Grain orientation and depositional ramps as flow direction indicators of a large-scale pyroclastic flow deposit in Japan

Abstract

Flow lineation was measured using the orientation of the elongated crystals and lithic fragments in the large-scale welded Ata pyroclastic flow deposit in southwestern Japan in order to analyze the effect of preflow topographic relief. Three different types of lineations were obtained. Flow lineations radially away from the source were obtained only for samples collected from the surface of the pyroclastic flow plateau. Samples collected along the floor of narrow valleys have lineations parallel to the axis of valley. Samples collected from valley walls tend to be parallel to the slope of the valley. This evidence suggests the following process. Earlier flow material moves along the valley axis in the narrow valley bottom. When the flow is trapped within a valley, it moves toward the bottom of the valley just prior to the final settlement. When the basement relief is filled up with the earlier flow units, the flow maintains its original radial movement until final settlement. Asymmetrical depositional ramps on the original surface of the pyroclastic flow provide an independent indicator of the flow direction.