2024 Volume 130 Issue 1 Pages 205-221
Sedimentary rocks containing molluscan and cetacean fossils occur along the Narashi–Moto Coast in Iwado, Muroto City, Kochi Prefecture, southwestern part of Shikoku Island, Japan. Here we describe the Narashi Formation of the Tonohama Group and estimate its depositional age based on planktonic foraminifera and calcareous nannofossil assemblages. The depositional age indicated by planktonic foraminifera is 8.58–4.37 Ma, and the age inferred from the calcareous nannofossil assemblage is 5.53–3.82 Ma. Therefore, the Narashi Formation is estimated to have been deposited at 5.53–4.37 Ma. The formation contains a pectinid-dominated molluscan association (i.e., the Amussiopecten–Mimachlamys–Amusium association). Based on the extant species present in the association, we estimate that the formation was deposited at depths shallower than 20–30 m. This association includes several characteristic species of the Kakegawa fauna, a Pliocene to early Pleistocene molluscan fauna on the Pacific side of southwest Japan. In contrast, the assemblage does not contain characteristic species of the Zushi fauna, which is a warm-marine molluscan fauna of the late Miocene to early Pliocene. The results suggest the Kakegawa fauna became established in Shikoku before 4.37 Ma.
