2023 Volume 129 Issue 1 Pages 469-487
The lower-middle Pleistocene Kazusa Group (forearc basin fill) and lower Pleistocene Chikura Group (trench slope basin fill) on Boso Peninsula of central Japan have been studied intensely and are the type sections for the northwestern Pacific region due to the abundant tephra beds and robust chronological framework obtained using biostratigraphy and magnetostratigraphy. We demonstrate a magneto-tephrostratigraphic correlation between the lower part of the Kazusa Group and the Hata Formation of the Chikura Group. We identified two zones of normal polarity, in the uppermost Katsuura Formation and in the upper Ohara Formation through to the lower Kiwada Formation of the Kazusa Group, which can be correlated with the Feni and Olduvai normal polarity subchrons, respectively. The horizons of reversed polarity between these subchronozones and below the Feni belong to the Matuyama reversed polarity chron. Based on the stratigraphic positions, petrographic characteristics, and major and trace element compositions of volcanic glass, the Kmj-3, Kmj-10, Kmj-18, Kmj-29, Kmj-41, Kmj-53, Kmj-68, and Kmj-71 tephra beds in the Hata Formation can be correlated with the Kr31, KRm, KH2, IW2, OFN2, KB, HS C, and HS A tephra beds in the Kazusa Group, respectively. These tephra correlations are consistent with the magnetostratigraphy of both groups. Our results highlight some important marker tephra beds, including IW2 in the Feni normal subchronozone and HS C and HS A, immediately below and above the lower Olduvai reversal boundary, respectively. Although the thickness of the lower Kazusa Group is generally several times that of the corresponding Hata Formation, both forearc and trench slope basins would have developed in parallel with no apparent erosional gap between 2.2 and 1.9 Ma.
