Geoscience Reference
In-Depth Information
Fig. 4.8
Flagstaff Point,
Wollongong, Australia. This
headland, which is 20 m above
sea level, was overridden and
severely eroded by a paleo-
tsunami that approached from the
southeast (white arrows). Main
features (white lines) are as
follows: a boulder piles,
b incipient whirlpool with fluted
rim, c plug, d canyon, and
e smear deposit over headland.
Sediment was transported across
the bay and deposited as a sand
sheet in the distance (f)
f
e
d
c
a
b
humate-cemented sediment from water depths of 100 m at
the edge of the continental shelf. For this to happen, distinct
from the capability of storm waves, the tsunami had to be
over 5 m high when it reached the shelf edge.
Tsunami-deposited barriers in the Jervis Bay region
consist of clean white sand that originated from the leached
A2 horizon of podsolized dunes, formed at lower sea levels
on the floor of the bay during the Last Glacial over
10000 years ago (Bryant et al.
1997
). The barriers form
raised platforms 1-2 km wide and 4-8 m above present sea
level on a tectonically stable coast. The barriers supposedly
were built up over the last 7000 years; however, the sands
yield an older age commensurate with their origin on the
floor of the bay. Again, the sands must have been trans-
ported to the coast suddenly in order to maintain an older
temporal signature. Transport occurred during one or more
tsunami events.
The effect of tsunami on the rocky sections of this coast
is even more dramatic. The scenic nature of many head-
lands is partially the consequence of intense tsunami ero-
sion. Two headlands, Flagstaff Point and Kiama Headland,
in the Wollongong area will be described here (Bryant and
Young
1996
). Both headlands are similar in that they pro-
trude seaward about 0.5 km beyond the trend of the coast.
Both were affected by the same erosive tsunami traveling
northwards along the coast. The raised dump deposit and
inverted keel-like stacks described in the previous chapter
respectively). Flagstaff Point consists of massively jointed,
horizontally bedded volcanic sandstones that have been
deeply
high-velocity tsunami has severely eroded the seaward
facets of both headlands, but with subtle differences. On
Flagstaff Point, the tsunami eroded the softer material,
forming a reef at the seaward tip and a rounded cliff along
the southern side (Fig.
4.8
). The wave planed the top of the
headland smooth, spreading a smear deposit that consists of
muds, angular gravel, and quartz sand across this surface.
The wave also moved massive boulders into imbricated
piles on the eroded platform surface on the south side.
However, the most dramatic features were created by tor-
nadic vortices on the northern side of the headland. These
vortices were 8-20 m deep and rotated in a counterclock-
wise direction. The largest vortex was shed from the tip of
the headland eroding a whirlpool with 20 m high sides into
the cliff. Fluted bedrock on the outer surface of erosion
outlines the vortex and its helical flow structure. The
whirlpool is incomplete, indicating that the vortex devel-
oped towards the end of the wave's passage over the
headland. As the wave wrapped around the headland, it
broke as an enormous plunging breaker, scouring out a
canyon structure along the northern side. The canyon is
separated from the ocean by a buttress that rises 8 m above
sea level at the most exposed corner, tapering downflow as
the wave refracted around the headland. The tsunami then
travelled across the sheltered bay behind the headland,
carving giant cusps into a bedrock cliff on the other side.
Finally, it swept inland depositing a large pebble- and
cobble-laden sheet of sand up to 500 m inland.
At Kiama, the tsunami wave developed broad vortices,
as water was concentrated against the southern cliff face
(Fig.
4.9
). While flow probably obtained velocities similar
to those at Flagstaff Point, the weakly weathered basalt was
weathered,
while
Kiama
Headland
consists
of
weakly
weathered,
resistant
basalt.
Overwashing
by
