CHAPTER NINE – TSUNAMI DEPOSITIONAL PROCESSES REFLECTING THE WAVEFORM IN A SMALL BAY: INTERPRETATION FROM THE GRAIN-SIZE DISTRIBUTION AND SEDIMENTARY STRUCTURES

Tsunami depositional processes reflecting the waveform are reconstructed from detailed analyses of the grain size and the sedimentary structures of tsunami deposits in a Holocene buried valley on the Pacific coast of east Japan. These tsunami deposits are 10- to 40-cm thick and consist of a stack of four units, Tna to Tnd in an ascending order. Unit Tna, erosionally covering the bay muds, is composed of fine to coarse sand with plane bed and antidune structures, and includes abundant rip-up clasts. Unit Tnb is a couplet of hummocky cross-stratified (HCS) sand units including outsized clasts and plant debris-rich mud drapes, erosionally overlying Unit Tna. Each HCS sand unit was deposited by high-density currents. Mud drapes covering the HCS sand units are due to settling from suspension during the long stagnant stages of the tsunami floods. Unit Tnc overlies Unit Tnb gradually, and shows cycles of plant-debris laminae and thin, sandy silt layers. Unit Tnd gradually overlies Unit Tnc and is composed of a silt bed with intercalations of layers with wood and plant fragments. The succession of Units Tna to Tnd documents the repeated arrival of high-density currents, which were separated by long stages of stagnant water. This cyclic deposition cannot be explained by storm waves with wave periods of a few to 10 s, but must be attributed to a tsunami with a wave period of the order of 10 min. The four units represent the four stages of a tsunami: the early stage (Tna), large waves of the middle stage (Tnb), small waves of the later stage (Tnc) and subsequent foundering of wood and plant debris (Tnd), respectively.