Geoscience Reference
In-Depth Information
noting that the Pacific Ocean has an average of
one to two destructive tsunamis each year; i.e.
they are geologically common. Although many
continental shelves are clearly influenced by
tsunami, there are few existing data from modern
shelves on their flows or their deposits.
needed applying such concepts to modern shelf
sedimentary processes, because shelf sediments
can reflect, to varying degrees, sedimentary pro-
cesses that are no longer active, and there can be
a variety of grain sources.
Increasingly, routine use of laser-diffraction and
other high-resolution techniques indicates that
many shelf sediments have polymodal grain-size
distributions. Size modes, once identified, can
reveal dispersal patterns. For example, on the
Californian shelf, the distribution of the domin-
ant size modes shows that Columbia River sedi-
ments are transported north-north-west across the
shelf, and decrease in size from fine sand to coarse
silt (Fig. 10.8). Very fine sands are held close to
the coast and accumulate at rates below
c
. 1.4 mm
yr
−1
, whereas coarse silts dominate along a well-
defined axis towards the north-north-west, rep-
resenting the main transport path of suspended
sediment. Accumulation rates decrease along
the main transport path from about 7 mm yr
−1
to 3mmyr
−1
. The sediment transport rate of
volcanic ash across the shelf has been calculated,
10.2.3.8 Inferring sedimentary processes from
sedimentary facies
Where direct measurements of shelf sediment-
ary processes do not exist, the nature of long-
term shelf sedimentation can be inferred from
the characteristics of the deposits, using aspects
including the spatial distribution of sediment-
ary facies and bedforms, gradients of sediment
texture and composition, and the shallow stra-
tigraphy. It is of course very helpful to date the
sediments and hence infer the timing of the main
sedimentary events. Long-term patterns of sedi-
ment dispersal sometimes can be demonstrated
from regional variations in sedimentation. On
the central inner-shelf of the Great Barrier Reef,
Holocene sediment accumulation rates decrease
with distance from the main source, the Burdekin
River, which discharges sediment onto the shelf
at an average rate of 3-9
0 20 km
10
6
tyr
−1
(Neil et al.
2002). Holocene sediment accumulation rates
decrease along the regional shelf-parallel sediment
transport pathway, from
c
. 0.7 to
×
0.1 mm yr
−1
(Woolfe & Larcombe 1998) as a function of the
regional trapping ability of a set of north-facing
coastal embayments. On the larger scale of the
entire central and northern GBR shelf, the texture
and composition of terrigenous sediments along
the 10 m depth contour, although complex, also
indicate general net northward shelf sediment
dispersal (Lambeck & Woolfe 2000).
Distance along a transport path sometimes
can be reflected in terms of two sedimentary
gradients:
1
minerals of low resistance to abrasion and
breakage (e.g. carbonate grains) give way to more
resistant minerals (e.g. heavy minerals);
2
grains become less angular and more rounded.
Concepts of compositional and textural maturity
thus result (Pettijohn et al. 1987), with immature
sediments tending to be close to source. Care is
Shelf
edge
<
Astoria Canyon
Accumulation rates (mm yr
−
1
)
> 4.2
2.8 - 4.2
1.4-2.8
< 1.4
Coast
Fig. 10.8
The Oregon-Washington shelf: accumulation rates
of sediments. (Adapted from Nittrouer et al. 1979.)
