9:30 AM - 9:45 AM
[MIS20-02] A late Aptian mega-tsunami in the western Pacific and its candidate source
Keywords:tsunami deposit, Aptian, mega-tsunami, Miyako Group
We reexamined the tsunami deposit that is previously reported from the Lower Cretaceous (Aptian-Albian) Miyako Group by Fujino et al. (2006), and estimated the relative magnitude of the tsunami by comparison with major modern tsunamis. A new outcrop of the tsunami deposit was discovered at the Masaki area, ~15 km from the Tanohata area where the tsunami deposit was already known, at the same lithostratigraphic horizon. The maximum observed thickness of the tsunami deposit in the Tanohata area is 8.5 m. The tsunami deposit in the Masaki area is 1.5–5.5 m thick, and the tsunami eroded more than 4 m of the underlying cross-stratified sandstone. The tsunami deposit contains rip-up clasts from the underlying cross-stratified sandstone, suggesting that the tsunami eroded the substrate to a depth where it had been compacted and cemented to some extent. The thickness and extent of erosion of the Lower Cretaceous tsunami deposit far exceed erosion and deposition by modern major tsunamis. In addition, the fact that the Cretaceous tsunami deposit is found at only one stratigraphic horizon in the entire Miyako Group indicates that that type of tsunami was much less frequent than subduction-zone earthquakes and tsunamis, which occur at intervals of one hundred to a few hundred years. Therefore, this Early Cretaceous tsunami should be called a mega-tsunami. Ammonite biostratigraphic study of the Miyako Group indicates that the mega-tsunami deposit was formed in the late Aptian. This age overlaps that of the Kirigishiyama Olistostrome bed in the Yezo Group, which was deposited in the same forearc basin as the Miyako Group. The formation event of the Kirigishiyama Olistostrome bed is a highly probable candidate for the source of the mega-tsunami.