Depositional Process of the Holocene Nobi Plain, Central Japan, Reconstructed from Drilling Core Analysis

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  • 濃尾平野ボーリングコア解析にもとづく完新統の堆積過程
  • ノウビ ヘイヤ ボーリングコア カイセキ ニ モトヅク カンシントウ ノ タイセキ カテイ

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Abstract

The Nobi Plain is located in the lower reaches of three Japanese major rivers-the Kiso, Nagara, and Ibi-which run from the Central Alpine Region toward Ise Bay, Central Japan. This plain is typical in its high subsidence rate and relatively strong fluvial processes with well-developed fluvial landforms, such as alluvial fans and natural levees.<br>Holocene depositional processes of the Nobi Plain, which is 45km long and 30km wide, were reconstructed from detailed AMS 14C dating and facies analyses of drilling cores. The Holocene sediments, which are divided into six sedimentary units (based on their facies)-basal gravel, incised valley, prodelta, deltafront, delta plain, and tidal or fluvial channel, in ascending order-were deposited under sea-level and tectonic influences. Accumulation rates in the Nobi Plain were relatively high compared with other Japanese Holocene sedimentary basins, and varied from 1 to 53mm/yr; the variation is related to the relative sea-level change, which is attributed to high sediment flux from the rivers and rapid subsidence of the basin. The lower two depositional facies were deposited in the transgressive stage from 12, 000yrs BP to 5, 900cal yrs BP, and the upper four depositional units forming the Kiso river delta covering the large area of western part of the Nobi plain prograded in the regressive stage that followed. Progradation rates have been low (6m/yr) during 5, 900-4, 200cal yrs BP, high (10m/yr) during 4, 200-2, 800cal yrs BP, and low again (5m/yr) from 2, 800cal BP to the present, corresponding to changes in the geometry of the sediment supply and depositional systems.

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