Genesis of Mixed Molluscan Assemblages in the Tsunami Deposits Distributed in Holocene Drowned Valleys on the Southern Kanto Region, East Japan

DOI Web Site 11 Citations 48 References
  • Fujiwara Osamu
    Tono Geoscience Center, Japan Nuclear Cycle Development Institute
  • Kamataki Takanobu
    Active Fault Research Center, National Institute of Advanced Industrial Science and Technology
  • Fuse Keisuke
    Daiwa Geological Laboratory Co. Ltd.

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Other Title
  • 津波堆積物中の混合貝類化石群の形成プロセス－南関東における完新世の内湾の例－
  • ツナミ タイセキブツ チュウ ノ コンゴウ カイルイ カセキグン ノ ケイセイ プロセス ミナミカントウ ニ オケル カンシン セイ ノ ナイワン ノ レイ
  • 南関東における完新世の内湾の例

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Abstract

Many sandy gravel beds with densely packed molluscan shells are intercalated in the bay muds deposited in the Holocene drowned valleys on the southern Boso Peninsula, east Japan. These event deposits, which erosionally overlie the bay muds and show upward fining sequence, were deposited by high-density currents. The shell beds in these event beds show different fossilization processes (Taphonomy), such as occurrence and species compositions from the storm beds as follows: 1) Lenticular shell beds alternate with muddy sand layers in the event deposits, though shell concentration density decreases upward from the base in the storm beds, reflecting waning currents and waves. This cyclic occurrence of shells in the event deposits indicates the repetition of shell concentrations by winnowing of oscillating currents and suspension fallout during the long current stagnation stages.<br>2) Highly diverse and well-preserved molluscan shells derived from various habitats such as mud bottom and rocky shore are mixed in the event beds. These mixed assemblages are attributed to the tearing up and rapid burial of the molluscs in and around the bays by strong currents. The molluscs inhabiting a deeper sea floor than the storm wave base are also yielded in the event deposits and indicate landward transport of shells from the offing.<br>The above-mentioned occurrences and species compositions of the mixed assemblages cannot be explained by storm waves with short period (about 10 seconds), but can be explained by a great tsunami which has a 10minute order wave period and disturbs the deep sea water.

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