PROXIMAL PYROCLASTIC DEPOSITS FROM THE 1989-1990 ERUPTION OF REDOUBT VOLCANO, ALASKA - STRATIGRAPHY, DISTRIBUTION, AND PHYSICAL CHARACTERISTICS

More than 20 eruptive events during the 1989-1990 eruption of Redoubt Volcano emplaced a complex sequence of lithic pyroclastic-flow, -surge , -fall, ice-diamict, and lahar deposits mainly on the north side of t he volcano. The deposits record the changing eruption dynamics from in itial gas-rich vent-clearing explosions to episodic gas-poor lava-dome extrusions and failures. The repeated dome failures produced lithic p yroclastic flows that mixed with snow and glacial ice to generate laha rs that were channelled off Drift glacier into the Drift River valley. Some of the dome failures occurred without precursory seismic warning and appeared to result solely from gravitational instability. Materia l from the disrupted lava domes avalanched down a steep, partly ice-fi lled canyon incised on the north flank of the volcano and came to rest on the heavily crevassed surface of the piedmont lobe of Drift glacie r. Most dome-collapse events resulted in single, monolithologic, massi ve to reversely graded, medium- to coarse-grained, sandy pyroclastic-f low deposits containing abundant dense dome clasts. These deposits var y in thickness, grain size, and texture depending on distance from the vent and local topography; deposits are finer and better sorted down flow, thinner and finer on hummocks, and thicker and coarser where pon ded in channels cut through the glacial ice. The initial vent-clearing explosions emplaced unusual deposits of glacial ice, snow, and rock i n a frozen matrix on the north and south flanks of the volcano. Simila r deposits were described at Nevado del Ruiz, Columbia and have probab ly been emplaced at other snow-and-ice-clad volcanoes, but poor preser vation makes them difficult to recognize in the geologic record. In a like fashion, most deposits from the 1989-1990 eruption of Redoubt Vol cano may be difficult to recognize and interpret in the future because they were emplaced in an environment where glacio-fluvial processes d ominate and quickly obscure the primary depositional record.