Significance of the remarkable unconformity in the Plio–Pleistocene of the Japanese Islands

The Kurotaki Unconformity is located in the Boso Peninsula between the subduction zone in the Pacific Plate and the Philippine Sea Plate. The underlying stratum (Miura Group) and overlying stratum (Kazusa Group) bordering the Kurotaki Unconformity are greatly different with respect to geologic structure and age. The influence of the Pacific Plate is reflected in the Kazusa Group, and that of the Philippine Sea Plate in the Miura Group. Thus, the Kurotaki Unconformity gives a geological constraint on tectonic conditions around the Japanese Islands. This study has clarified of the formation age of the Kurotaki Unconformity based on tephrochronology. The age of the unconformity is ca. 3.0–2.8 Ma. The formation age of the unconformity is specified based on tephrochronology. Common and widespread tephra layers were found in strata bordering the unconformity. The formation of the regional Kurotaki Unconformity indicates a widespread event in the vicinity of the newly designated Pliocene–Pleistocene boundary in the Japanese Islands.