Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 120

Table 5.1 Diagnostic criteria used to identify tsunami deposits (from Goff et al.
1998 , 2001 ).
1. Sediments within sheet generally fine inland and upwards. Deposits often rise in altitude
with distance inland
2. Each wave can form a distinct sedimentary unit, although this is not often recognised in
the sedimentary sequence
3. Distinct upper and lower subunits representing run-up and backwash can be identified,
but investigation of recent tsunami deposits indicates that there is still considerable
uncertainty about when most deposition occurs (during run-up or backwash) and so these
subunits may be related to other processes
4. Lower contact is unconformable or erosional
5. Can contain intraclasts of reworked material, but these are not often reported
6. Often associated with loading structures at base of deposit
7. Particle and grain sizes range from boulder layers (up to 750 m 3 ), to coarse sand to fine
mud; however, most deposits are usually recognised as anomalous sand units in peat
sequences
8. Generally associated with an increase in abundance of marine to brackish water diatoms,
but reworking of estuarine sediments may simply produce the same assemblage.
Preservation of frustules can be excellent, although many are often broken
9. Marked changes in foraminifera (and other microfossil) assemblages. Deeper water species
are introduced with catastrophic saltwater inundation
10. Increases in concentration of sodium, sulphate, chlorine, calcium and magnesium occur
in tsunami deposits relative to under- and overlying sediments; indicates saltwater
intrusion
11.Pollen concentrations are lower (diluted in the deposit)
12. Individual shells and shell-rich units are often present
13.Often associated with buried vascular plant material and/or buried shell
14. Shell, wood and less dense debris often found 'rafted' near top of sequence
15. Dating of tsunami sediment is problematic. Best results for dating are from units above
and below to 'bracket' the event. Radiocarbon ages often equivocal because of older
reworked carbon; dating of introduced marine macrobiota is preferred (and successful).
Optical dating (OSL) is the best method available assuming the sediments were exposed to
daylight during reworking by the tsunami
16.Often associated with reworked archaeological remains (e.g. middens). In some cases
occupation layers are separated by a palaeotsunami deposit
foraminifera and diatoms that live at greater water depths can be incorporated
within the tsunami flow and be deposited onshore, whereas storm waves tend
to entrain shallower water species.
Sato et al .(1995)suggested that the spatial distribution and thickness of
tsunami sand sheets depends on the size of the tsunami inundation, in par-
ticular tsunami run-up, and the availability of sediment for entrainment. Based