Geoscience Reference
In-Depth Information
Table 4.3
Comparison of tsunami and storm wave heights required to transport boulders in the Bahamas
Volume (m
3
)
Location
Length (m)
Boulder
width (m)
Thickness (m)
Weight (tonnes)
Height of tsunami
at shore (m)
Breaking storm-
wave height (m)
Palaeo-
boulders
13.0
11.5
6.5
972
1,846
5.9
23.7
14.0
7.3
6.7
685
1,301
2.6
10.5
9.3
6.0
4.0
223
424
2.8
11.3
8.1
5.7
5.5
254
482
1.9
7.7
7.2
5.7
5.0
205
390
2.1
8.2
Modern
7.8
4.7
2.5
92
174
2.7
10.9
4.9
4.5
1.2
26
50
4.0
16.1
3.8
2.5
2.0
19
36
1.0
4.0
3.8
3.2
1.5
18
35
1.9
7.8
6.4
2.7
0.5
9
16
3.9
15.7
Source Based on Hearty (
1997
)
8-25 m high, increasing in elevation towards their tip.
Some ridges indicate that waves must have run up to ele-
vations of 40 m above sea level at the time of deposition.
There are up to 30 ridges, some tucked into each other.
Nowhere does this involve more than four ridges. The rid-
ges have a consistent orientation to the west-southwest that
varies by no more than 10 along 300 km of coastline,
despite a 60 swing in the orientation of the shelf edge.
Internally, the ridges contain low-angle cross-beds, scour-
and-fill pockets, pebble layers, and bubbly textures char-
acteristic of rapid deposition found at the swash limit of
accreting sandy beaches. The steepest-dipping beds are
found towards the landward margin of the ridges. Because
of their shape and the fact that smaller ones lie nestled
within large forms, they have been termed chevron ridges. It
was similarities to these ridges, found at Jervis Bay, New
South Wales, that led to this term being used to identify one
of the prominent signatures of large tsunami.
The chevron ridges on Eleuthera Island are associated
with huge boulders up to 970 m
3
in size and weighing up to
2,330 tonnes (Hearty
1997
). These have been transported
over the top of cliffs more than 20 m high. The boulders
were emplaced at the end of the Last Interglacial sea level
highstand. They were transported up to 500 m landward—a
distance greater than present-day boulders have been trans-
ported. The modern boulders are much smaller, averaging
22 m
3
in volume and weighing less than 175 tonnes
(Table
4.3
). However, some of these modern boulders are
anomalous and suggestive of recent tsunami. For example,
some have been transported up to 200 m landward and more
than 10 m above sea level. Again, Eqs. (
3.3
) and (
4.2
) can be
used to resolve the difference between the capacity of the-
oretical tsunami and storm waves to transport these boulders.
Note that in these calculations a density of 1.9 g cm
-3
has
been used for the coral boulders. The largest of the mod-
ern coral boulders requires a storm wave about 16 m in
height to be transported, whereas the paleo-boulders require
maximum storm waves of about 24 m in height. Both of
these sizes are difficult to obtain close to shore without
breaking even under storm surges of 7-8 m that can be
generated here by tropical cyclones. Local storms also do not
account for the formation and consistent alignment of the
chevron ridges. Tropical cyclones have winds that rotate
around an eye that rarely exceeds 100 km in diameter. For
the ridges to be produced by a cyclone, the storm would have
had to maintain consistently strong winds and moved par-
allel to the islands over a much longer distance than is
observed at present. The echelon nature of some chevron
ridges, tucked one inside the other, also requires more than
one storm—which appears unlikely.
Tsunami with a distant origin appear more feasible. The
paleo and modern boulders require maximum tsunami wave
heights of only 6 and 4 m respectively. Tsunami waves, as
small as 1.0-2.0 m in height, could have moved some of the
boulders. Tsunami generated by submarine landslides or
comet/asteroid impact with the ocean produce up to four
waves in their wave train. This fact could easily account for
up to four chevron ridges nestled one within another—all
with the same orientation. While the Bahamas are subject to
submarine landslides along the shelf margin, a local source
can be ruled out because waves would have radiated out-
wards from this source and not been able to produce the
consistent ridge alignment over such an extended distance.
Either distant submarine landslides or an comet/asteroid
impact in the Atlantic Ocean accounts for the ridges and
boulder deposits in the Bahamas. These possibilities will be
discussed further in subsequent chapters.
A third example comes from Leeward Lesser Antilles,
consisting of the islands of Aruba, Curaçao and Bonaire
(Fig.
4.7
c). The islands stand slightly elevated above pres-
ent sea-level but are devoid of a fringing reef in an envi-
ronment where coral should have no problem growing
