Fission track ages of eight tuffs in the upper part of the Awa Group, Boso Peninsula, central Japan.

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  • 房総半島安房層群上部の８凝灰岩のフィッション・トラック年代
  • ボウソウ ハントウ アワソウグン ジョウブ ノ 8 ギョウカイガン ノ フィッショントラック ネンダイ

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Abstract

Though a number of tuffs are intercalated in Amatsu, Kiyosumi and Anno Formation in ascending order, the upper part of the Neogene Awa Group, Boso Peninsula, central Japan, very few radiometric ages for these tuffs have been reported. The present authors report the fission track ages (FT ages) of zircon crystals from eight marker tuffs in those formations, i.e. Am19, Am40, Am61 Am78(Ok tuff), Am94, Ky21(Hk tuff), An49 and An 73 in ascending order.The followings are the main results of the present study.1) Dated FT ages for eight tuffs are as follows : Am 19 : 11.7±0.3Ma, Am 40 : 8.5±0.5Ma, Am 61 : 6.3±0.5Ma, Am 78 (Ok tuff) : 5.7±0.4Ma, Am 94 : 5.2±0.3Ma, Ky 21 (Hk tuff) : 4.5±0.2Ma, An 49 : 3.9±0.4Ma, An 73 : 3.7±0.2Ma. These data are the first FT ages following the proposal of IUGS Subcomission in 1990 for the tuffs in those formations.2) The present FT ages are a little younger than previously reported FT ages, while coincide well with the latest K-Ar ages for a few tuffs among eight tuffs dated in this paper.3) The present FT ages for tuffs in the lower to middle parts of the Amatsu Formation and in the Anno Formation coincide relatively well with biostratigraphic ages based on some key species of planktonic foraminifers and nannofossils, while those in the upper part of the Amatsu Formation and in the Kiyosumi Formation are significantly younger compared with those biostratigraphic ages. The cause of this difference is not known.

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