Anatomies of Sandy Tsunami and Storm Deposits

Because large tsunamis and storms occur infrequently, the geologic record is needed to accurately assess their frequency and potential magnitude. Fundamental to using the geologic record is the ability to discriminate between tsunami and storm deposits. Field and laboratory investigations after 7 recent tsunamis and 2 storms reveal similarities and differences between sandy deposits from these extreme wave events. Here we focus on one aspect of the deposits— vertical changes in grain size distributions. Vertical grading in tsunami deposits ranges from normal (fining upward), which is commonly observed, to inverse (coarsening upward). Massive or inversely graded intervals are usually found at the base of each layer (bed). At the tops of layers a specific type of grading, suspension grading, is often found in tsunami deposits. Suspension grading occurs when the entire distribution shifts to finer grain sizes moving upward in the deposit. Storm deposits typically exhibit inverse vertical grading or are massive. Grading for both tsunami and storm deposits reflects both changes in source and process during deposition. It is the process information that can be exploited in quantitative models for interpreting these extreme wave events. Suspension grading in tsunami deposits is a signature of sediment falling out of suspension and is the predicted outcome for clearing a water column charged with suspended sediment with an exponential decrease in concentration (e.g. produced by an equilibrium between upward diffusion and downward settling of sediment). The shift occurs because of the timing of when larger and smaller grains are deposited. Grains with higher settling velocities (larger particles for a given density and shape) deposit first and are absent in the water column during the later stages of deposition. The grains with lower settling velocities, which take longer to reach the bed, are absent from the bottom of the suspension-graded interval and deposit later on the top of it. The massive nature (inversely graded) storm deposits, and portions of tsunami deposits, can be explained by gradual deposition from sediment transport convergences (more sediment coming into an area than leaving it) created by, for example, flow deceleration with a constant (coarsening) source. This process is often associated with bedload transport. Improving the understanding of the anatomies of tsunami and storm deposits forms the basis for forward and inverse modeling approaches to learn extreme wave event characteristics and for increasing the utility of the geologic record in tsunami and storm hazard assessment.;