越後平野の沖積層形成における火山性洪水イベントの影響

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 largescale dam-break flood after the Numazawako eruption (about 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 ( -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 . to . km per 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.

2掘削地点のコアの岩相解釈に基づく福島県南相馬市小高区における完新世エスチュアリー埋積堆積物中の津波堆積物

“…4F and G), as described by Kakubari et al (2017). The depositional environment was identified as a closed anoxic basin floor, such as the central basin of the estuary (Zaitlin et al, 1994;Urabe et al, 2006). Core depth 8.63-0.38 m: Two sand beds were present at depths of 8.63-8.60 m and 8.53-8.50 m, and represent the grading structures of coarse and medium sand to very fine sand.…”