The Tsunamite Problem

摘要

The genetic term tsunamite is used for a potpourri of deposits formed from a wide range of processes (overwash surges, backwash flows, oscillatory flows, combined flows, soft-sediment deformation, slides, slumps, debris flows, and turbidity currents) related to tsunamis in lacustrine, coastal, shallow-marine, and deep-marine environments. Tsunamites exhibit enormous variability of features (e.g., normally graded sand, floating mudstone clasts, hummocky cross stratification, etc.). These sedimentary features may also be interpreted as deposits of turbidity currents (turbidites), debris flows (debrites), or storms (tempestites). However, sedimentary features play a passive role when these same deposits are reinterpreted as tsunamites on the basis of historical evidence for tsunamis and their triggering mechanisms (e.g., earthquakes, volcanic explosions, landslides, and meteorite impacts). This bipartite (sedimentological vs. historical) approach, which allows here classification of the same deposit as both turbidite and tsunamite, has blurred the distinction between shallow-marine and deep-marine facies. A solution to this problem is to classify deposits solely by a descriptive sedimentological approach. The notion that tsunami waves can directly deposit sediment in the deep sea is unrealistic because tsunami waves represent transfer of energy and they are sediment starved. During tsunamis and major storms, submarine canyons serve as the physical link between shallow-water and deep-water environments for sediment transport. Tsunami-related deposition involves four progressive steps: (1) triggering stage (offshore), (2) tsunami stage (incoming waves), (3) transformation stage (near the coast), and (4) depositional stage (outgoing sediment flows). In this progression, deep-water deposition can commence only after the demise of incoming tsunami waves due to their transformation into outgoing sediment flows. Deposits of these sediment flows already have established names (e.g., debrite and turbidite). Therefore, the term tsunamite for these deposits is obsolete.