HOW DO DEPOSITS FROM THE 2004 TSUNAMI IN SUMATRA CHANGE OUR VIEWS OF TSUNAMI DEPOSITION?

Sumatran tsunami deposits from the 2004 South Asia tsunami are more complex than those of previously studied events, complicating efforts to “typify” the deposits of tsunamis. Tsunami and paleotsunami deposit studies over the past ten years have produced a paradigm for tsunami deposition—sandy tsunami deposits occur when nearshore material is suspended in the flow, and rains out as the flow converges or decelerates, producing a deposit that generally fines and thins landward, and is normally graded.

In contrast, 2004 Sumatra deposits show complex internal stratigraphy, display not only normal grading but also reverse grading and massive deposition, show evidence for bedload grain motion, contain sedimentary units possibly associated with concentrated or high-shear flows, and have complex thickness patterns. These deposits suggest that tsunami hydraulics are more complicated than previously envisioned, in ways that we currently do not fully understand. For example, does reverse grading result from flow-backflow within a single wave, from tapping of multiple sediment sources, or from deposition through some mode other than suspension, such as grain flow or sheet flow?

Additionally, we cannot yet determine how “typical” the 2004 Sumatra deposits are, either of deposition from this event as a whole, or of tsunami deposition in general. Is this complexity common in tsunami deposits, but either obscured or eroded by postdepositional alteration? Alternatively, is there some fundamental difference between the hydraulics of a very large, near-source wave and the hydraulics of a smaller wave or one far from its source?

To answer these questions, tsunami sedimentologists will need to shift from facies paradigms for tsunami deposit recognition and interpretation to a more process-oriented understanding of the hydraulics that prevail during deposition. Such a shift will require collaboration with many other disciplines, but has the potential to answer important questions about the size of paleotsunamis (and the earthquakes that spawned them) from the deposits they left behind.