"沖積層"の形成史からみた湖沼 : 東京港地区を例として  [in Japanese] The Buried Lake and Marsh Viewed from Geohistory of Alluvial Deposits in the Port of Tokyo  [in Japanese]

東京港における約18,000年前の最終氷期の埋没地形によれば, 古東京谷と古神田川谷の2大主谷が認められ, それらの谷を埋めるようにして七号地層が分布する。そして, 前者の谷には七号地層下部の典型的層相とされる砂・粘土の互層がみられるのに対して, 後者の谷には認められない。一方, 谷に狭窄部が存在している古神田川谷の埋没地形からは, この古神田川谷系に埋没湖沼が存在したようにみえるが, 微化石分析などの結果からは, 古東京谷の方に湖沼または潟があったことをうかがわせている。諸資料を総合的に検討すると, 両主谷の七号地層下部の層相の違いは, 双方の古河川水系の規模による堆積物供給量の差に起因すると考えられる。

Viewing coastal land forms and coastal plains we recognize various kinds of topography such as deltas, lagoons, bays, crescent lakes, back marshes and natural levees. These land forms give us clues as to the paleotopography buried under Alluvial Deposits or Alluvial Ground, which have been produced by the interaction of sea level change and the amount of deposits supplied by rivers since the beginning of the postglacial age. The buried topography of the port area at about 18,000 y.B.P. shows two main valleys, namely, the Paleo-Tokyo Valley and the Paleo-Kandagawa Valley. The typical facies showing the alternation of sand and clay strata, in the Lower Nanagouchi Formation (Nal), which buried the paleo-Alluvial valley, can be found clearly in the former valley. On the contrary, they are not found in the latter valley. An isthmus of the Paleo-Kandagawa Valley suggests to us that a kind of buried lake or marsh had existed there. However, the results of micro fossil analysis shows us that a buried lake or lagoon had existed in the Paleo-Tokyo Valley. The facies' difference in the Lower Nanagouchi Formation between the two main paleo-valleys can be attributed to the difference in the amount of deposits supplied by each paleo-river. Because of the large mass of deposits supplied by the upper stream, a lake or lagoon environment was likely to appear on the Paleo-Tokyo River. On the other hand, the lack of such deposits caused the inner bay environment to remain for a long time in the Paleo-Kandagawa River.