Abstract
New evidence is provided to correlate of eleven key tephra layers in the Lower Pleistocene strata between the Inubou Group in the Choshi district and the Kazusa Group in the Boso Peninsula. On the basis of petrographic and chemical characteristics of key tephra layers, the Ob1, Ob2, Ob4a, Ob4c, Ob5a, Ob5.1, Ob5.6, Ob5.7, Ob6a, Ob6c and Ob7b in the Obama Formation of the Inubou Group can be correlated with the Kd39, Kd38, Kd25, Kd24, Kd16 and Kd5A in the Kiwada Formation, the O7 and O3 in the Otadai Formation and the U8, U7 and U6A in the Umegase Formation of the Kazusa Group, respectively. Calcareous nannofossil datums detected in the Obama Formation also support this result. Tephrostratigraphic and biostratigraphic data indicate that the Obama Formation can be assigned to the lower part of the Kiwada to the middle part of the Umegase Formations. Chronological data demonstrate the sedimentation rates markedly decreased in the sandy interval, corresponding to the middle part of the Obama Formation. Our study suggests that further paleoecological and paleoceanographical studies in the Lower Pleistocene strata will be needed to clarify the cause of the extremely slower sedimentation in the middle part of the Obama Formation.
