Planktonic foraminiferal and diatom biostratigraphy of the upper part of the Miocene Ichishi Group in Mie Prefecture, central Honshu, Japan

We present new biostratigraphic data from an exposed Miocene sedimentary section of the upper part of the Ichishi Group in Mie Prefecture, central Honshu. Mudstone samples were collected from outcrop exposures, mainly found along a river, where we mapped lithologies and structures. Planktonic foraminiferal assemblages consisting mainly of warm water species were obtained from 21 samples above and below the lithostratigraphic boundary between the Mitsugano Member (upper part of the Oi Formation) and the overlying Chaya Member (lower part of the Katada Formation). The sedimentary sequence is correlated to zones N.7 and N.8 of the planktonic foraminiferal zonal scheme. Importantly, our results from high-resolution sampling show that the N.7/N.8 zonal boundary is located within a horizon ~19 m below the Oi/Katada formation boundary. Diatom fossils of the Crucidenticula sawamurae Zone (NPD2B; 18.1-17.0 Ma) were found in calcareous concretions within the Mitsugano Member below the N.7/N.8 zonal boundary. Our results suggest that the section is a suitable candidate for assessing the validity of early-middle Miocene planktonic foraminiferal biochronology.