Geoscience Reference
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Fig. 3.8 The chenier-like ridges
in the Cullendulla Creek
embayment at Batemans Bay,
New South Wales, Australia
(from Bryant et al. 1992 )
Cullendulla
Creek
Age of ridges
ranked
youngest to
oldest
7
6
9
8 6
5
4
3
2
1
Surfside
Beach
Long
Beach
N
Batemans Bay
Tollgate Is.
Marsh or
mangrove
Rock
platform
Relict sand
plains
0 1 2 km
Depths in meters
periods are 4.5, 13.4, 22.5, and 31.0 min, values that fall
within the tsunami window measured in harbors. The che-
niers in the bay consist of a series of six, landward asym-
metric ridges deposited in a sheltered embayment presently
occupied by Cullendulla Creek (Fig. 3.8 ). The ridges rise
1.0-1.5 m above the surrounding estuarine flats and
increase in width and volume towards the bay. They consist
of shell-rich sand overlying estuarine muds. The formation
of these chenier ridges by storm waves is difficult to justify
because wave refraction reduces wave heights by 80-90 %.
Waves would also have to travel in the most convoluted
pathway possible to deposit ridges up the re-entrant within
Cullendulla Creek. The simpler explanation is that tsunami
have deposited chenier-like ridges and banks in this shel-
tered re-entrant. There is chronological evidence for at least
three tsunami events operating within the bay 300, 1300,
and 2800 years ago. While Batemans Bay contains the best
example of tsunami ridge development along this coast,
similar ridges and banks exist in the Port Hacking, Middle
Harbor, and Patonga Beach estuaries in the Sydney area
200 km to the north. Spits and ridges lying within sheltered
estuarine environments should be examined closely for
evidence of a tsunami origin.
Fig. 3.9 Fabric of sand and gravel deposited in a chevron-shaped
dune by paleo-tsunami at Steamers Beach, Jervis Bay, Australia. Car
keys are for scale. The rounded ball to the left of the keys consists of
humate eroded elsewhere from the B-horizon of a podsolic soil profile.
This deposit lies 30 m above sea level, while the dune itself crests
130 m above sea level
130 m above sea level, contains gravels and mud clasts
(Fig. 3.9 ) (Bryant et al. 1997 ). Because these tsunami swash
features are symmetrical, they form v-shaped dunes that
may be stacked relative to each other. They are also called
chevrons after the inverted v used in heraldry or the stripes
on the arms of a military uniform used to denote rank. This
term comes from similarly shaped dunes found in Barbados
and dating to the Last Interglacial (Hearty et al. 1998 ).
The chevron feature in Jervis Bay is contiguous in form,
from the present beach to a peak 130 m above present sea
level (Bryant et al. 1997 ). It is an exemplary signature of
mega-tsunami because its limits are far beyond those of
either storm swash or earthquake-generated tsunami repor-
ted in the literature. Four waves are envisaged to have
formed the feature (Fig. 3.10 ). The first wave was deposi-
tional, overwashing the hill 130 m above sea level and
3.2.5
Chevrons
Tsunami can also create parabolic-like dunes that are sandy,
but contain angular clasts or mud layers that cannot be
transported or deposited by wind. These types of ridges
exist on Hateruma Island in the South Ryukyus of Japan,
where they consist of sand, gravels, and large boulders up to
2 m in width (Ota et al. 1985 ). They were deposited by the
great tsunami of April 24, 1771, which had a possible
maximum run-up height in the region of 85.4 m. Another
example occurs at the south end of Jervis Bay, New South
Wales (Fig. 3.3 a). Here a parabolic dune, rising more than
 
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