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Volcanic Debris Flows: Facies and Sediment Transformations

Richard V. Fisher

Calc-alkaline stratovolcanoes commonly occur in magmatic arcs related to oceanic crust subduction. The signature of this tectonic environment is a sedimentologic spike that indicates an increased accumulation rate of clastic sediments above some background level. This increase appears in the stratigraphic record as an increase in thickness, volume, coarseness, and volcanic content of the sediments--subaerial and subaqueous--and a lithofacies dominated by deposits of sediment gravity flows with volcanic debris flows and their sedimentologically transformed equivalents being dominant. Debris flows form widespread fan-aprons surrounding stratocones that dominate the sedimentologic environment, providing source materials to all associated watersheds and contiguous subaqueous asins. As the andesitic volcanic cones grow in volume and height, the lithofacies associations in marine environments--at first dominated by pyroclastic fallout layers and pelagic muds--become increasingly flooded by turbidites and subaqueous debris flows. Debris flows are non-Newtonian fluids that are essentially nonturbulent. On land, flowing debris maintains an internal texture and structure that is preserved in the final deposit. Debris flows that move into rivers or into standing bodies of water undergo sediment transformations that have the effect of extending coarse-grained debris farther into basins than debris flows alone. If increased volcanism can be directly related to active subduction, then dating sedimentologic spikes within adjacent basins would give a direct method for d termining rates and timing of motion along subduction zones.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.