海洋酸素同位体ステージ５‐６境界に降下した飯縄上樽テフラ群とその編年学的意義

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タイトル別名
  • Iizuna-Kamitaru Tephra Group Erupted from the Iizuna Volcano of Myoko Volcano Group in the Transition from Marine Isotope Stage 6 to 5, and Its Significance for the Chronological Study of Central Japan.
  • カイヨウ サンソ ドウイタイ ステージ 5 6 キョウカイ ニ コウカ シタ イイズナ カミタル テフラグン ト ソノ ヘンネンガクテキ イギ

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The Iizuna-Kamitaru Tephra Group, which consists of three distinctive fallout tephras (Iz-KT a to Iz-KT c in descending order), erupted from the Iizuna volcano, a Quaternary stratovolcano of the Myoko Volcano Group in central Japan. The purpose of this paper is to describe the characteristic properties, distribution, and stratigraphic position in several areas of this group, to discuss its age, and to show the possibility of applicative studies. Iz-KT a (9.8km3), the most widespread tephra of the group, is a plinian deposit distributed in the boundary area between Nagano and Niigata prefectures, north part of Gunma and Tochigi prefectures. Iz-KT a is petrographically identified by min eral composition, characteristic shape of orthopyroxene, refractive indices of orthopyroxene (γ: 1.707-1.716) and hornblende (n2: 1.673-1.687), and chemical composition of titanomagnetite (FeO: 82.0-82.5%, TiO2: 5.9-6.0%). Stratigraphic positions of the Iizuna-Kamitaru Tephra Group in the tephra sequence of distal areas and radiometric ages by the K-Ar dating method carried out in proximal areas show that the deposition of the group occurred between 150ka and 125ka of the transition from the marine isotope stage 6 to 5. This group is useful for identification of climatic fluvial terraces formed in the glacial stage of marine isotope stage 6, and provides a datum plane for eruptive histories of several Quaternary volcanoes in this area.

収録刊行物

  • 第四紀研究

    第四紀研究 40 (1), 29-41, 2001

    日本第四紀学会

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