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

Abstract

Abstract Tsunami have probably deposited sand sheets that are intercalated in Holocene bay-floor mud exposed on the Boso and Miura Peninsulas, facing the convergent 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 sorted muddy sand and well-sorted sand layers in fining upward sequences, containing abundant transported shells, rip-up clasts and wood fragments. The sheets erosionally overlie bioturbated bay-floor mud that contains molluscan shells in life position. Most of the sheets are less than 20 cm thick and rarely more than 50 cm thick. Some molluscan shells are older in these layers than in underlying mud. Both landward and seaward paleocurrents are shown in a few cases by imbrication of shells and by low-angle wedge shaped lamination. At least five of the sand sheets contain molluscan fossils derived 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 dominated 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 Holocene marine terraces. These correlations suggest that great earthquakes triggered the inferred tsunami. The tsunami occurred at intervals of 300–2000 years beginning about 10,000 years ago.