Fission-Track Age and Correlation of the Takatsukayama Volcanic Ash Layer Distributed at the Western Foothills of the Rokko Mountains, Western Japan.

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  • 六甲山地西麓に分布する高塚山火山灰層のフィッション・トラック年代とその対比
  • ロッコウ サンチ セイロク ニ ブンプ スル タカツカヤマ カザンバイソウ ノ フィッション トラック ネンダイ ト ソノ タイヒ

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Abstract

The Takatsukayama Member of the Meimi Formation is distributed at the western foothills of the Rokko Mountains, western Japan, and has been known as the Middle Pleistocene deposit intercalating a marine bed. The Takatsukayama volcanic ash layer is intercalated in the terrestrial bed immediately below the marine sediment. It was dated to be 0.41±0.12Ma by the fission-track method from 30 zircon crystals, indicating the eruption age of the ash to be ca. 0.4Ma. Petrologic characteristics (such as heavy mineral composition and types of volcanic glass shards), and refractive index of glass shards and green hornblende crystals were examined for this volcanic ash layer.<br>On the basis of this eruption age, and of the petrologic characteristics and refractive index of glass shards, the Takatsukayama volcanic ash layer is correlated with the Minatojima II volcanic ash layer in the Ma 9 marine clay of the Osaka Group (Miyakawa et al., 1996), as was suggested by Inoue and Yoshikawa (1998). The BT 76 volcanic ash layer in the Takashima-Oki core sample from Lake Biwa (Yoshikawa and Inouchi, 1991), which erupted at ca. 0.39Ma (Yoshikawa and Inouchi, 1993), is also considered to be equivalent to the Takatsukayama volcanic ash.<br>The correlation and age estimation of the Takatsukayama volcanic ash layer led to the conclusion that the marine bed of the Takatsukayama Member is equivalent to the Ma 9 marine clay and was deposited during the period corresponding to the oxygen isotope stage 11.

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