Holocene depositional systems along the Agano River of the Echigo Plain, central Japan

Based on the boring database, soil distribution, N-value and various engineering properties were revealed in alluvial subsurface layers of Niigata City. In northern area of the city, soft fine-grained soils and organic soils are contained in the shallow layer between dune deposits. In the depth of about 10-40 m, coarse-grained soil layers with average N-value over 30 are dominantly. These layers are composed of fine or medium sands with average fines content Fc under 10% in shallow, while Fc tends to increase at the depth of more than 20m. In the southern back mash area, subsurface layers contain fine-grained soils and coarse-grained soils in same degree from shallow layers to deep deposits and average N-values are smaller than in the northern area. As Fc is higher in coarse-grained soil, and as clay content is higher in fine-grained soil, the layer indicates smaller wet density, larger void ratio and natural water content. Fine-grained soils have about 70% average liquid limit and most of them are distributed near the A line of the plasticity chart. Unconfined compression strength and consolidation yield stress increase with depth, and they tend to be overconsolidation in shallow layers, while they are close to normal consolidation in deeper 30m.

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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

Urabe

Fujimoto

Kataoka

2011

Jour. Geol. Soc. Japan

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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.

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Holocene depositional systems along the Agano River of the Echigo Plain, central Japan

Cited by 4 publications

References 9 publications

Millennial-scale reworking of tephra in alluvial to shallow marine settings: Distinguishing pseudo-isochrons from genuine ones

Millennial-scale reworking of tephra in alluvial to shallow marine settings: Distinguishing pseudo-isochrons from genuine ones

Grain size distribution and engineering properties of subsurface layers in Niigata City area based on boring database

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

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