Abstract

The Pliocene Mushono tephra bed, central Japan, is characterized by fluvio-lacustrine volcaniclastic resedimentation at distal locations from the volcanic source. Sedimentary responses to an explosive eruption are well recorded in the resedimented Mushono tephra bed. Immediately after the eruption, large volumes of primary volcaniclastic material were reworked from proximal areas and discharged into the distal basin. Overloading of fluvial systems by pyroclastic debris resulted in the development of low-sinuosity active fluvial channel systems that carried sediment-laden, hyperconcentrated flows. The fluvial system flowed into a lake more than 150 km from the inferred source location, resulting in delta formation and progradation. Primary pyroclastic fall deposits are only a few centimeters thick but are overlain by reworked volcaniclastic deposits more than 5 m thick, which show coarsening-upward successions representing fluvio-lacustrine deltaic progradation. The reworked succession consists of six sedimentary facies, which can be interpreted as prodelta, mouth bar, distributary-channel, hyperconcentrated-flow, inter-distributary-lowland or floodplain, and slump deposits. Rounded, cobble-sized pumice clasts in prodelta and distributary-channel deposits and thick, reworked deposits imply that the majority of resedimented volcaniclastic material was derived from voluminous proximal ignimbrite, rather than thin, fine-grained distal fallout ash material. Vertical facies changes are concordant with a decrease in sediment yields with time. Lateral facies distributions correspond to episodic influxes of volcaniclastic sediments that controlled the morphology of discrete fluvio-lacustrine deltas. The distal sedimentary response to an explosive silicic eruption is thus clearly recorded in vertical and lateral facies distribution of reworked volcaniclastic sediments.

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