Abstract
The Kazusa Group is widely distributed in the Boso Peninsula and is composed of forearc basin marine sediments with a total thickness of about 3000 m. This group represents the type stratigraphy of the marine Pleistocene of the Japanese Islands and has been extensively investigated in the past. More than 500 tephra layers are intercalated in the Kazusa Group, and a detailed tephra stratigraphy has been established from the lower Kiwada Formation to the upper Kasamori Formation. Correlations of the tephras within the Kazusa Group with many widespread tephra layers have resulted in the construction of a detailed tephrostratigraphic framework for the Pleistocene tephrochronology of the Japanese Islands.
In this study, we compiled the most widespread tephras of the Kazusa Group identified by previous research. More than 20 tephra layers span the period 2.0-0.4 Ma. Our results indicate possible problems regarding the currently accepted tephra stratigraphy of the Kiwada Formation, including the possibility of double-counting and the order reversal of its tephra layers.
We also investigated the less well known tephras of the Katsuura, Namihana, and Ohara Formations of the lower Kazusa Group, resulting in the discovery of new fine glassy tephra layers. Two widespread tephra layers, namely, Fup-KW2 (2.2 Ma) and Bnd2-O1 (2.1 Ma) were identified in the lower Kazusa Group as a result of considering the correlations between new tephra layers and widespread tephras. These new tephra correlations constrain the base of the Kazusa Group to be older than 2.3 Ma.
