Abstract
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.
