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Pyroclastic flow deposits of May 13, 1980, found high on the steep (14°-28°) slopes of Mount St. Helens, show large-scale internal cross-stratification and upper surface bed forms. Antidune structures of up to 0.8 m (2.5 ft) amplitude migrated sourceward and developed from free-surface flows that lay in or close to a supercritical condition. Sigmoidal, upflow-migrating bed forms, up to 2 m (6.5 ft) in amplitude and with internal angles up to 65°, formed in response to high aggradation flows. Stoss-side truncations of low-angle dune forms are also very common. The depositional products of the passage of one pyroclastic flow can be identified by erosional surfaces between bed sets.
To produce and preserve these bed forms we must appeal to special conditions of deposition. These conditions include steep slopes, excessive sediment load, velocities of 30-100 m/sec, flow densities of 1.0-1.2 for the particle and gas mixture, and minimum bed load thicknesses of 1-2 m (3-6 ft), equivalent to individual flow units. Turbulence, which plays an important role in the formation of these bed forms, is believed to be enhanced by the increase in velocity and possible thinning of the flows as they rushed down the steep, rough slopes of Mount St. Helens.
Data from bed-form measurements and grain-size analysis are presented to document and help understand the development of these bed forms. These data will be compared to and contrasted with other cross-stratification types in primary pyroclastic flows from the Laacher See volcano of West Germany and Tambora volcano in Indonesia.
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