Reexamination of the stratigraphic position of the Upper Cretaceous Haraokayama Andesite, northwestern Yamaguchi Prefecture, Japan

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  • 山口県北西部，後期白亜紀原岡山安山岩の層序学的位置の再検討
  • ヤマグチケン ホクセイブ,コウキ ハクアキ ゲン オカヤマ アンザンガン ノ ソウ ジョガクテキ イチ ノ サイケントウ

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Abstract

<p>The stratigraphic positions of the Upper Cretaceous Haraokayama and Tenjōgatake andesite members of the Abu Group, located in the Nagato-Hōhoku area of Yamaguchi Prefecture in Japan, were reexamined through field survey, petrography, whole-rock geochemistry, and plagioclase K-Ar dating. We found that: 1) the Haraokayama and Tenjōgatake andesite members unconformably overlie a volcanic basin covered by the Abu Group; 2) the andesite members yielded ages of 72.8±1.6, 72.6±1.6, 71.4±1.6, 70.5±1.6, and 69.9±1.6 Ma, which are significantly younger than that of the Abu Group (90-80 Ma); and 3) the Haraokayama andesites have slightly different geochemical characteristics to the intercalated andesites within the Abu Group. Andesites with K-Ar ages of 73-70 Ma should therefore be separated from the Abu Group, and redefined as the Upper Cretaceous Haraokayama Andesite, which unconformably overlies the Abu Group from Nagato City to Hōhoku-Cho of Shimonoseki City. These units were designated previously as the Haraokayama Andesite Member of the Nagato Formation and the Tenjōgatake Andesite Member of the Shiratakiyama Formation. We suggest that the transition from ignimbrite flare-up in the Abu Group to steady-state arc volcanism, constrained by the Haraokayama Andesite, dates back to at least 70 Ma.</p>

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