Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan

Tsunami have probably deposited sand sheets that are intercalated in Holoce ne hay-door mud exposed on the Boso and Miura Peninsulas, facing the conver gent boundary of the Pacific, Philippine Sea, and Eurasian Plates. We have identified seven sand sheets at four drowned valleys, and correlated these by 137 radiocarbon dates of shells and wood. The sheets consist of poorly s orted muddy sand and well-sorted sand layers in fining upward sequences, co ntaining abundant transported shells, rip-np clasts and wood fragments. The sheets erosionally overlie bioturbated bay-floor mud that contains mollusc an shells in life position. Most of the sheets are less than 20 cm thick an d rarely more than 50 cm thick. Some molluscan shells are older in these la yers than in underlying mud. Both landward and seaward paleocurrents are sh own in a few cases by imbrication of shells and by low-angle wedge shaped l amination. At least five of the sand sheets contain molluscan fossils deriv ed from rocky coasts or shore platforms, although they intercalated in mud deposited within bays, at depths of 10-15 m. Two other sand sheets are domi nated by open seashore ostracode assemblages, although they were deposited in the brackish inner bay and muddy central bay. Five of the layers may correlate with emergences recorded by nearby Holocen e marine terraces. These correlations suggest that great earthquakes trigge red the inferred tsunami. The tsunami occurred at intervals of 300-2000 yea rs beginning about 10,000 years ago. (C) 2000 Elsevier Science B.V. All rig hts reserved.