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The Kazusa Group is widely distributed in the Boso Peninsula and is composed of forearc basin marine sediments with a total thickness of about 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 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 tephra layers span the period .-. 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-KW (. Ma) and Bnd-O (. 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. Ma.

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A small but explosive eruption occurred from Asama Volcano from 01:51 a.m. to around 08:00 a.m. on February 2, 2009 and the diluted tephra covered an elongated area extended SE direction passing Chichibu, Hamura, Fuchu, Inagi, Kawasaki and Yokohama Cities and reached Boso Peninsula. In Tokyo Metropolis and Saitama and Kanagawa Prefectures, we carried out a questionnaire survey about the distribution and amount of ash-fall deposit by this eruption. The result shows that most of the respondents found ash deposit in the area enclosed by the isopach of 1 g/m 2 and the ratio of the respondents who recognize ash deposit decrease remarkably out of the area. In the questionnaire, the respondents reported the semi-quantitative volume of ash deposit, judged from comparing their observations with a photograph showing an ash fall of 6 g/m 2. The result is almost consistent with the quantitative field survey by geologists, suggesting the possibility of semi-quantitative evaluation of ash fall with questionnaire.

The basement of the Tokyo metropolitan area consists of the Miocene–Pleistocene forearc basin fills that are well exposed around Tokyo Bay, especially on the Miura and Boso peninsulas. The forearc basin fills on these two peninsulas are called the Miura and Kazusa groups, and they were deposited during the late Miocene–Pliocene and Pliocene–middle Pleistocene, respectively. Because many biostratigraphic datum planes, paleomagnetic reversal events, and other chronostratigraphic tools are available for these deposits, they provide the “type stratigraphy” of other equivalent sedimentary sequences on the Japanese islands and in the northwest Pacific. However, the use of such stratigraphic markers has not been fully applied to understand the architecture of a basin-wide unconformity between the Miura and Kazusa groups called the Kurotaki unconformity. For our study, we made correlations among the Pliocene vitric tephra beds based on their stratigraphic levels, lithologic characteristics, the chemical compositions of glass shards, and calcareous nannofossil biostratigraphy. As a result, we were able to correlate tephra beds Ng-Ky25 just above the C3n.3n normal subchronozone (4.7 Ma), IkT16-An157.5 and IkT19-An158.5 near the top of the Mammoth reverse polarity subchronozone (3.21 Ma), and Ahn-Onr (2.6–2.7 Ma) across Tokyo Bay on the Miura and Boso peninsulas. We were able to recognize erosional surfaces and coeval mass-transport deposits immediately below the top of the Mammoth reverse polarity subchronozone, which suggests that submarine landslide(s) may have produced the lack of stratigraphic horizons (4.5–3.2 Ma) in the Miura and eastern Boso regions. Basal pebbly sandstone beds pervasively cover the erosional surfaces, and they show lateral variations into the thick (up to 60 m) mass-transport deposits and overlying turbidite sandstones. The lateral variations in sediment thickness of the post-failure deposits suggest that the basin-wide erosion was associated with the initial growth of a basin-bounding structural high that separates two distinct sub-basins in the forearc basin, which resulted in the subsequent onlapping deposition in the earliest stage of the Kazusa forearc basin. The basin-wide erosion marks the initiation of tectonic reconfigurations that led to segmentation of the forearc basin around the Tokyo Bay region.