Geoscience Reference
In-Depth Information
debris, exposing the underlying bedrock to large lift forces.
Blocks, controlled in size by the thickness of bedding planes
and the spacing of joints, may be too large to be entrained by
this tsunami flow, but the lift forces can continually jar blocks
until they eventually fracture into smaller pieces. This pro-
cess occurs in preparation zones at the front of cliffs. When
blocks are small enough, they then are transported across
headlands or down platform and ramp surfaces. The lack of
percussion marks or chipping on most boulders, some of
which are highly fretted by chemical weathering, is sugges-
tive of boulder suspension in sediment-starved flows without
bed contact until the boulder is deposited. The minimum flow
depth or tsunami wave height along the coast of New South
Wales required in this scenario is 4 m, although larger waves
must certainly have been present to move such material up
cliffs 30 m high (Fig.
3.12
).
There is no general consensus on whether or not coastal
boulders are moved by tsunami or storms (Dawson et al.
2008
; Scheffers et al.
2009
; Paris et al.
2011
). In both cases,
the favored mechanism has often never been witnessed nor
monitored. Observations along coasts where storm waves
dominate, and have been observed, indicate that boulder
weight, altitude, and landward distance of transport mimic
those of some tsunami deposits (Etienne and Paris
2009
;
Goto et al.
2010
; Fichaut and Suanez
2011
). Determination
of the mechanism of transport of boulders must consider
their environmental context and internal fabric—if any.
Boulders relate to tsunami if the following ten criteria are
met: (1) the boulders are in groups; (2) the boulder deposits
exclude other sediment sizes; (3) the boulders are imbri-
cated and contact-supported; (4) the points of contact-sup-
port lack evidence of percussion and thus implicate
suspension transport; (5) the boulders show lateral transport
or shifting distinctive from in situ emplacement due to cliff
collapse under gravity; (6) the boulders are elevated above
the swash limit for storm waves; (7) the boulders are situ-
ated away from any shoreline where storm waves impacting
on the coast could have simply flicked material onto shore;
(8) the evidence of transport by tsunami rather than storm
waves is unequivocal based upon the size of boulders—
usually above 100 tonnes—and hydrodynamic determina-
tions; (9) the direction of imbrication matches the direction
of tsunami approach to the coast regionally; and (10) there
are other nearby signatures of tsunami.
turbidites (Masson et al.
1996
). As a submarine landslide
moves downslope under the influence of gravity, it disin-
tegrates and mixes with water. The sediment in the flow
tends to separate according to size and density, forming a
sediment gravity flow called a turbidity current. The slurry
in a turbidity current moves along the seabed at velocities
between 20 and 75 km h
-1
, and can travel thousands of
kilometers onto the abyssal plains of the deep ocean on
slopes as low as 0.1. As current velocity decreases, splays
of sediment, known as turbidites, are deposited in sub-
marine fans. Turbidite thickness depends upon the distance
of travel and the amount of sediment involved in the ori-
ginal submarine landslide. In the Atlantic Ocean, individual
turbidites have volumes of 100-200 km
3,
values implying
submarine landslides that are sufficient to have generated
tsunami of several meters amplitude at their source. Tur-
bidity currents have not been directly observed; however,
there is substantial indirect evidence for their existence.
One of the best of these is the sequential breaking of tele-
graph cables laid across the seabed. The first noteworthy
record occurred following the Grand Banks earthquake on
November 18, 1929 off the coast of Newfoundland (Heezen
and Ewing
1952
). Similar events have occurred off the
Magdalena River delta (Colombia), the Congo Delta, in the
Mediterranean Sea north of Orléansville and south of the
Straits of Messina, and in the Kandavu Passage, Fiji.
Turbidites generally are less than 1 m in thickness and
form a distinct layered unit known as a Bouma sequence
(Bouma and Brouwer
1964
). The upward structure of a
Bouma unit (Fig.
3.16
) shows erosional marks in the
underlying clays called sole marks, overlain by a massive
graded unit (T
a
), parallel lamination (T
b
), rippled cross-
lamination or convoluted lamination (T
c
), and an upper unit
of parallel lamination (T
d
). This latter unit contains gravels
and pebbles close to the source, and fine sand and coarse silt
out on the abyssal plain. The unit is overlain by pelagic
ooze (T
ep
) that has settled under quiet conditions between
events. The basal contact below the coarse layer is sharp,
while that above is gradational. The coarse layer is also well
sorted and contains microfossils characteristic of shallow
water. Interpretation of this sequence, supported by labo-
ratory experiments, indicates deposition from a current that
initially erodes the seabed, and then deposits coarse sedi-
ment that fines as velocity gradually diminishes. More has
been written about sediment density flows in sedimentology
than on any other topic, and the deposits form one of the
most common sedimentary sequences preserved in the
geological record. Each turbidity deposit preserved in this
record potentially could be a diagnostic signature of a tsu-
nami event. Submarine landslides and their resultant tsu-
nami
3.2.9
Turbidites
There are few geomorphic features linked to tsunami
described in the ocean. However, one of the most notable,
turbidites, has received considerable attention in the litera-
ture.
have
been
very
common
features
in
the
world's
Submarine
landslides
generate
both
tsunami
and
oceans throughout geological time.
