Event deposits associated with tsunamis and their sedimentary structure in Holocene marsh deposits on the east coast of the Shima Peninsula, central Japan

Abstract

In Japan, many earthquakes have occurred during historic times. Many tsunamis caused by these earthquakes are recorded in ancient documents. Some researchers have studied tsunami deposits in lake and marsh sediments in coastal areas of Japan. However there are only very few st udies on tsunami deposits at the Kii Peninsula. The coastal area of this peninsula faces the Nankai Trough. Tsunamis triggered by great Tokai and/or Tonankai Earthquakes (M ≥ 8), whose source is distributed along the Nankai Trough, have been causing serious damage to this area. We examined some cores related to some eve nt deposits (sand layers) possibly caused by the Tokai and/or Tonankai Earthquakes. Studied cores (A0-6, B1, P1-2) were excavated at the coastal marsh in the east side of the Kii Peninsula. In this marsh deposits, some researchers reported diatom, foraminifera, radiocarbon age and general lithofacies. We summarized these previous studies and carried out detailed lithofacies descriptions and additional radiocarbon dating to elucidate paleoenvironments of the studied area and origin of sand layers. Tempor al changes of Holocene depositional environments of the studied area were elucidated as follow: Around 7,000 cal yr BP, marine (inner bay) environment; ca. 6,500-3,000 cal yr BP, freshwater marsh; ca. 3,000-1,000 cal yr BP, influence of marine water to the marsh; after ca. 1,000 cal yr BP, fresh-water marsh; the Edo Period- the beginning of the Showa Period, the paddy cultivation in studied marsh; After the beginning of the Showa Period, fresh-water marsh (stopping of the paddy cultivation). Lithofacies and distribution of event deposits strongly s uggested that sand layers were formed by tsunamis.