Sedimentology of tsunami inflow and backflow deposits: key differences revealed in a modern example

Bahlburg, H. Spiske, M.

Zusammenfassung

Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes canrange from clay to boulders of several metres in diameter. Grain-size distributionsreflect the mode of deposition and may be used to explore the hydrodynamic conditionsof transport. The absence of unique sedimentary features identifying tsunami depositsmakes it difficult in some cases to distinguish inflow from backflow deposits.On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow andbackflow deposits of highly variable character. Tsunami inflow entrained sands, gravelsand boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to12 t were transported up to 300 m inland and 13 m above sea-level. Thin veneers ofcoarse sand were found up to the maximum runup at 600 m inland and 19 m above sealevel.Backflow re-mobilized most of the sands and gravels deposited during inflow. Theorientation of erosional structures indicates that significant volumes of sediment wereentrained also during backflow. A major feature of the backflow deposits are widespreadprograding fans of coarse sediment developed downcurrent of terrace steps. Fansediments are mostly structureless but include cross-bedding, imbrication and ripples,indicating a deposition from bedload traction currents. The sediments are poorly sorted,grain sizes range between medium to coarse sand to gravel and pebbles. An assessmentof the backflow transport conditions of this mixed material suggests that bedloadtransport at Rouse numbers >2.5 was achieved by supercritical flows, whereasdeposition occurred when currents had decelerated sufficiently on the low gradientlower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists ofbackflow deposits to more than 90%. Due to the lack of sedimentary structures, manyprevious studies of modern tsunami sediments found that most of the detritus wasdeposited during inflow. This study demonstrates that an uncritical use of thisassumption may lead to erroneous interpretations of palaeotsunami magnitudes andsedimentary processes if unknowingly applied to backflow deposits.