A tephra bed named CB2038 was found at a depth of 2038.26-2038.21 m in a boring core sample (hereafter called Chiba core) drilled at Chiba City, east of Tokyo Bay. The calcareous nannofossil assemblage from CB2038-bearing siltstone is characterized by the presence of small (< 3 μm) gephyrocapsids and Calcidiscus macintyrei, and the absence of discoasters, suggesting that CB2038-bearing strata correspond to the Ohara to the lower Kiwada formations (lower Pleistocene) exposed on the Boso Peninsula. Possible candidates to be correlative with CB2038 are Oh16A, B in the Ohara Formation, and Kd48.2, Kd48.1, Kd48, and Kd40 in the lowermost Kiwada Formation, because of the similarity of the shapes of glass shards and heavy mineral composition (hornblende > orthopyroxene). Among these tephra beds, Kd48 was considered the best candidate to be correlative with CB2038 based on refractive index, and major and trace element compositions of glass shards. Kd48 identified from deep underground near Tokyo Bay would make a significant contribution to the subsurface stratigraphy of basement strata around the Tokyo metropolitan area.
