The Quaternary Research (Daiyonki-Kenkyu)

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Contributions of marine diatom fossils as a stratigraphic indicator and age determination tool for the Quaternary sediments are reviewed, focusing on research examples from the Quaternary sediments around Japan, and current achievements of this research and potential perspectives are discussed. Planktonic diatom stratigraphy can provide a useful age index for oceanic Quaternary sediments, and it has possibility to improve its precision and accuracy. On the other hand, shallow marine deposits play an important role as a key bed in the Quaternary formations of coastal areas around Japan. Although diatom fossils are a valuable indicator to recognize shallow marine sediments in such formations, biostratigraphy based on the evolution of coastal shallow marine diatoms has not been established yet. At present, the genus Lancineis is the only effective stratigraphic marker diatom fossil on the Quaternary coastal shallow marine deposits. In order to reveal the biostratigraphy of coastal shallow marine diatoms, diatom fossil assemblages should be refined both taxonomically and stratigraphically in the future.

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Geological evidence of tsunamis recorded in the Kiritappu marsh, Hokkaido, Japan, provides a unique opportunity to study past giant earthquakes along the Kuril Trench. The landward limits of tsunami deposits in the northern marsh have been reported previously, whereas their limits remain uncertain in the southern marsh. Here we present the distribution of tsunami deposits at Ichibansawa in the southern Kiritappu marsh. Tephrochronology and ¹⁴C age data indicate that the sand bed in the seaward area of Ichibansawa is correlated with tsunami deposits of the 17^th century or the 13-14^th centuries. The sand bed in inland Ichibansawa is correlated with tsunami deposits of the 13-14^th centuries. The sheet-like distribution of the inland sand bed is also observed in a tributary of Ichibansawa. These results may constrain models of faults that generate large tsunamis along the Pacific coast of eastern Hokkaido.

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