Facies analysis of a pumiceous terrace beside klutlan Glacier, Yukon Territory

Abstract

Dacitic pumice forms discontinuous terraces along both sides of Klutlan Glacier, which emanates from Mount Churchill, Alaska. Mount Churchill is the eruptive source of the White River Ash, an extensive tephra deposit accumulated ~1200 and 1900 BP during two plinian eruptions. Composition, texture, primary structures, and lack of induration suggest that, apart from a locally preserved cover of air-fall tephra, the Klutlan pumice deposits are resedimented proximal equivalents of the White River Ash. The pumice terraces display large-scale crossbedding, normal and inverse graded bedding, channels, and both linguoid and climbing ripples, all sedimentary structures characteristic of subaqueous deposition. In addition, many of the pyroclasts are subround and show a wide variation in sorting from bed to bed, in contrast to the uniformly angular to subangular texture of well-sorted pyroclasts in an air-fall ash layer that caps the terraces. This uppermost tephra unit, up to 1 m thick, is attributed to the last major eruption of Mount Churchill. The underlying resedimented pumice deposits are attributed to deposition by meltwater produced as a result of substantial melting of the snow and ice fields below Mount Churchill, the headward region of Klutlan Glacier, in response to increased heat flow immediately before the last eruption. The terraces stand more than 100 m above the present surface of Klutlan Glacier, indicating that substantial melting has occurred since the time of terrace deposition. In comparison to present-day conditions, this implies a prolonged cold climate before accumulation of the pumice terraces in glacier-margin channels. An alternative explanation is that Klutlan Glacier may have been catastrophically thickened as a result of glacial surging in response to elevated heat flow during eruptions of Mount Churchill. Meltwater backed up behind one or more ice dams could have created temporary lakes in which detached segments of Klutlan Glacier locally abutted against and (or) scoured the shorelines, thus explaining the discontinuous nature of the pumice terraces. Rapid deposition of the pumice terraces, perhaps during a single winter, is inferred from an almost complete lack in the terraces of lithoclasts derived from the steep talus-covered slopes of Klutlan's U-shaped valley.