Depositional facies and sedimentary successions of earthquake-induced tsunami deposits in Upper Pleistocene incised valley fills, central Japan

Abstract

Two ‘anomalous’ deposits with well-preserved sedimentary structures were discovered in a drowned valley and in relict barrier sand body deposits in the Furuya Mud and Kyomatsubara Sand of the Upper Pleistocene incised valley fills. The sedimentary features of both deposits are interpreted as ‘earthquake-induced tsunami deposits’, which were deposited in incised valley fills. The tsunami deposit in the drowned valley (Furuya Mud) is 150 cm thick and shows a contorted structured bed in the lower part and cross-laminated sand in the upper part. The tsunami deposit in barrier sand body (Kyomatsubara Sand) is 270 cm thick and shows structureless sand with mud clasts in the lower part, cross-laminated sand with mud-drapes in the middle part and mud with small wave-ripples in the upper part. Taken together, these sedimentary features suggest a model for the composite succession, which is comprised of three units: a lower earthquake-shock-influenced (anomalous) convoluted part, a middle part dominated by tsunami-generated tractional currents, and an upper suspended-sediment part. The lower part is interpreted as earthquake event deposits (seismites) and the middle and upper parts as tsunami current deposits (tsunamiites).