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(a)
(b)
(2)
Upper
continental
slope
(1)
A
A '
A'
(3)
0
10
20
30
40
50
(1)
(1)
(2)
(2)
(2)
(2)
(2)
(3)
0 5 10 15 20 25 30 35
Distance across shelf (km)
Townsville
A
(3)
Coral reefs
Inner shelf - terrigenous sands/muds
Middle shelf - shelly muddy sands
Outer shelf - carbonate sands
Case Fig. 9.2 Simplified plan (a) and cross-sectional (b)
views showing the effects of cyclones on sediment generation
and dispersal across the central Great Barrier Reef. (Adapted
from Larcombe & Carter 2004.)
Cyclone impacts:
(1) Breakage of reef framework and mobilization of reef sediment
(2) Cyclone 'pumping' of sediment along and from the middle shelf
(3) Fluvial sediment inputs onto the inner shelf
1 causing the breakage of reef framework on the outer shelf, which contributes carbonate sand
and gravel on the leeward flanks of individual reefs;
2 generating floods in rivers and discharge of terrigenous sediment to the inner shelf;
3 eroding the sea-floor of parts of the inner and middle-shelf zones (Case Fig. 9.2a & b).
Cyclones also, however, exert a major control on the mobilization and transport of sedi-
ment on the shelf. Under cyclonic conditions, fast wind-driven currents (
60 cm s −1 ) have been
recorded and, in the case of Cyclone Joy (1990), these formed a persistent flow of 9 days dura-
tion. Under such conditions, extensive along-shelf transport of sand and gravel occurs, and the
cyclones act as a 'pump', transporting large volumes of sediment northwards along the shelf
(Larcombe & Carter 2004).
Sedimentary evidence for such transport occurs across much of the inner and middle shelf,
in the form of large (up to 2 m high) subaqueous sand and gravel dunes, sediment ribbons and
large dune fields. Extensive erosion of the sea-floor also occurs across the middle shelf, which in
combination with high rates of cyclone-driven sediment transport may explain the lack of reef
development on the middle shelf throughout the Quaternary. The cyclones also redistribute
large amounts of the suspended terrigenous sediment that is discharged onto the shelf during
storm events, driving this sediment along and onshore, and thus contributing to the develop-
ment of the terrigenous sediment-dominated inner shelf. In this way, cyclones act both as a
control on cross-shelf sediment accumulation and along-shelf sediment transport.
>
Relevant reading
Gagan, M.K., Chivas, A.R. & Herczeg, A.L. (1990) Shelf-wide erosion, deposition, and suspended sediment
transport during Cyclone Winifred, central Great Barrier Reef, Australia. Journal of Sedimentary Petrology
60 , 456 -70.
Larcombe, P. & Carter, R.M. (2004) Cyclone pumping, sediment partioning and the development of the Great
Barrier Reef shelf system: a review. Quaternary Science Reviews 23 , 107-35.
Smithers, S. & Larcombe, P. (2003) Late Holocene initiation and growth of a nearshore turbid-zone coral reef:
Paluma Shoals, central Great Barrier Reef, Australia. Coral Reefs 22 , 499-505.
 
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