Geoscience Reference
In-Depth Information
52
o
W 51
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52
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W 51
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6
o
6
o
Physically stratified sand
Interbedded mud/sand
Proximal-shell sandy silt
Faintly laminated mud
Mottled mud
Organic-rich laminae
0 m
Sediment accumulation rate mm yr
−
1
5
o
5
o
6 m
4
o
4
o
0
0m
1 m
0 m
3
o
0 m
10
3
o
5
100 m
100 m
3
2
o
5 m
2
o
5 m
5 m
2
9 m
1
0.1
0 m
0.5 m
1
o
1
o
Amazon
River
Amazon
River
0
o
0 m
0
o
3 m
Marajo Island
Marajo Island
1
o
1
o
0 100 km
0 100 km
2
o
S
2
o
S
Fig. 10.7
Sedimentary facies and accumulation rates on the continental shelf off the Amazon River. (Adapted from Kuehl et al. 1986.)
10.2.3.5 Oceanic current-influenced shelves
scale bedforms generated by waves and tides
(Testa & Bosence 1999).
Although sedimentation on many shelves is influ-
enced to some degree by oceanic currents, such
as on the Amazon, Newfoundland (Barrie &
Collins 1989) and outer Saharan shelves (Newton
et al. 1973), several cases occur where the intrud-
ing oceanic current (generally a western boundary
current) is a major contributor to sediment dis-
persal, especially on the outer shelf, and in places
on the mid-shelf (Case Study 10.2). The long
NW-SE-orientated shelf of north-east South
America is influenced by the Guiana Current,
which flows at 35-75 cm s
−1
to the north-west,
and which supplements strong tidal currents
and waves. To the extreme east is the north-east
Brazilian shelf, which is relatively starved of land-
derived sediments, but which contains a suite
of coast-parallel zones of large-scale carbonate
and siliciclastic bedforms, driven by oceanic
and wind-driven flows, with superposed smaller
10.2.3.6 Wind-driven-current influenced shelves
Except during storms, wind-driven currents are
generally only weak, but they may be sedimento-
logically significant because they may carry fine
sediment placed into suspension by tides or
fair-weather waves. On the Great Barrier Reef
shelf, during fair-weather conditions, trade-winds
blow from the south-east, along the shelf. Shelf
sediment transport results primarily from wave-
induced resuspension, combined with north-
ward-directed wind-driven currents and coastal
longshore drift (Belperio 1983, 1988; Larcombe
et al. 1995; Orpin et al. 1999). Suspended sedi-
ment concentrations of 10 -100 mg L
−1
, caused
by resuspension of sea-bed mud, occur in a
2-10 km wide coastal belt, which drifts north
at rates of up to 15 cm s
−1
under the influence of