Keywords:Bioturbation, Burrow, Ichnology
Tsunami deposits provide important information on the magnitudes and recurrence intervals of the causative tsunami events. However, such deposits might be modified or obliterated by subsequent physical disturbances and/or biomixing of the sediment (bioturbation). For a clear understanding of the post-depositional alteration of tsunami deposits, it is necessary to monitor changes in sedimentary structures of the deposits several years after a tsunami event. Thus, we conducted field survey in the 2011-tsunami affected sea bottoms in 2016, to investigate preservation potential of the event layer. We obtained sediment core samples from ria coasts, northeastern Japan: i.e., from Onagawa Bay (Miyagi Prefecture, Seike et al., 2016, 2017), Samenoura Bay (Miyagi Prefecture), Kamaishi Bay (Iwate Prefecture), Otsuchi Bay (Iwate Prefecture), and Funakoshi Bay (Iwate Prefecture). From the all-sampling sites, tsunami deposits (sandy layer with parallel laminations) were recognized. In contrast, upper part of the layers was heavily bioturbated and lacks any physical sedimentary structures; the original sedimentary structures (parallel laminations) produced by the 2011-tsunami were obliterated by bioturbation. On the other hand, tsunamigenic coarse-grained deposit can be distinguished from ordinary background deposits (mud) based mainly on textural differences among the sediments in the semi-enclosed bays. Thus, recognition of the effects of post-depositional alteration of ancient tsunami deposits is important for the identification of paleotsunami events in the geological record.
Seike, K., Kitahashi, T. and Noguchi T., 2016, Sedimentary features of Onagawa Bay, northeastern Japan after the 2011 off the Pacific coast of Tohoku Earthquake: sediment mixing by recolonized benthic animals decreases the preservation potential of tsunami deposits. Journal of Oceanography, 72, 141–149.
Seike, K., Kobayashi, G. and Kogure, K., 2017, Post-depositional alteration of shallow-marine tsunami-induced sand layers: A comparison of recent and ancient tsunami deposits, Onagawa Bay, northeastern Japan. Island Arc, doi:10.1111/iar.12174