利根川支流，鏑川流域における飯縄火山起源の中期更新世テフラ

摘要

Three major treads of fluvial terraces (lower, middle, and higher terraces) are well developed along the Kabura-gawa river, tributary of the Tone-gawa river, in the southern part of Gunma prefecture, northern Kanto region. We detected cryptotephras in overbank and aeolian deposits covering the middle terrace gravels, and examined refractive indices and chemical compositions of some of amphiboles in the cryptotephras. We also investigated refractive indices and chemical compositions of amphiboles in the Iizuna-Kamitaru tephra (Iz-KT) and the Iizuna-Nishiyama (Iz-NY) tephra extracted from outcrops near the Iizuna volcano, northern Nagano prefecture. Based on these data, wernidentified possibly Iz-KT and Iz-NY tephra at the bottom of overbank and aeolian deposits of the middle terrace of the Kabura-gawa river. This indicates that the middle terrace was formed through aggradation at the glacial age of marine oxygen isotope stage (MIS) 6 and subsequent degradation at interglacial of MIS 5e. The Kabura-gawa river basin is located out of the previously reported distribution area of Iz-KT. This fact implies that Iz-KT is distributed more widely in the northern Kanto and southern Tohoku regions, and that Iz-KT is useful as marker tephra of the middle Pleistocene in these regions.%利根川支流の鏑川流域における中位段丘面構成層の堆積年代を明らかにするため，被覆層を連続採取し，クリプトテフラの検出を試みた．そのうち，いくつかの層準で，角閃石とカミングトン閃右の屈折率，カミングトン閃石の主成分化学組成などの岩石学的特徴を記載した．その結果と，長野県北部の妙高火山群の1つである飯縄火山付近の露頭における飯縄上樽テフラ層，飯縄西山テフラ層の岩石学的特徴とを比較した．その結果，鏑川流域の中位段丘面を覆う細粒堆積物の最下部付近に，飯縄上樽テフラ，飯縄西山テフラの可能性が高いテフラが検出された．このことにより，鏑川流域の中位段丘ほ酸素同位体ステージ6 に形成された堆積段丘である可能性が高いことが示された．本稿で飯縄上樽テフラが確認された地点は，これまで指摘されていた飯縄上樽テフラの分布域から南へ大きく外れている．これは飯縄上樽テフラが関東北部～東北南部にも広く降灰した可能性を示し，この地域の中期更新世の指標テフラとして，飯縄上樽テフラが広く利用できることを示す．