Geoscience Reference
In-Depth Information
0 (km) 80
0
Africa
Walvis Bay
Maputo
100
−
20 m
−
40 m
Durban
Cape
To w n
80
−
60 m
Study
area
−
80 m
Benguela
Current
60
40
−
200 m
20
Return
Agulhas
Current
0
Subtropical Convergence
Modern sand prism
Relict gravels
Underwater dunes
222 x
Sand ribbons
22 x
Direction of littoral drift
0
1 x
Scale of distortion
5
Direction of dune
sand transport
10
Case Fig. 10.2B
Location of Agulhas shelf with regional ocean current patterns and detail of the shelf-parallel sedimentary
facies. (Adapted from Flemming 1980, 1981.)
55 cm s
−1
to the speed of near-bed currents, initiating movement of the rhodoliths and thus
forming a major control upon the presence of this facies on this shelf.
A particularly well-documented example of shelf sediment transport dominated by an
intruding oceanic current is the south-east African shelf (Flemming 1978, 1980, 1981, 1988)
(Case Fig. 10.2B). Here, the western boundary current is the Agulhas Current, which flows to
the south-south-west, parallel to the south-east African coast, at speeds up to 2.5 m s
−1
. Such
speeds easily entrain sand and gravel on the sea bed and induce the development of large
sedimentary bedforms such as large and very large transverse dunes (
sensu
Ashley et al. 1990)
in symmetrical, asymmetrical and 'cat-back' forms (i.e. those where the crest is reversed in
orientation compared with the overall bedform).
There is also a severe swell-wave regime, with regular large waves of long period. Given
the microtidal regime (i.e. maximum tidal ranges
2 m), the impact of the Agulhas Current is
the dominant mechanism driving sediment transport pathways. At major offsets in the con-
tinental margin, the Agulhas Current overshoots the edge of the margin, and, at some distance
downstream, the flow 'reattaches' to the margin (Darbyshire 1972). Flow divides at these
reattachment zones, so that some sediments to its north are transported back towards the offset
in a major eddy, and to its south are transported along the margin. As might be expected, these
zones tend to be relatively coarse-grained, ranging from fine sand up to gravel lag pavements.
Long-term fluctuations in the current lead to changes in the location of the bedload parting
zones of 10 -100 km, so that in places a complex suite of sedimentary bedforms occurs which
indicates that bedload sediment transport reverses its direction. The suite of sedimentary facies
and bedforms on the shelf is very similar to that observed on many tidally dominated shelves,
such as the UK shelf (section 10.2.3.1).
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