広島県三原市西部に分布する未命名古第三系のジルコンU-Pb年代

抄録

Paleogene formations locally distributed in the southern Chugoku region and the eastern coastal areas of the Seto Inland Sea, southwest Japan, were analyzed in this study. The Paleogene strata were roughly divided based on their major constituents into a formation comprising non-marine and marine sandstones and mudstones and a second formation composed of fluvial conglomerates that are lithologically similar to the previously named “mountain gravels”. However, detailed age information is unavailable for some of these strata. The first reliable zircon U-Pb age was obtained using laser ablation-inductively coupled plasma-mass spectrometry on zircon grains from a fine-grained sandy tuff intercalated with an unnamed conglomerate bed that, based on lithology, can be correlated with the “mountain gravels” distributed in the western part of Mihara city, Hiroshima Prefecture. The zircon grains yield a U-Pb age of 40.8±0.1 Ma (weighted mean age of the youngest cluster), which is interpreted as the maximum depositional age of the sandy tuff, thus establishing a middle Eocene age for this formation in the southern areas of the Chugoku region.