U-Pb zircon ages of the lower part of the Eocene Nogata Group in the Chikuho Coalfield, Fukuoka Prefecture, northern Kyushu, Japan

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  • Miyata Kazunori
    Fukui Prefectural Dinosaur Museum Institute of Dinosaur Research, Fukui Prefectural University
  • Nagata Mitsuhiro
    Graduate School of Science and Engineering for Education, University of Toyama
  • Niki Sota
    Geochemical Research Center, University of Tokyo
  • Hattori Kentaro
    Division of Earth and Planetary Sciences, Kyoto University
  • Obayashi Hideyuki
    Division of Earth and Planetary Sciences, Kyoto University
  • Hirata Takafumi
    Geochemical Research Center, University of Tokyo
  • Otoh Shigeru
    Academic Assembly, Faculty of Sustainable Design, University of Toyama

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  • 九州北部福岡県筑豊炭田の始新統直方層群下部のジルコンU-Pb年代
  • キュウシュウ ホクブ フクオカケンチクホウ タンデン ノ シ シントウノオガタソウグン カブ ノ ジルコン U-Pb ネンダイ

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Abstract

<p>We report new uranium-lead (U-Pb) zircon ages for strata in the Sanjakugoshaku Formation in the lower part of the Eocene Nogata Group of the Chikuho Coalfield, Fukuoka, northern Kyushu, western Japan, to provide a better geochronological control for the group than a previously reported fission-track date (44.2±3.4 Ma) from the Uwaishi Formation in the upper part of the Nogata Group. The U-Pb zircon dates were obtained via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) for four different tuff and tuffaceous layers in the Sanjakugoshaku Formation, yielding ages of 46.18±0.59, 44.21±0.49, 41.53±0.28, and 40.34±0.20 Ma, in stratigraphic order. These new dates indicate that deposition of the Sanjakugoshaku Formation lasted for more than 5 Myr (including hiatuses), with this period corresponding to most of the chronological interval of the Okinoshiman molluscan Stage (middle middle Eocene to late Eocene?) in Kyushu. Furthermore, the new dates suggest that the basal part of the Sanjakugoshaku Formation and underlying Oyake Formation (the lowest unit of the Nogata Group) are chronologically correlated with the older Takashiman molluscan Stage (early Eocene to early middle Eocene). Therefore, the tectonic processes responsible for the formation of the sedimentary basin of the Chiku-ho Coalfield appear to have started at latest by the early middle Eocene.</p>

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