Medieval coastal inundation revealed by a sand layer on the Ita lowland adjacent to the Suruga Trough, central Japan

There is an exotic sandy layer associated with a marine incursion beneath the Ita lowland, a small basin adjacent to the eastern shore of Suruga Bay, central Japan. This poorly sorted, coarse sandy deposit, which contains brackish-marine diatoms, is interbedded with organic-rich mud and was likely deposited in a freshwater swamp or marsh environment subject to occasional marine influence (e.g., storm or tsunami). According to historical documents, there has been at least one large storm in Suruga Bay every 100 years; however, the local geology records deposition of only two extensive sand sheets between ca. 500 BC and ca. AD 1460. This difference in the historical and geological records of the frequency of marine incursion events suggests that infrequent tsunamis or extraordinarily large storms are the events most likely to have deposited such coarse sediments on the lowland. The depositional age of the sand is constrained by radiocarbon ages of plant macrofossils and charred material sampled from below and above the sand: AD 1140-1340 below and AD 1090-1390 above. These ages are consistent with deposition of the sand by known tsunamis in 1096, 1099, 1293, or 1361, or another undocumented overwash associated with tsunami or storm.