岡山県北部, 細池湿原のテフラについて Stratigraphic and Petrographic Properties of the Tephra Layers in Hosoike Moor, Okayama Prefecture, Japan

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岡山県北部の細池湿原の堆積物に挾まれるテフラの検討を行なった.細池湿原の地表面は南へ平均1.4°傾斜しており,堆積物の上部は泥炭層,その下位は有機質シルト質粘土層,有機質砂質シルト層よりなる.堆積物中には10層のテフラが挾まれており,それらのうち2層は広域テフラの姶良Tn火山灰(AT)・鬼界アカホヤ火山灰(K-Ah)であり,3層は大山起源の弥山軽石(MsP),上のホーキ火山灰(Uh),三瓶山起源の浮布軽石(U<sub>2</sub>)に対比される.U<sub>2</sub>とMsPの降下年代は<sup>14</sup>C年代値より,それぞれ15,800±250yrs BPの直後,17,400±<sup>1,080</sup><sub>960</sub>yrs BP頃と推察できる.また,湿原内におけるテフラの分布状態およびテフラと堆積物の関係から,細池湿原はAT降下以前から存在しているが,MsP降下以前に湿原を侵食し,谷を形成する時期があったこと,その谷はMsP降下以前に埋積が開始され,MsP降下以降,湿原が拡大したことが明らかになった.

Hosoike Moor, in the northern part of Okayama Prefecture, is one of the largest moors in western Japan, and is important as it provides information about long-term environmental changes since the Late Pleistocene. Our purpose was to study the stratigraphy and petrography of the tephra layers found in this moor.<br>The stratigraphical examination of 12 samples extracted by boring show that 10 tephra layers are intercalated in and under the moor sediments. The stratigraphical order is Hs-5, Hs-6, Hs-7, Hs-8, Hs-2, Hs-3, Hs-4 (Hs-9: to be correlated), Hs-10, Hs-1 (Hs-11) and Hs-12, from upper to lower.<br>Their petrographic properties show that Hs-5 is correlated to the Kikai-Akahoya ash (K-Ah: 6, 300yrs BP) derived from the Kikai Caldera in Southern Kyushu; Hs-3 is correlated to Ukinuno Pumice (U<sub>2</sub>) derived form the Sanbe volcano ; Hs-4 (Hs-9) and Hs-10 are respectively correlated to Misen Pumice (MsP) and Uenohoki (Uh) derived form Daisen Volcano in the Chugoku Mountains. Hs-1 (Hs-11) is correlated to Aira-Tn ash (AT: about 25, 000yrs BP) derived from the Aira Caldera in Southern Kyushu. Hs-12 is correlated to the Middle or Lower Volcanic Ash of Daisen Volcano. In addition, several thin tephra layers are deposited between the AT and K-Ah layers. Their source volcanos, however, could not be pinpointed. We also obtained some <sup>14</sup>C ages which show the ages of the tephras. The age at deposit of MsP is estimated to be about 17, 400±<sup>1, 080</sup><sub>980</sub>yrs BP and that of U<sub>2</sub>, about 15, 800±250yrs BP. These tephra layers are useful time markers for the Late Pleistocene and Holocene.

収録刊行物

  • 第四紀研究

    第四紀研究 34(1), 1-8, 1995-03-31

    Japan Association for Quaternary Research

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各種コード

  • NII論文ID(NAID)
    10002632372
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    3610334
  • NDL 刊行物分類
    ME41(火山・岩石)
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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