Flow Lineation of the Ata Pyroclastic Flow Deposit, Southern Kyushu, Japan

Abstract

Microscopic textures showing flow lineation were measured on oriented thin sections of the welded, late Quaternary Ata pyroclastic flow deposit, southern Kyushu, Japan. Obtained results indicate three factors which control the flow lineation. The radial orientation away from a source as mentioned by ELSTON and SMITH (1970) was obtained only at the sites where basement relief was completely filled up with the pyroclastic flow deposit. The lineations which are obtained from the samples collected in the floor of narrow valleys tends to be parallel to the direction of the valley. The samples which were collected from the wall of valley show the lineation parallel to the slope of the wall. These results suggest that the flow direction of the pyroclastic flow is strongly affected by the existing topographic relief. The earlier surges tend to flow along the valley floor or settle towards the bottom of the valley. After the basement relief was filled up with the earlier flow units, the later fluidized layer preserves its original radial movement until its final settlement.