Geoscience Reference
In-Depth Information
Relevant reading
Ashley, G.M., Dalrymple, R.W., Elliott, T., et al. (1990) Classification of large-scale subaqueous bedforms:
a new look at an old problem.
Journal of Sedimentary Petrology
60
, 160 -72.
Belderson, R.H., Johnson, M.A. & Kenyon, N.H. (1982) Bedforms. In:
Offshore Tidal Sands
(Ed. A.H. Stride),
pp. 27-57. Chapman and Hall, London.
Darbyshire, J. (1972) The effect of bottom topography on the Agulhas Current.
Pure and Applied Geophysics
101
, 208 -20.
Flemming, B.W. (1978) Underwater sand dunes along the southeast African continental margin - observations
and implications.
Marine Geology
26
, 177-98.
Flemming, B.W. (1980) Sand transport and bedform patterns on the continental shelf between Durban and Port
Elizabeth (southeast African continental margin.
Sedimentary Geology
26
, 179 -205.
Flemming, B.W. (1981) Factors controlling shelf sediment dispersal along the southeast African continental
margin.
Marine Geology
42
, 259 -77.
Flemming, B.W. (1988) Pseudo-tidal sedimentation in a non-tidal shelf environment (southeast African con-
tinental margin). In:
Tide-influenced Sedimentary Environments and Facies
(Eds P.L. De Boer, A. van Gelder
& S.D. Nio), pp. 167- 80. Reidel, Dordrecht.
Harris, P.T., Tsuji, Y., Marshall, J.F., et al. (1996) Sand and rhodolith-gravel entrainment on the mid- to outer-
shelf under a western boundary current: Fraser Island continental shelf, eastern Australia.
Marine Geology
129
, 313 -30.
the wind-driven, northward, along-shelf current.
Where the coastline is relatively straight, the
muddy sediment is transferred northward along
the shelf, but where it is indented, fine sediments
become trapped in inner-shelf embayments behind
headlands or sand spits.
Trade winds also dominate sedimentation on
the western New Caledonian shelf, in the south-
west Pacific Ocean. Here, the shelf lagoon is
100 km in length, averages
c
. 18 m in depth,
and has a funnel shape, narrowing from 40 km
wide in the south-east to about 5 km in the north-
west, constrained to seawards by a series of long
barrier reefs. The maximum tidal range is
c
. 1.7
m, and maximum tidal currents are
c
. 0.2 m s
−1
.
The prevailing winds in the south-west lagoon
are moderate to strong south-east trade winds
(
c
. 18 knots) that blow for more than 200 days
a year (November-May), and which drive shelf
currents north-west along the lagoon (Douillet
et al. 2001). Wind-driven currents dominate the
oceanography when wind speeds are
sediments, confining the accumulation of muddy
material to embayments in the complex coast-
line (Debenay 1987).
10.2.3.7 Additional shelf sedimentary processes
As noted above, sedimentary processes on muddy
shelves are influenced by factors such as dissolved
oxygen concentration, sediment organic matter
and bioturbation. On other shelves, especially
where carbonate producers are the primary sedi-
ment source, various oceanographic factors asso-
ciated with the water column (nutrient supply,
water clarity, water temperature) are all import-
ant factors (Reading 1996). Regarding sedimen-
tary 'disturbance', natural hydrocarbon seeps
can be important on some continental shelves,
because pockmarks produced by escaping fluids
can be 0.5 to 20 m deep and 1 m to 1 km in
diameter (Hovland & Judd 1988). Pockmarks
represent material released from the sea bed and
may contain an aggregation of biogenic material
in the centre (Dando 2001). Finally, in the light
of the (magnitude 9.0) earthquake of 26 Decem-
ber 2004 beneath the sea bed off north-west
Indonesia, which created a destructive tsunami
that killed more than 150,000 people, it is worth
2ms
−1
(Douillet 1998). Although data are relatively few,
resuspension and resedimentation can account
for more than 80% of total sedimentation (Clavier
et al. 1995), and wind-driven currents assist
the shelf to remain relatively free from muddy
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