Diversified grain-sizes, thickness and sedimentary structures of terrestrial tsunami deposits

Abstract

Outcrop-scale characteristics of terrestrial tsunami deposits are non-uniform, because of their diversified grain-sizes, thickness and sedimentary structures. Grain-sizes of tsunami deposits are influenced a great deal by that of source area. Therefore some tsunami deposits may be composed mainly of mud and others gravel, and may be accompanied by boulders (Figs. 1-4). Grain-sizes of tsunami deposits also vary according to some factors such as the distance from the source area, even if they were made by the same tsunami in the same area (Figs. 1, 2, 4). Their thickness varies from a few millimeters to a meter or more. Some tsunami deposits are massive but others show a variety of sedimentary structures. These include parallel lamina, grading structure, inverse grading structure (Figs. 1, 2, 3), rip-up clasts (Fig. 2) and multiple graded units that reflect several wave inundation (Nanayama and Shigeno, 2006).Some sedimentary processes other than tsunami can form many of these characteristics. Therefore, it is required to exhibit as many criteria as possible along with fossil content and geochemical proxies when we identify paleo-tsunami deposits in strata.