Zircon FT and U–Pb ages of Neogene tuffs and tuffaceous sandstones and their stratigraphic implication in Memanbetsu and adjacent areas, Hokkaido, Japan

<p>The Neogene lithostratigraphy in the Memanbetsu area, eastern Hokkaido, Japan, has been studied since the 1950s. However, stratigraphic reconstructions are hampered by a lack of age data. In this study, the fission-track (FT) and uranium–lead (U–Pb) ages of zircon grains were obtained via LA-ICP-MS for three tuff or tuffaceous sandstone samples from the Memanbetsu and adjacent areas. Considering previous study of lithostratigraphic correlations, the samples were collected from the lower Miocene Tokoro Formation (ID No. 2089), the lower part of the middle Miocene Mito Formation (ID No. 19091812-1), and the Pliocene Misaki Formation (ID No. 19061601). The weighted mean U–Pb and pooled FT ages of the youngest zircon grains were 11.9±0.2 Ma (2σ) and 10.8±0.9 Ma (1σ) for ID No. 2089, 12.4±0.2 Ma (2σ) and 13.4±0.8 Ma (1σ) for ID No. 19091812-1, and 8.7±0.1 Ma (2σ) and 8.2±0.4 Ma (1σ) for ID No. 19061601, respectively. Therefore, the stratum formerly referred to as the Tokoro Formation is equivalent in age to the upper Miocene Notoro Formation. In addition, on the basis of its lithological characteristics, geological continuity with adjacent areas, and previously reported ages from dinoflagellate cysts, the stratum is attributed to the Toika Formation. The ages obtained from the lower part of the Mito Formation correspond to the middle–upper Miocene Abashiri Formation, which supports the existing stratigraphic correlations. The ages obtained from the Misaki Formation suggest deposition during the late Miocene. Based on the radiometric ages, lithological characteristics, and geological distribution, the stratum is attributed to the Abashiri Formation. The results enabled us to re-evaluate the Neogene stratigraphy in the Memanbetsu area.</p>