Correlation of the tephra beds in the Middle Pleistocene Older Ontake Volcano with those in the Kazusa Group in Boso Peninsula, central Japan

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  • 古期御岳火山起源の中期更新世テフラと房総半島上総層群中のテフラとの対比
  • コキ オンタケ カザン キゲン ノ チュウキ コウシン セイ テフラ ト ボウソウ ハントウ カズサソウグン チュウ ノ テフラ ト ノ タイヒ

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Heavy mineral assemblage and chemical compositions of hornblende of the nine Middle Pleistocene tephra beds (Ku6E, Ku5C, Byakubi tephra, Ka2.4A, Ka2.4B, Ch3, Ch1.5, Ks18, Ks12) from the Kazusa Group, in Boso Peninsula were examined in order to correlate them with the tephra from the Older Ontake Volcano in central Japan. Hornblende composition from the nine tephra beds can be distinguished and two tephra beds, the Byakubi tephra (BYK) and the Ks12 tephra, were correlated with the tephras from the Older Ontake Volcano, the YUT4 or 5 of the H Substage and KZT of the Lava Stage, respectively. The age of these tephra beds of the Kazusa Group can be inferred from the stratigraphic relationships with the dated lavas on the foot of the Older Ontake Volcano, and from the well-known widespread tephras and magnetostratigraphy in Boso Peninsula. These two correlated tephra beds became valuable marker tephras for geochronological studies of both inland and marine sediments from central Japan. It should be also emphasized that the BYK and YUT4 or 5 can provide a datum plane of the Early-Middle Pleistocene boundary in both regions. The present study indicates that hornblende chemistry can be a useful tool for correlation of highly weathered tephra, in which volcanic glass is strongly altered.<br>

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