報告書番号

N12007

タイトル（和文）

段丘の対比・編年の信頼性向上のための風化指標の検討（その２）－段丘礫の風化状態の把握と対比指標の適用条件の提示－

タイトル（英文）

Study on weathering index for improving the reliability of terrace correlation and chronology, Part 2 - Understanding weathering condition of terrace gravel and induction of application requirement for correlation index -

概要 （図表や脚注は「報告書全文」に掲載しております）

地形・テフラ層序のデータが豊富な新潟県中越地方で，河成段丘の地形調査，地表地質踏査，ボーリング調査，河成段丘礫の風化性状の調査・分析を行った．その結果から，段丘礫の風化程度を段丘対比指標として見なすことの妥当性，および観察・測定する上での留意点、試料採取方法を示した．
ボーリング調査では，風化皮膜厚，有効間隙率が大きく、風化がより進んだ段丘礫が分布するのは、孔口から5m以浅に限られる．すなわち，段丘礫層の風化は深部に及んでおらず，風化程度の調査・評価では，浅層部を対象とする必要がある．段丘礫層内の礫の風化皮膜厚，有効間隙率は，値にばらつきがあるが，風化皮膜厚は安山岩礫に限定することにより，また、有効間隙率は礫径を揃えることにより，測定値のばらつきは低減する．これにより，調査地域の多くの地区では，形成年代に対する変化傾向が明瞭となり，対比指標としての有効性が認められる．段丘礫層内では各データのばらつきが大きいが，近接する同時代の段丘では露頭間の風化程度の顕著な差はない。これらのことは、段丘礫の風化皮膜厚と有効間隙率が形成年代の定量的指標として妥当であることを示す．風化皮膜厚が対比指標としての有効な地区では，安山岩礫の風化皮膜の形成速度は，0.04mm/1000年程度であり，風化皮膜厚からMIS6段丘とMIS8段丘は区別が可能である．この値は，本州内の既往報告と整合的であり，本州の広い範囲で適用が期待される．

概要 （英文）

Geomorphographic survey of fluvial terraces, geological exploration, borehole drilling and investigation, and analysis of weathering condition of terrace gravels were carried out in Chuetsu area, Niigata prefecture, where a great deal of geomorphostratigraphic and tephrostratigraphic data are available. The results of these surveys and investigations indicate that weathering degree of terrace gravels can be considered as an index of the terrace age, and also provide points to remember for sampling and method of sampling and observation. The effective porosity and the thickness of weathering rind of gravels, which are indexes for weathering degree evaluation, in boring core, increase above the depth of about 5m from the top of the hole. Weathering doesn't reach the deep portion, therefore, investigation and evaluation for the weathering degree of terrace gravels must be carried out on the upper portion. Weathering rind thickness and effective porosity of the gravels are dispersive. Dispersion of the weathering rind thickness can be reduced by confining to andesite, and dispersion of the effective porosity can be reduced by limiting range of gravel size. Reducing dispersion, increase trend with age becomes clear in change of the weathering rind thickness and the effective porosity in many of the studied area. It shows that weathering rind thickness and effective porosity are effective for terrace correlation. Dispersion of data in an outcrop isn't small, but data from neighboring terraces with the same age are not different each other. It indicates that weathering rind thickness and effective porosity can be quantitative indexes for terrace age evaluation. In area where weathering rind is effective for terrace correlation, the rate of the weathering rind formation of andesite gravels is about 0.04mm/1000 years. Therefore, MIS6 terraces and MIS8 terraces can be distinguished each other by means of thickness of the weathering rind. This formation rate falls inside the range of previous reports from Honshu region, so it can be expected to apply in widespread area of Honshu region.

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