Diatom Biostratigraphy of the Kazusa Group, Boso Peninsula, Honshu, Japan.

Abstract

Since the Kazusa Group is considered as the candidate to stratotype of the Early-Middle Pleistocene boundary in the Pacific region, it is urgent to investigate the Kokumoto Formation as a member formation of the Kazusa Group. The paleomagnetic boundary between C1r (Matuyama) and C1n (Brunhes) is recognized at the ash marker bed “Shirao” in the middle part of the Kokumoto Formation. The diatoms from the Otadai, Umegase, Kokumoto, Kakinokidai, and Chonan Formations along the Yoro River, in the middle part of the Boso Peninsula, about 80km southeast of Tokyo, have been studied. The marine sequences are dominated by shelf-water Paralia sulcata (Ehrenberg) Kützing indicating shallow water sedimentation. The seven diatom datum levels (biohorizons) allow precise correlation in the transitional region of the northwest Pacific Ocean. Among them the four biohorizons appear synchronous near the Early-Middle Pleistocene boundary in the Kokumoto Formation: LO (last occurrence) of Actinocyclus oculatus Jousé and LO of Rhizosolenia matuyamai Burckle at the lower part of the boundary, and LO of Nitzschia fossilis (Frenguelli) Kanaya and LO of Nitzschia reinholdii Kanaya at the upper part of the boundary. The regressions were recognized twice at the basal part of C1r. 1n (Jaramillo) in the Otadai Formation and across the C1r and C1n (Early-Middle Pleistocene boundary) in the Kokumoto Formation. The Kokumoto Formation is suitable as the candidate to stratotype of the Lower-Middle Pleistocene boundary of the Pacific region based on diatom biostratigraphy.