Facies variations east-northeast of Mount St. Helens preserve a record of depositional processes in the 18 May 1980 lateral blast cloud. This paper reports new field, grain-size and component data from the ENE sector of the timber-blowdown zone and presents a model for blast flow and sedimentation. The first-erupted ejecta was rich in juvenile components and extends to the distal blowdown limit. The last-erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge. The blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a ‘high-density’ turbidity current. The possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns. Depositional conditions were strongly influenced by the rate of suspended-load fallout from the blast. Within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered-settling zone able to detach gravitationally and drain into local depressions. The resulting proximal facies resembles a low-aspect-ratio ignimbrite. Fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered-settling zone. Component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers. With increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions, building up antidune-like bedforms. Wavelengths of these bedforms range from 20 to <1 m, and decrease away from vent. There is a systematic relationship between antidune migration direction and depositional slope. The transition from proximal (ignimbrite-like) to distal (surge-like) facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high-velocity pyroclastic flows.

Emplacement of the 18 May 1980 lateral blast deposit ENE of Mount St. Helens, Washington

Settling behaviour of concentrated dispersions and some volcanological applications

The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass-ballotini sand of particle size 35·5 ?m

An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits

Experimental evidence is presented which shows that both uniform and differential sedimentation are unstable flows for relatively concentrated fluid suspensions containing particles of two different densities or sizes. The instability takes the form of a new type of fingering phenomenon which develops as a result of the two kinds of particles segregating laterally from each other.

On instabilities arising during sedimentation of two-component mixtures of solids

Sedimentation of Binary Particle Mixtures

Analyse semi-empirique du processus d'extraction de solvant pour les sables asphaltiques d'Athabasca en vue de definir des criteres de conception du procede. Obtention d'une equation d'extraction generalisee. Application aux donnees experimentales sur le kerosene dans un contacteur rotatif ainsi qu'a des resultats sur plusieurs solvants en appareils agites

Colin R. Phillips

Papers

Solvent extraction of mined athabasca oil sands

Absorption of sulfur dioxide in calcium hydroxide solutions

Modes of Settling and Relative Settling Velocities in Two-Species Suspensions