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

この論文にアクセスする

この論文をさがす

著者

抄録

房総半島南部の溺れ谷に堆積した完新世の内湾性のシルト層には,基底に侵食面を持ち,上方へ細粒化する高密度流から堆積した砂礫層(イベント堆積物)が多数挾まれる.多くのイベント堆積物では,レンズ状の貝化石層と粘土質の地層とが細互層をなす.これは,ストームによる化石層が波の減衰過程を反映して,貝殻の集積密度が基底から上方へ低下する状況とは異なる構造で,貝殻を吹き分け集積させる振動流と,浮遊物質が沈積する流れの長い停滞期の繰り返しを示す.イベント堆積物に含まれる貝類遺骸群は保存がよく,泥底から岩礁までさまざまな底質に生息した種が混合している.これは,普段は低エネルギーの内湾周辺に生息した貝類が強い流れで剥ぎ取られ,急速に埋積されたためと考えられる.これらの混合化石群は,ストームウェーブベースより深い海底に棲む貝類の化石も含むことがあり,沖から湾奥への貝殻の運搬を示す.これらの産状と種構成は,周期が10秒前後のストーム波では説明が困難だが,周期が10分オーダーの波動の繰り返しで,かつ深い海底まで海水を動かす津波であれば形成可能である.

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.

収録刊行物

  • 第四紀研究 = The Quaternary research

    第四紀研究 = The Quaternary research 42(6), 389-412, 2003-12-01

    Japan Association for Quaternary Research

参考文献:  48件中 1-48件 を表示

被引用文献:  7件中 1-7件 を表示

各種コード

  • NII論文ID(NAID)
    10012511215
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    6784879
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
ページトップへ