IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Poster presentation 1838
Miocene conglomerates formed by Huge tsunami
Dr. Toru Tachibana
Reserch org. Environmental Geology of Setouchi
Yoshinobu Tsuji, Yuichi Namegaya
Some tsunami sediments need huge tsunami to interpret the sedimentary processes. The Miocene
conglomerates,Tsubutegaura Conglomerates, is one of them. These conglomerates including meterorder
gravels were deposited in deep water (about several hundred meters depth) , and show
sedimentary structures formed under not only gravity flows but also strong traction currents. Most
possible agents of such traction currents are tsunamis. Nevertheless, there is a doubt whether tsunamiinduced
currents are able to move large gravels in deep water environments. We attempted two
researches; (1) the tsunami simulation to estimate the velocity of tsunami-induced currents, based on
the paleogeograhy and the reconstructed tectonic setting, (2) the estimation of the threshold velocities
of large gravels. Results of research (1) using similar parameters of the 2004 Sumatora Earthquake
showed that maximum velocity of the estimated bottom current reached 6m/s in 500m depth.
Considering results research (1) and (2), tsunamis are possible to transport large gravels in deep water.
Tsubutegaura Conglomerates are interpreted to be formed by the huge tsunami caused by large
earthquake like the Sumatora Earthquake. Tsunami have been thought to be no major agents in deep
water environments except extraordinary events like meteorites-induced tsunami.But tsunami may have
occasionally deposited the sediments in deep water environments.
Keywords: deep water environments, conglomerates, miocene