Geoscience Reference
In-Depth Information
3.2.8
Large Boulders and Piles of Imbricated
Boulders
giant ripples spread over a distance of at least 1.5 km atop
80 m high cliffs. These mega-ripples have a relief of
6.0-7.5 m, are asymmetric in shape, and spaced 160 m
apart. The mega-ripple field is restricted to a 0.5-0.7 km
wide zone along the cliffs, and is bounded landward by a
linear ridge of sand several meters high paralleling the
coastline. This ridge is flanked by small depressions. Fur-
ther inland, deposits grade rapidly into hummocky topog-
raphy, and then a 1-2 m thick sandsheet. The mega-ripples
were produced by sediment-laden tsunami overwashing the
cliffs, with subsequent deposition of sediment into bedforms
along the cliff top. Flow then formed an overwash splay as
water drained downslope. The flow over the dunes is the-
orized to have been 7.5-12.0 m deep and to have obtained
velocities of 6.9 m s
-1
-8.1 m s
-1
. The second example
occurs at Point Samson, Western Australia (Fig.
3.3
c). Here
gravelly mega-ripples have infilled a valley with bedforms
that have a wavelength approaching 1,000 m and an
amplitude of about 5 m (Nott and Bryant
2003
). Flow depth
is theorized to have been as great as 20 m with velocities of
over 13 m s
-1
.
Large tsunami can also generate dune bedforms on the
seabed. The T ¯hoku Tsunami of March 11, 2011 generated
landward asymmetric dunes in 10-15 m depths in Kesen-
numa Bay, Japan (Haraguchi et al.
2013
). The wavelength
of the dunes increases seaward from 8 m to 25 m over a
distance of more than a kilometer. The dunes are up to
1.5 m high and were formed by landward flow velocities of
about 8 m s
-1
. They consist of silt and sand covered with
gravel derived from the surrounding beaches. The gravel
was transported as bedload.
Tsunami differ from storm waves in that tsunami dissipate
their power at shore rather than within any surf zone. The
clearest evidence of this is the movement of colossal
boulders onshore. For example, the Flores Tsunami of
December 12, 1992 destroyed sections of fringing reef and
moved large coral boulders shoreward (Fig.
3.6
) (Shi et al.
1995
), often beyond the zone where trees were ripped up by
the force of the waves. The Sea of Japan Tsunami of May
26, 1983 produced a tsunami over 14 m high. A large block
of concrete weighing over 1,000 tonnes was moved 150 m
from the beach over dunes 7 m high (Minoura and Nakaya
1991
). Boulders transported by tsunami have also been
found in paleo-settings. For example, on the reefs of
Rangiroa, Tuamoto Archipelago in the southeast Pacific,
individual coral blocks measuring up to 750 m
3
have been
linked to tsunami rather than to storms (Bourrouilh-Le and
Talandier,
1985
). Figure
3.11
also shows boulders that have
been scattered by tsunami across the reefs at Agari-Hen'na
Cape on the eastern side of Miyako Island in the South
Ryukyu Islands (Kawana and Pirazzoli
1990
; Kawana and
Nakata
1994
). On Hateruma and Ishigaki Islands in the
same group, coralline blocks measuring 100 m
3
in volume
have been emplaced up to 30 m above present sea level,
2.5 km from the nearest beach. These boulders have been
dated and indicate that tsunami, with a local source, have
washed over the islands seven times in the last 4500 years.
Two of the largest events occurred 2000 years ago and
during the great tsunami of April 24, 1771. In the Leeward
Islands of Netherlands Antilles in the Caribbean, boulders
weighing up to 280 tonnes have been moved 100 m by
repetitive tsunami most likely occurring 500, 1500 and
3500 years ago (Scheffers
2004
). In fact, detailed cata-
loguing of anomalous boulders from the literature indicates
that they are a prevalent signature of tsunami on most
coasts. The largest boulders theorized as being transported
by tsunami occur in Tonga and the Bahamas, with weights
of 1,600 tonnes and 2,330 tonnes respectively (Hearty
1997
;
Frolich et al.
2009
). Both relate to higher sea levels during
the Last Interglacial.
Along the east coast of Australia, anomalous boulders are
incompatible with the storm wave regime (Young and Bryant
1992
; Young et al.
1996a
). For example, exposed coastal
rock platforms along this coast display little movement of
boulders up to 1-2 m in diameter, despite the presence of 7 m
to 10 m high storm waves. At Boat Harbor, Port Stephens
(Fig.
3.3
a), blocks measuring 4 m 9 3m9 3 m not only
have been moved shoreward more than 100 m, but also have
been lifted 10-12 m above existing sea level. At Jervis Bay,
preparation zones for block entrainment can be found along
3.2.7
Smear Deposits
More enigmatic are deposits on headlands containing a mud
matrix. These deposits are labeled smear deposits because
they are often spread in a continuous layer less than 30 cm
thick over the steep sides and flatter tops of headlands.
Along the New South Wales coast, these deposits have been
found at elevations 40 m above sea level. These deposits
can contain 5-20 % quartz sand, shell, and gravel. Smear
deposits are not the products of in situ weathering because
many can be found on volcanic sandstone or basalt, which
lacks quartz. These smear deposits form the traction carpet
at the base of a sediment-rich tsunami-generated flow
overwashing headlands. The mud allows sediment to be
spread smoothly under substantial pressure over surfaces.
When subsequently dried, the packed clay minimizes ero-
sion of the deposit by slope wash on steep faces. The
deposit has only been identified on rocky coasts where
muddy sediment lies on the seabed close to shore.
