Physical criteria for distinguishing sandy tsunami and storm deposits using modern examples

Modern subaerial sand beds deposited by major tsunamis and hurricanes were compared at trench, transect, and sub-regional spatial scales to evaluate which attributes are most useful for distinguishing the two types of deposits. Physical criteria that may be diagnostic include: sediment composition, textures and grading, types and organization of stratification, thickness, geometry, and landscape conformity.

Published reports of Pacific Ocean tsunami impacts and our field observations suggest that sandy tsunami deposits are generally < 25 cm thick, extend hundreds of meters inland from the beach, and fill microtopography but generally conform to the antecedent landscape. They commonly are a single homogeneous bed that is normally graded overall, or that consists of only a few thin layers. Mud intraclasts and mud laminae within the deposit are strong evidence of tsunami deposition. Twig orientation or other indicators of return flow during bed aggradation are also diagnostic of tsunami deposits. Sandy storm deposits tend to be > 30 cm thick, generally extend < 300 m from the beach, and will not advance beyond the antecedent macrotopography they are able to fill. They typically are composed of numerous subhorizontal planar laminae organized into multiple laminasets that are normally or inversely graded, they do not contain internal mud laminae and rarely contain mud intraclasts. Application of these distinguishing characteristics depends on their preservation potential and any deposit modifications that accompany burial.

The distinctions between tsunami and storm deposits are related to differences in the hydrodynamics and sediment-sorting processes during transport. Tsunami deposition results from a few high-velocity, long-period waves that entrain sediment from the shoreface, beach, and landward erosion zone. Tsunamis can have flow depths greater than 10 m, transport sediment primarily in suspension, and distribute the load over a broad region where sediment falls out of suspension when flow decelerates. In contrast, storm inundation generally is gradual and prolonged, consisting of many waves that erode beaches and dunes with no significant overland return flow until after the main flooding. Storm flow depths are commonly < 3 m, sediment is transported primarily as bed load by traction, and the load is deposited within a zone relatively close to the beach.