Abstract
We present a revised Cenozoic chronostratigraphy of the Yatsuo Area, Toyama Prefecture, based on U-Pb and fission-track (FT) dating of zircon grains from tuff beds, mineralogical analysis of tuff beds, and diatom biostratigraphy. The results reveal that syn-rift volcanism (represented by the Iwaine Formation) began at ~17.5 Ma and that the Ikahama unconformity between the syn-rift Higashibessho Formation and the post-rift Tenguyama Formation represents a relatively short-duration (~0.5 m.y.) event ~15 Ma. The Otogawa Formation unconformably overlies the Tenguyama Formation and is divided into middle Miocene (Serravallian) and late Miocene, with a possible unconformity in between. The 2-4 Ma Mita Formation unconformably overlies the Otogawa Formation. These results require previous local correlations of a key tuff bed (MT2) in the Mita Formation to be reassessed.
Facies and sequence stratigraphic analysis of the lowermost Nirehara Formation confirm that this formation comprises multiple depositional cycles generated by water-level fluctuations in a coastal or lacustrine fan delta. The results also suggest that the Nirehara Formation, which is unconformably overlain by the syn-rift Yatsuo Group, was formed in an early rift basin. Detrital zircon U-Pb and FT dating of sandstone within the Nirehara Formation suggests that the sandstone may be derived from the Nohi Rhyolite.
Regional correlations of onshore and offshore Cenozoic chronostratigraphy in the Hokuriku district show multistage rifting during the Oligocene-middle Miocene opening of the Sea of Japan. Regional correlations also suggest that the post-rifting compressive regime was associated with fluctuations in the strength and direction of compressive stress fields.
