Welded block and ash flow deposits from Mount Meager, British Columbia, Canada

Abstract

The 2360 years B.P. eruption of Mount Meager, British Columbia, produced a succession of rarely observed, welded block and ash flow deposits and non-welded equivalents. We report on these rhyodacitic block and ash flow deposits by describing the deposits, documenting the nature of the welding process, and establishing the origins of these pyroclastic deposits. Variations in welding intensity are tracked by image analysis of field texture maps, petrographic study of shard morphology, and measurements of sample density and porosity. Pyroclast oblateness from image analysis of texture maps provides a qualitative record of welding intensity, but underestimates the amount of compactional (e.g., volume) strain when compared to estimates from physical property measurements (e.g., density, porosity). Unconsolidated deposits have an average matrix porosity of 41% (3845%) and clasts have an average porosity of 32% (2440%). In unconsolidated deposits, isolated porosity is pervasive 8%) in juvenile clasts, but is near absent in samples of matrix 1%). Welding and compaction cause a reduction in both connected and isolated porosity. Clasts and matrix record equivalent amounts of strain 38%) and, thus, are fully coupled during the welding and compaction process. The average integrated strain for vertical sections of the deposit is 31% implying > 50 m shortening of the thickest deposit (from 162 m to the current 112 m). The pyroclastic flow deposits result from accumulation in a narrow, confined river valley; the accumulation was sufficiently rapid to keep the deposits above their glass transition temperature thereby allowing the succession to weld as a single cooling unit. The nature and distribution of isolated porosity in the juvenile clasts of rhyodacite suggest an explosive (rather than effusive) origin for these block and ash flow deposits that may be analogous to some Vulcanian-eruption-triggered dome-collapse processes observed at Soufrière Hills Volcano, Montserrat (rather than gravitational collapse).

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