Grain-size Distribution of Tsunami Deposits Reflecting the Tsunami Waveform: An Example from a Holocene Drowned Valley on the Southern Boso Peninsula, East Japan:An Example from a Holocene Drowned Valley on the Southern Boso Peninsula, East Japan

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Tsunami deposits in an early Holocene drowned valley on the southern Boso Peninsula, Pacific coast of east Japan, comprise a stack of four units, Tna, Tnb, Tnc, and Tnd.Unit Tna is composed of poorly sorted coarse to very coarse sand with high mud content, and erosionally it overlies the bay mud. This unit was deposited under the influence of landward paleocurrents in an upper flow regime, forming plane beds and antidunes.Unit Tnb consists of a stack of several subunits and well reflects the tsunami waveform in detail. Each subunit, bounded by first-order erosion surfaces of hummocky cross-stratification, is attributed to a oscillating high-density current. The subunits are composed of lower inverse and upper normal graded intervals. Sorting becomes best in the middle horizon of the subunits. Mud content decreases upward in the inverse graded interval, and increases upward up to 20% in the normal graded interval. The inverse graded interval seems to be traction carpet and/or transport-lag. The normal graded interval suggests a long stagnant stage of current, which allows fallout of mud particles. Paleocurrents, reconstructed from the sedimentary structures, show that each subunit corresponds to the up-flow and/or return-flow.The stack of subunits showing saw-toothed curve in grain-size parameters indicates the oscillation of high-energy currents with long period. This long period oscillating flow can be explained by a tsunami with a 10-minute order wave period. An extremely coarse-grained subunit intercalated in the middle horizon of unit Tnb can be explained as deposition from the largest wave of a tsunami wave train. Delayed arrival of the largest wave in a wave train is due to edge wave effect, indicating reflection on the continental shelf, and is a remarkable feature of tsunami that originate in deep ocean.Unit Tnc also shows cyclic deposition of plant debris laminae and poorly sorted sandy silt layers, and is attributed to the relative small waves in the later stage of a tsunami.Unit Tnd is composed of sandy silt layer including subsequent sinking of wood and plant debris after the tsunami.
日本第四紀学会の論文

Grain-size Distribution of Tsunami Deposits Reflecting the Tsunami Waveform: An Example from a Holocene Drowned Valley on the Southern Boso Peninsula, East Japan:An Example from a Holocene Drowned Valley on the Southern Boso Peninsula, East Japan

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