SEDIMENTARY STRUCTURES AND MOLLUSCAN ASSEMBLAGES WITHIN A TSUNAMI DEPOSITS FROM HOLOCENE BAY SEDIMENTS IN THE BOSO PENINSULA, CENTRAL JAPAN

Many tsunami deposits are intercalated in the Holocene bay sediments on the Boso Peninsula, central Japan. These tsunami deposits are subdivided into four units (Tna, Tnb, Tnc, and Tnd). Unit Tna is composed of poorly sorted coarse to very coarse sand containing numerous rip-up clasts, and erosionally overlies the bay mud. It contains sedimentary structures formed under the upper flow regime, such as plane beds and antidunes. Landward paleocurrent was estimated from these sedimentary structures. Large and thick shells are convex-up orientation and lie flat on the basal erosion surface. This unit shows a fining upward sequence and is draped by plant debris. Unit Tnb is composed of fine to gravely very coarse sand, and overlies subunit Tna with an erosion surface. It consists of a stack of some hummocky cross stratification (HCS) units. Each HCS unit is draped by plant debris laminae or a thin clay layer. This cyclic structure shows the repeated arrival of high-density turbidity currents forming the HCS with long stagnant stages permitting the deposition of mud drapes. Shell concentration density is high in this unit. One or two HCS units in this unit are extremely coarse and include outsized clasts. These HCS units are thought to be formed by the largest of the succeeding waves. This structure indicates the delayed arrival of the largest waves in a wave train. The delayed largest waves are explained as an edge-wave effect which is attributed to the reflection of long waves on the continental shelves. Unit Tnc shows cyclic deposition of plant debris laminae and poorly sorted sandy silt layers, and is attributed to the small waves in the later stage of tsunami. Unit Tnd is composed of sandy silt layer including subsequent sinking of wood and plant debris after the tsunami. This cyclic deposition sandwiching the long stagnant stages cannot be explained by storm waves with 10 to 20-second wave periods. However, tsunamis have 10- minute-order wave periods and can form such cyclic structures.