Abstract
The Pliocene Mita Formation outcrops in the Yatsuo-machi area of Toyama City, Toyama Prefecture, Japan. This formation containsthree key tuff beds (OT3, MT1, and MT2); two of these beds, OT3 and MT2, previously yielded ages of 5.2 ± 0.5 Ma and 2.2-2.3 Ma, respectively. This study focuses on the geology of the area surrounding the Akae-gawa River in the Hirabayashi region, and provides new fission track ages for the MT1 tuff exposed at two study sites in this area. These new data are combined with new fossil ostracod and planktonic foraminiferal faunal data to reconstruct the paleoenvironment during deposition of the MT1 tuff. This study presents new ages of 3.5 ± 0.2 and 3.4 ± 0.2 Ma for the MT1 tuff at the two sites that form the focus of this study. Integrating these data with the results of previous studies indicates that the first warm-water molluscs within the Mita Formation lived between 3.66 and ca. 3.4 Ma. Microfossil assemblage data from the study area also indicate that the paleoenvironment in the study site changed from a cool-water bay or shallow sea to an open temperate shallow sea setting, with warm-water ostracod species present in the uppermost horizon of the formation.
