| Abstract: | The grainsize characteristics of dune-bedded pyroclastic surge bedsets are surveyed. The variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets, and it increases as the grainsize of the coarsest bed increases, reflecting an increasing velocity of emplacement. Deposits of wet surges, identified as those which contain accretionary and ash-coated lapilli, tend to be finer and show less variance, this partly because wet ash is cohesive, but mainly because wet surges tend to be weaker. Dry surge bedsets are strongly fines-depleted, wet ones less so. The lack of erosion of underlying ash layers shows that the environment is a strongly depositional one. Individual bedsets are demarcated by thin intervening fine ash-fall layers, which are the complementary ash-cloud deposits settled or flushed out after the passage and decay of each turbulent surge. Surge deposits are generally less coarse than the coarsest associated airfall deposits, which shows that they are formed by generally weaker events.This study helps interpret the dune-bedded parts of the landscape-mantling May 18th 1980 “blast” deposit of Mount St. Helens. The blast was a very violent event, but the variance and the grainsize of the coarsest bed are those of a relatively weak surge. This suggests that the dune-bedding was produced by a weak effect, such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs. |
