during the winter is provided by two stratigraphic sections of ash along the Alaska Highway. The first of these localities is in the southern bank of Bock's Creek, 100 m west of the Alaska Highway, and 100 km east of Mount Churchill. The vertical section is predominantly composed of layers of silt and gravel, with minor admixtures of sand. Discontinuous interlayers of peat occur throughout the silt. The gravel layersconsistently fine upwards, and the silt layers are typically laminated. The ash occurs in a distinctive white unconsolidated granular layer with sharp boundaries, approximately 70 cm above the base of the succession. Consisting entirely of pumice, this layer has an average thickness of 3.5 cm, and is laterally continuous for more than 8 m. The ends of the ash layer are abruptly truncated by channel-fill deposits of sand and gravel. The ash layer displays normal graded bedding and faint horizontal laminations. The entire succession is capped by 30 cm of dark brown peat. The eastern-lobe ash at the Bock’s Creek locality was deposited as a distinct and laterally extensive layer. Its compositional homogeneity, horizontal lamination, and normal graded bedding indicate that it is anairfall deposit. Preservation of such an ash layer between beds of fluvial sand and gravel requires that the ash had to have been frozen before being buried during the subsequent spring flood, thus escaping reworking by high-energy fluvial currents. Deposition of the ash layer during early winter would allowfreezing and accumulation of substantial snow cover, so that early thawing would be unlikely. The preserved ash layer is inferred to have been deposited on an overbank flood plain which remained protected by snow and ice in winter, and then was buried (while still frozen) by sand and gravel during thesubsequent spring flood. Minor irregularities along the boundaries of the ash bed record post-depositional loading by epiclastic sediment concurrent with melting of pore-space ice in the ash bed. Additional evidence of winter emplacement for the eastern-lobe ash is provided by the Donjek River section, 200 m west of the Alaska Highway, and 130 km southeast of Mount Churchill. The exposed succession here consists of glacial till overlain by 60 cm of fluvial silt and sandy to pebbly silt. The upper 30 cm of the silt unit contains abundant tabular, ovoid and irregular clasts of unconsolidated granular pumice, most showing faint lamination parallel to elongation. Because of their compositional homogeneity, angularity and lack of consolidation, these pumice clasts are inferred to have been derived from a frozen layer of airfall ash, comparable to the Bock’s Creek occurrence, that was fragmented duringspring flooding. Minimal transport after fragmentation is dictated by the predominance of fragile shapes, and those clasts that are rounded must have been abraded while still cemented by ice. Furthermore, to preserve their morphological integrity, the pumice clasts must have remained ice-cemented during subsequent burial in the unit of fluvial silt. The season during which the White River Ash was emplaced is important to both geologists and archaeologists. Volcanic ash eruptions intensify the effects of erosion, landslides, and mudslides(Workman, 1979), and such effects are strongly influenced by the season in which an eruption occurs. Volcanism also has a large ecological and human impact. The most recent eruption that produced the eastern lobe of White River Ash is thought to have been sufficiently devastating to cause migration of theAthapaskan peoples into southern Canada and the United States (Workman, 1977). Further geological assessment of the two White River Ash eruptions will help to elucidate the cultural and ecological ramifications.

Bibliography of Canadian Geomorphology