地球科学
Online ISSN : 2189-7212
Print ISSN : 0366-6611
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  • 松岡 喜久次, 桑原 希世子
    原稿種別: 短報
    2021 年 75 巻 2 号 p. 119-124
    発行日: 2021/04/25
    公開日: 2021/06/30
    ジャーナル フリー

    The Capitanian (Permian) radiolarians were found from a siliceous tuff block within volcaniclastic rocks in the Sumaizuku Unit of the Northern Chichibu Belt in the Kanto Mountains. The Sumaizuku Unit is composed of chaotic rocks consisting of exotic blocks of chert and mafic rocks in Jurassic clastic matrix. The rocks of studied area consist of limestone-basalt conglomerate, lime-sandstone, volcaniclastic rocks and chert. The volcaniclastic rocks are composed of clasts of basalt lava and volcanic glass accompanied with clasts of limestone, siliceous tuff and fragment of plagioclase. The clasts of siliceous tuff containing radiolarian tests are angular pebble to boulder. The siliceous tuff is regarded as blocks which were mixed in volcaniclastic rocks by slumping. We consider that the volcaniclastic rocks deposited immediately after Capitanian age, and this deposition formed on the lower flank of a seamount.

  • 小松 宏昭, 小坂 秀王
    原稿種別: 短報
    2021 年 75 巻 2 号 p. 125-130
    発行日: 2021/04/25
    公開日: 2021/06/30
    ジャーナル フリー

    The Akashibuzawa Formation is distributed in the Okaya-Shimosuwa area, Nagano Prefecture, central Japan, and consists mainly of basalt and basaltic andesite lavas and some subordinate pyroclastic units. The Akashibuzawa Formation unconformably covers the Latest Miocene Tozawa Formation consisting mainly of volcanic rocks of quartz andesite, and is, in turn, unconformably overlain by the Enrei Formation of Early Pleistocene in age. A wide range of K-Ar ages between 1.54 ± 0.07 Ma and 2.24 ± 0.13 Ma has been previously reported from the Akashibuzawa Formation. Really fresh samples of basalt and basaltic andesite have been collected from the Akashibuzawa Formation in order to determine more precise timing of the volcanic activity. Whole-rock K-Ar ages of three samples are obtained to be 2.31 ± 0.04 Ma, 2.77 ± 0.06 Ma and 2.82 ± 0.05 Ma. These are significantly older than the ages previously reported, and likely represent a more accurate age of the volcanic activity of the Akashibuzawa Formation.

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