The lower part of the Jangki Group (Miocene), SE Korea consists of pyroclastic mass-flow-dominated facies and epiclastic stream-flow-dominated facies which reflect sedimentation during syn- and intereruption periods, respectively. On the basis of pyroclastic composition, sedimentary structures and bed geometry, they are organized into two facies associations: (1) dacitic and basaltic debris-flow and hyperconcentrated-flood-flow deposits of eruption periods, and (2) epiclastic stream-flow and interchannel deposits of intereruption periods. The lateral relationship between the syn- and intereruption deposits varies significantly over short distances (2 km). In the western part of the study area, syneruption deposits are predominant, and fluvial deposits occur as small-scale channel-fill gravelstone bodies encased within dacitic debris flow deposits. In the eastern part, however, intereruption deposits are dominated with thick sequences of interbedded channel and interchannel deposits. The abrupt lateral change indicates alternation of epiclastic axial fluvial system with pyroclastic-rich volcaniclastic aprons. The syneruption deposits are enriched in vitric ash but lack contemporary volcanic rock fragments (dacitic or basaltic). They are sharply differentiated from intereruption deposits that mostly consist of epiclasts and are deficient in vitric ash. The vertical transition suggests that streams drained a hinterland of igneous basement rocks during intereruption periods and became bulked with pyroclasts during syneruption periods.