TSUNAMI DEPOSITS IN A LACUSTRINE SEQUENCE OF THE SANRIKU COAST, NORTHEAST JAPAN

Lacustrine deposits of the Pacific coast of northeast Japan, consisting mostly of black organic mud, contain intercalated thin beds of well-sorted medium sand. Examination of the deposits from a coastal site of Sanriku has revealed that sand grains are of marine origin and are fractions of deposits in marshy ponds, transported from the littoral environment by a great flooding of seawater. The mode of sedimentation shown in the thin beds of sand implies that they were deposited by tsunamis, each with a maximum rise of 1 m or more above sea level.