東京都北区中央公園ボーリングコアにみられる更新統東京層の層序  [in Japanese] Stratigraphy of the Pleistocene Tokyo Formation in the Kita-ku Central Park (Chuo-Koen) core in Kita-ku, Tokyo, central Japan  [in Japanese]

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Abstract

<p>東京都区部の地下には東京層と呼ばれる更新統が分布し,模式地(渋谷区代々木公園)の東京層の形成年代はMIS 5eとされている.武蔵野台地東部に位置する北区中央公園で掘削されたボーリングコアの堆積相と珪藻化石群集を検討した結果,コアの深度33m以浅には礫層を基底としその上位に浅海成の泥層と砂層が累重する2つの堆積サイクルが認められた.テフラ層の対比によって,上部の堆積サイクルはMIS 7eに,下部の堆積サイクルはMIS 9かそれよりも古い時代に堆積したことが示された.少なくとも上部の堆積サイクルは従来から東京層として扱われていることから,これまで東京層とされていた地層にはMIS 5eだけでなく,より古い時期に形成された堆積物も混在することが示唆される.今後は東京層の分布形態を層序学的観点から全面的に見直す必要がある.</p>

<p>Central Tokyo is underlain by the Pleistocene Tokyo Formation, which is considered to have been deposited during Marine Isotope Stage (MIS) 5e based on stratigraphic examination of the core type section (Yoyogi Park core). In this study, we investigated the Kita-ku Central Park core from the eastern Musahino Upland to re-examine the stratigraphy of the Tokyo Formation in this area. Two depositional cycles, each of which is composed of a basal gravel bed and overlying shallow marine sandy and muddy sediments, are observed at depths shallower than 33 m in the core, based on sedimentary facies and diatom analyses. Tephrostratigraphy indicated that the upper cycle was deposited during MIS 7e, and the lower depositional cycle may have accumulated during MIS 9 or during an earlier stage. The upper depositional cycle is considered to represent the Tokyo Formation. These results suggest that the Tokyo Formation includes a number of sedimentary cycles that not only formed during MIS 5e but also during earlier MIS periods. The distribution of the Tokyo Formation across central Tokyo should be re-examined from a stratigraphic perspective in the future.</p>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 126(10), 575-587, 2020

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130007968382
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    031196920
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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