Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 138

eastwards across the headland, suggesting that waves over-ran the headland
from the west-northwest. The main channel on the northern side of the head-
land is 5--6 m across and as deep as 3 m. Minor tributaries feed into the channel
head. Clasts of the porcellanite lie within the drainage channel. Various species
of marine shells ( Melo Amphora, Phasianell sp .), oysters and coral fragments are
scattered across the headland. Dunes of sediment weathered from the Creta-
ceous strata also contain gravel derived from the shore platforms as well as
shell and coral. These dunes sit on a section of the headland about 30 m above
sea level (a.s.l.), above the general level of the dislodged porcellanite layer and
thewaveoverwash channels. Farther south along the western edge of the head-
land, drainage seems to have been toward the southwest. Here a network of
channels deepen up to 12 m and widen in the direction of flow. The channels
are carved into the weathered but still-consolidated Cretaceous strata (Fig. 5.6).
The channel walls have well-developed cavettos and potholes a few metres deep
are numerous. In the same part of the headland, vegetation has been removed,
and an obvious trim line runs south along the west side of the headland. Today
no vegetation grows west (seaward) of this trim line. Yet clearly vegetation used
to be here, for abundant iron-indurated root casts occur in the upper soil layer.
Wave-transported sediment at the back of the beach, 400 m northeast of
theheadland, includes dunes up to 8 m high. These dunes contain abundant
angular to rounded gravel of Cretaceous sandstone and porcellanite from the
headland, together with a variety of species of shells, including oysters, and
coral fragments. The evidence at Cape Leveque suggests the following sequence
of events:

(1) wavesran up over the headland to 20--30 m a.s.l., stripping several metres

of orange sandy regolith and maybe also disrupting the porcellanite
layer creating boulders of this material;

(2) partial burial of these boulders, either in this event or a later one, and

deposition of shell and gravel-rich sand dunes at 30 m a.s.l;

(3) channels cut below the level of the high-gravel and shell-rich dune.

The erosion of these channels may have been contemporaneous with
deposition of sediments on the headland.

The height of the headland and modifications to it by waves suggest tsunamis
to be the most likely cause. This site has experienced tsunamis with run-ups
as high as 6 m above mean sea level in AD 1977. A tsunami of this height is
toosmall to have caused the impact to the headland, suggesting that much
larger tsunamis have occurred here in prehistoric times. Whether these larger
tsunamis were caused by earthquakes in Indonesia as occurred in 1977 remains
to be determind. If earthquakes were responsible then they must have been of