Estimation of the depositional age of the lower part of Eocene Kyoragi Formation in the Uto Peninsula, western Japan

The Eocene non-accretionary strata in the Amakusa region of western Japan, vary in thickneess, suggesting differential tectonic subsidence during deposition.However, their depositional ages are not well constrained, particularly in the eastern part of the region where the strata are especially thick.This hampers our understanding of the style of Eocene tectonic subsidence in the region.To obtain additional age constraints, I obtained a zircon U-Pb age from a thin tuffaceous sandstone intercalated within the lower part of the Eocene Kyoragi Formation on the Uto Peninsula.Ten concordant zircon grains which comprise the youngest age cluster yielded a weighted mean age of 46.9±0.4Ma (2σ, MSWD = 1.0).This age is interpreted as the maximum depositional age of the tuffaceous sandstone bed, which does not conflict with microfossil assemblages from the Kyoragi Formation and its equivalent, the Shikiyama Formation.