Stratigraphic and Petrographic Properties of the Tephra Layers in Hosoike Moor, Okayama Prefecture, Japan.

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  • Nomura Ryotaro
    Institute of Geography, Faculty of Letters, Kobe University
  • Tanaka Shingo
    Institute of Geography, Faculty of Letters, Kobe University
  • Kashiwaya Kenji
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Soma Hidehiro
    Department of Geography, Faculty of Letters, Nara Women's University
  • Ogura Hiroyuki
    Graduate School of Geosciences, Faculty of Science, Osaka City University
  • Kawasaki Teruo
    The Graduate School of Humanities and Social Sciences, Kobe University

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  • 岡山県北部，細池湿原のテフラについて
  • オカヤマケン ホクブ ホソイケ シツゲン ノ テフラ ニ ツイテ

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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 (U2) 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 14C ages which show the ages of the tephras. The age at deposit of MsP is estimated to be about 17, 400±1, 080980yrs BP and that of U2, about 15, 800±250yrs BP. These tephra layers are useful time markers for the Late Pleistocene and Holocene.

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