Geoscience Reference
In-Depth Information
Fig. 3.6
Photograph of the
coastal landscape at Riang-
Kroko on the island of Flores,
Indonesia, following the tsunami
of 12, December 1992. The
greatest run-up height of 26.2 m
above sea level was recorded
here. Boulders and gravels have
been mixed chaotically into the
sand sheet. Note the isolated
transport of individual boulders.
Photo credit Harry Yeh,
University of Washington.
Source National Geophysical
Data Center
coastlines with stable sea level histories. Either storm waves
superimposed upon a storm surge or tsunami are known to
be responsible for such deposits. For example, Hurricane
Iniki in Hawaii in 1992 swept a thin carpet of sand con-
taining boulders inland beyond beaches (Bryant et al.
1996
),
while the tsunami that hit Flores, Indonesia, on December
12, 1992 did likewise (Fig.
3.6
) (Shi et al.
1995
). While
storms can exhume boulders lying at the base of a beach and
are known to move boulders alongshore, storm waves
cannot deposit sand and boulders together on a beach unless
overwash is involved.
The cause of boulder floaters in paleo-deposits is difficult
to determine unless such deposits lie above the limits of
storms. For example, south of Sydney along the east coast
of Australia (Fig.
3.3
a), 0.2-0.4 m boulder floaters are
common within sand deposits (Bryant et al.
1992
). The
coastline is tectonically stable, and sea levels have not been
more than 1 m higher than present. Here, many deposits lie
perched on slopes to elevations of 20 m above sea level,
well above the maximum limit of storm surges. Bouldery
sands also appear behind beaches where the nearest source
for boulders lies up to a kilometer away. Unfortunately,
boulder floaters as a signature of tsunami are not an easily
recognized one in the field. Boulder floaters often constitute
less than 0.1 % by volume of a deposit and lie buried
beneath the surface. These facts make them difficult to find
unless the deposits are trenched or intensively cored.
wash, glowing volcanic avalanches, and human disturbance.
Dump deposits were first identified along the south coast of
New South Wales, Australia (Fig.
3.3
a) (Bryant et al.
1992
,
1996
). Because ice, volcanic activity, and ground freezing
are not present along this coast, catastrophic tsunami, as
proposed by Coleman (
1968
) became a viable mechanism
for the transportation and deposition of large volumes of
sediment with minimal sorting in a very short period.
Coarser dump deposits, containing an added component of
cobble and boulders, often are plastered against the sides or
on the tops of headlands along this coast (Fig.
3.7
). Many
recent deposits may also contain mud lumps. It would be
tempting to assign these deposits to storms but for three
facts. First, storm waves tend to separate sand and boulder
material. Storm waves comb sand from beaches and trans-
port it into the nearshore zone in backwash and rips. Storm
swash, however, moves cobbles and boulders landward and
deposits them in storm berms. Certainly, storms do not
deposit mud and angular mud lumps in coarse sediment
deposits on steep slopes. On the other hand, tsunami,
because of their low height relative to a long wavelength,
form constructional waves along most shorelines, trans-
porting all sediment sizes shoreward. Second, while it may
be possible for exceptional storms to toss sediment of
varying sizes onto cliff tops more than 15 m above sea level
(Solomon and Forbes
1999
), tsunami dump deposits can be
found not only much higher above sea level (Fig.
3.7
), but
also in sheltered positions. Finally, there is substantial
observation from Hawaii and elsewhere of tsunami laying
down dump deposits (Fig.
3.6
) (Bryant et al.
1996
).
The internal characteristics or fabric of tsunami dump
deposits allude hydrodynamically to their mode of transport
and deposition. This fabric is identical to that formed by
3.2.3.1 Dump Deposits
Chaotic sediment mixtures, or dump deposits, are emplaced
in coherent piles or layers above the limits of storm waves,
mainly on rocky coasts. They can be problematic. Such
deposits can also be formed by solifluction, ice push, slope
