抄録
東京都中央部の文京区と練馬区で掘削された温泉井,「豊島園温泉井」と「東京ドーム温泉井」の石灰質ナンノ化石調査を行い,地質年代の決定と房総半島に分布する最上部新生界との対比を行った.産出した石灰質ナンノ化石群集は,東京ドーム温泉井の下部で下部鮮新統〜上部鮮新統に,中部以浅で1.21〜1.73Ma間の下部更新統に対比されること,豊島園温泉井で最下部を除く大部分が1.21〜1.73Ma間に対比されることを示す.これらの結果は,江戸川区内で認められた大田代層と梅ヶ瀬層が,より西方に位置する東京ドームおよび豊島園地域までは分布せず,東京都中央部では黄和田層およびそれ以下の地層のみで構成されることを示す.
We discuss the uppermost Cenozoic calcareous nannofossil biostratigraphy of two hot spring wells (Toshimaen and Tokyo-dome) located in central part of Tokyo. The lithology of the upper Pliocene to lower Pleistocene is mainly composed of silty sandstone to sandy siltstone, pumiceous tuff, and siltstone which are correlated to the Kazusa Group in the Boso Peninsula. Calcareous nannofossil assemblages of the Tokyo-dome well are characterized by abundant occurrences of Reticulofenestra spp. (small form). Discoaster spp., which indicates the early to late Pliocene age, and the early Pliocene marker species Reticulofenestra pseudoumbilicus are found in the lower part of the section. Four early Pleistocene nannofossil datums between 1.73 Ma and 1.21 Ma are traceable to the lower part of the section. Nannofossil assemblages found in the Toshimaen well are dominated by the latest Pliocene to early Pleistocene species. Four Quaternary nannofossil datums which are recognized in the Tokyo-dome well are also detected in the lower part of the sequence. This indicates that the Quaternary sequences found in both wells clearly correlate to the lower Pleistocene Kiwada Formation of the Boso peninsula. The middle to upper Pleistocene marine sequence above the Kiwada Formation was not detected in these wells.
