Abstract
The Echigo Plain, which faces the Sea of Japan in northeast Japan, is a typical coastal plain with numerous sand dunes. We examined the sedimentology of pumiceous sediments (lahar deposits) in the Agano River area of the Echigo Plain, derived mainly from a large-scale dam-break flood after the Numazawako eruption (about 5 ka), using an existing borehole dataset and facies analysis of sediment cores.
The lahar deposits show different sedimentary facies in coastal areas versus the shallow marine environment. Coarse-grained pumiceous sediments (5—8 m thick) are interpreted to have been deposited under the delta front or in a delta plain environment in a coastal area. Sandy silt deposits (pro-delta facies), including pyroclastic material (horizons of concentrated volcanic glass shards and heavy minerals), are distributed in the shallow sea area.
The large volumes of volcaniclastic sediment delivered during the flood event induced the rapid progradation of the delta system in the Agano River region. The deposition of volcaniclastic sediments in the shallow sea also caused progradation of the delta system and changed the coastal landforms along the margin of the Echigo Plain.
The flood event resulted in an increase in the progradation rate of the depositional system from 0.9 to 2.5 km per 1000 years. An understanding of large-scale pyroclastic resedimentation can contribute to evaluating and predicting the volume of sediment discharge and the nature of hazards created by a volcanogenic flood and its aftermath in an alluvial plain setting.
