Sedimentary features of AD1741 tsunami deposit and some older event deposits in the narrow lowlands along the eastern margin of the Japan Sea, northern Hiyama District, Hokkaido, Japan

Abstract

Abstract Deposits of the AD1741 Oshima-Ohshima tsunami and some other events were identified by pit excavations in two narrow coastal lowlands along the eastern margin of the Japan Sea, northern Hiyama District, Hokkaido, Japan. The AD1741 tsunami deposits were consisted of volcanic-ash sand grains derived from the AD1640 Ko-d tephra, and were characterized by landward thinning bed geometry, landward paleocurrent, normal grading, erosional base with rip-up clasts, and higher ratio of marine diatoms. In the Kumaishi-Ayukawa lowland, well-preserved out-flow layer indicates that the tsunami entered through the river mouth reached the terrace cliffs and returned along the river. Whereas the tsunami deposits in the Hirahama lowland lack return flow layer and were overlain by marsh muds, suggesting that the tsunami overtopped the coastal sand dunes and seawater ponded the vast lowland for a period. The sand grains of AD1741 tsunami deposits in Hirahama were not much rounded compared with those in the Kumaishi-Ayukawa, which probably reflected sediment source from tephra fell on the sand dunes and inland. We also recognized event deposits younger than AD1741 of flash flood origins in both lowlands. In addition, two older event deposits (672–561 cal yBP and 2333–1999 cal yBP) existed in Hirahama, that were consisted of rounded pebbles to cobbles similar with present beach gravels. Although such gravelly event deposits may be formed by storm surge or tsunami, the details of their origins are currently unclear.