HR: 1330h
AN: OS22B-1152    [PDF]
TI: Deep-sea tsunami deposits in the Miocene Nishizaki Formation of Boso Peninsula, Central Japan
AU: * Lee, I T
EM: itlee63@hanmail.net
AF: Dr Lee In Tae, Department of Oceanography, Chonnam National University, Kwangju, Cho 500-757 Korea, Republic of
AU: Ogawa, Y
EM: yogawa@arsia.geo.tsukuba.ac.jp
AF: Dr Yujiro Ogawa, Institute of Geoscience, University of Tsukuba, Tsukuba, Iba 305-8571 Japan
AB: Many sets of deep-sea deposits considered to be formed by return flow of tsunami were found from the middle Miocene Nishizaki Formation of Boso Peninsula, Central Japan, which is located near the convergent plate boundary at present as well as in the past, and has been frequently attacked by tsunami. The characteristics of the tsunami deposits in the Nishizaki Formation are as follows. Each set consists of 10-20 beds with parallel laminations formed under upper plane regime composed of alternated pumiceous beds in white and black colors. The white bed comprises coarse sands and pebbles with thickness of 5-10 cm. In contrast, the black bed is made of silts with thickness less than 1 cm. Among the 10-20 beds, the grain size is coarsest in the middle part of the set in general. The uppermost bed of each set shows cross-lamination formed by lower plane regime, gradually changing into finer graded bed on top. Sometimes, the lower part of the parallel laminated bed is associated with an underlying debrite or turbidite bed. Each set of these parallel-laminated beds is lenticular in shape thinning to the east in consistent with the generally eastward paleocurrent of the cross-lamination at the top. Such sedimentary characteristics are different from any event deposits reported in deep-sea but similar to the deep-sea K/T boundary deposits in the Caribbean region. Statistically, tsunami waves occur totally 12-13 times. Among them the height of 5-6th wave is known to be strongest. Interval time of each return flow is known to be 30-40 minutes, enough to settle the finer clastics at each bed top. The parallel-laminated parts have common dish structure and never trace fossils, indicating rather rapid deposition for the whole parts of the set. Consequently, the sedimentary characteristics shown from the parallel-laminated beds of the Nishizaki Formation are attributed to the return flow of tsunami to the deep-sea. We considered that such deep-sea parallel-laminated deposits of pumiceous clastics occur just after a large earthquake which forms the debrite or turbidite at the lowermost part.
DE: 3022 Marine sediments--processes and transport
DE: 4512 Currents
DE: 4558 Sediment transport
DE: 7299 General or miscellaneous
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting