Paleogene formations in the northwestern part of Mihara City, Hiroshima Prefecture, south–central part of the Chugoku Region, Japan: Lithofacies and zircon U–Pb ages of interbedded fine tuff

Abstract

We studied the chronology of Paleogene formations in the northwestern part of Mihara City, Hiroshima Prefecture, Japan. Paleogene rocks in the study area consist primarily of arkosic granular conglomerates interbedded with sandstone, mudstone, and fine tuff. We obtained zircon U–Pb ages from tuff and silicified wood and performed a preliminary investigation of pollen fossils. A weighted mean U–Pb age of 39.2 ± 0.1 Ma was obtained, indicating that the tuff layer is Eocene (Bartonian) in age. This age, if interpreted as the Paleogene depositional age for the study area, is older than previously assumed, suggesting that the chronology of the Paleogene should be revised. The silicified wood is white and poorly preserved; therefore, it could not be comprehensively assessed. However, we determined that the wood belongs to the Taxodioxylon sp. of the family Cupressaceae. Pollen fossils include Fagaceae, which are similar to the evergreen type Quercus sp. and Castanopsis sp. These results provide the first firm evidence for the conventional age estimation of Paleogene formations in the study area and indicate the existence of terrestrial and fluvial depositional environments in the south-central part of the Chugoku region during the Eocene (Bartonian).