Refined chronostratigraphy of the Miocene Nanto and Yatsuo Groups in the Toyama basin, SW Japan, based on radiometric dates and stratigraphy

Yamada, Raiki; Nagata, Mitsuhiro   ; Takahashi, Toshiro*; Ueda, Hayato*

In this study, lithostratigraphy and zircon U-Pb dates for the Miocene Nanto and Yatsuo Groups in the Toyama basin, SW Japan, were examined. Obtained lithostratigraphy and zircon U-Pb dates were compared to radiometric dates, magnetostratigraphy, and microfossil biostratigraphy by previous studies. Depositional and emplacement ages of the target strata can be summarized as following stages on the basis of those results: (1) Nanto stage (23.0-22.3 Ma), (2) Iwaine stage (17.5-17.3 Ma) after formation of an unconformity (22.3-17.5 Ma), (3) Iozen-Kurosedani stage (17.3-16.6 Ma), and (4) Higashibessho-Fukuhira stage (16.6-15.2 Ma). Formation ages of each formation were estimated more narrowly. Additionally, formation ages of several formations could be extrapolated in 0.01-0.001 Myr scales. These ages were constrained much narrowly than recent works. Moreover, these results propose that combination between developing radiometric dating techniques and detailed stratigraphic data enable us to discuss highly resolved chronostratigraphy and formation processes of Miocene sedimentary basins in 0.1 Myr scale.