Chapter 4 - Bedforms and Sedimentary Structures Characterizing Tsunami Deposits

Abstract

Abstract Tsunami deposits show a wide spectrum of bedforms and sedimentary structures, which are related to the physical properties of the flows, such as speed, density, viscosity and dominant grain size. However, several common characteristics are recognized in tsunami deposits from various depositional sites. This results from the fact that tsunamis have a much longer wavelength and period than wind-generated waves do, such as storm waves. This remarkable physical property of tsunamis is, under the favourable depositional and preservation conditions, recorded in tsunami deposits as a unique stacking pattern of bedforms and sedimentary structures: (1) an alternation of sediment sheets deposited from high-energy sediment currents and mud drapes, (2) repeated reversal of current directions and (3) a fining and thinning upwards series of sediment sheets. Thick tsunami deposits comprise sometimes four depositional units, Tna to Tnd in ascending order, which reflect the changes of wave amplitude with time. Unit Tna corresponds to the relatively small waves in the early stage of a tsunami. The extremely coarse-grained Unit Tnb evidences the occurrence of large waves in the middle stage of the tsunami. A fining- and thinning-upwards sequence from Unit Tnb to Unit Tnc indicates the waning process of the tsunami. Unit Tnd, a muddy layer with plant debris, is formed by suspension fallout, indicating the return to low-energy conditions after the tsunami. A combination of these sedimentary properties provides the best criterion for distinguishing tsunami deposits from other event deposits such as tempestites and sediment gravity-flow deposits.