Geoscience Reference
In-Depth Information
Table 5.4
Wave sizes required to transport boulders from various pre-transport
settings, Boulder Ridge, Exmouth, Western Australia (from Nott
2004
)
a
axis
b
axis
c
axis
Vol
T (sm)
S (sm)
T (sa)
S (sa)
T (lift)
S (lift)
3
2.1
0.7
4.4
1.4
6
1.3
5.3
4.5
17.9
2.4
1.8
0.7
3.0
1.0
4
1.1
4.2
3.6
14.3
2.8
1.8
0.6
3.0
1.2
5
1.2
4.9
4.2
16.7
2.2
1.5
0.5
1.7
1.0
4
0.9
3.6
3.3
13.1
2
1.2
0.7
1.7
0.5
2
0.9
3.6
3.0
12.0
Boulder axes (
a
,
b
and
c
) measured in metres, vol
=
volume of boulder (m
3
),
T
=
tsunami, S
=
storm wave, (sm)
=
submerged boulder, (sa)
=
subaerial boulder,
(lift)
=
boulder lifted from joint bounded block position through lift force only.
Density of boulders measured at 2.1 gm m
−
3
.Waveequations presented in Nott
(2003).
lands along with displacement of layers of cap rock strata, and deposits of
poorly mixed sediments (diamicts) including sand, shells, corals and lithic frag-
ments several centimetres in diameter (Bryant
et al
., 1992, 1997;Bryantand
Nott, 2001).
Bedrock erosional features along the coast remain a more equivocal form of
evidence for past tsunamis. Their similarity to such forms in fluvial and glacio-
fluvial environments, and the fact that they often cut across structures and
bedding planes, suggests that flowing water may be responsible. However, the
precise physical mechanisms for the formation of these features in coastal envi-
ronments remain to be determined. Until recently their presence in the coastal
environment had not been discussed in the literature (Bryant and Young, 1996).
Bedrock sculptured features in fluvial environments most likely form during
extreme flow velocity conditions, and the same is likely in the coastal environ-
ment. The critical question is how rapidly can they form? Certainly, some of
these forms were present in the sand barrier at Sissano Lagoon following the
10--15 m high tsunami of July 1998. Here, depressions ranging in size from 0.5 to
1.0mdepthand0.5to8mdiameterwereeroded into the rear of the barrier. But
these features formed in unconsolidated sands, not bedrock. At the moment the
origin of the coastal bedrock erosional features is far from resolved; numerical
modelling may help to resolve this situation.
Astudy of marine inundations at Cape Leveque, Western Australia found all of
theabove mentioned signatures of palaeowave deposits, in this case most likely
attributable to tsunamis (Nott and Bryant, 2003). Cape Leveque is about 200 km
north of Broome on Australia's northwest coast (Fig. 5.12). Evidence for large
marine inundations here include fields of imbricated boulders, sedimentary
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