In Rokken-gawa lowland, located near Lake Hamana in central Japan, along the Nankai Trough, a sand sheet dated at around 1400–1300 BC (∼3400 cal BP) was identified along a coring transect. The sand sheet is possibly associated with a high-energy event such as a tsunami or storm. The sand sheet, composed of cross-stratified well-sorted fine to very fine sand is characterized by a maximum thickness of ∼25 cm and extends over 600 m inland from the former coastline. It erosively covers a brackish muddy substrate and is overlaid with freshwater peat deposits. In some cores, the sand sheet is classified into lower and upper sub-layers by a mud drape between them. The lower sub-layer is characterized by current ripples indicative of a landward flow direction and shows inversely graded bedding. Based on the following sedimentological and paleoecological diagnostic criteria it is suggested that the sand sheet was deposited by a tsunami:
(1) The sand sheet shows a clear fining- and thinning landward trend, suggesting an extremely long inundation distance.
(2) The lower and upper sub-layers comprising the sand sheet indicate a turnover of sediment flows (possibly up flow to return flow). Mud drape separating the lower and upper sub-layers indicates a slack water condition during the transitional stage between up flow and return flow.
(3) The sand sheet yields an open marine diatom assemblage suggesting the intrusion of seawater into the freshwater marsh environment.
(4) The presence of the sand sheet in the sedimentary sequence can be linked to an abrupt paleoenvironmental change along the coastline, whereby the study area evolved from a semi-open brackish coastal environment into a freshwater peaty marsh environment due to the closure of a tidal inlet.
2013 公益社団法人 東京地学協会