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Further, the oscillatory action of waves would
have dominated in shallow water. Hence, fast
flows would have mobilized the sands and silts
of the inner shelf south-west along the shelf, and
the oblique offshore currents would have trans-
ported sediments offshore, with finer sediments
deposited in deeper water.
The seawards-fining and seawards-thinning
storm bed described above, however, is not a
universal feature. Shelves of different morpho-
logies and sedimentary regimes can have dis-
tinctively different storm sediment dynamics on
physical scales of tens to hundreds of kilometres.
For the Great Barrier Reef (GBR) shelf, the storm
bed coarsens and thickens seawards (Gagan et al.
1988, 1990; Larcombe & Carter 2004). Further,
and also in marked contrast with the Texas-
Louisiana shelf, the GBR storm bed is best
preserved on the inner shelf, where it is buried
beneath the depth of subsequent bioturbation by
immediate post-storm sedimentation. Storm-bed
preservation is a function of cyclone recurrence
interval (Case Study 10.1), storm-bed thickness
and the rate and depth of between-cyclone bio-
turbation (Gagan et al. 1988, 1990). In passing,
it has long been recognized that the frequent
passage of storms is not incompatible with the
presence of abundant and apparently delicate
benthic organisms on and in shelf sediments
(e.g. Vaughan et al. 1987).
Case study 10.1 Time-scales of cyclone-driven sedimentary processes on the northern Great Barrier
Reef shelf
Like most regions of the Great Barrier Reef shelf, the shelf off Cairns (latitude 16°50
S) can be
divided into three sedimentary regions(Case Fig. 10.1A), which in this region consist of:
1
the inner-shelf sediment prism (at depths of 0 -20 m and out to 10 -15 km from the coast)
formed of bioturbated muddy sand;
2
the middle shelf (depths of 20 - 40 m), narrow at this point at only 12 km wide, which is
mostly formed of bioturbated mixed quartz and calcareous gravelly muddy sands;
3
the outer-shelf reef complex, formed of a series of steep-sided patch reefs between which are
shelly calcareous sandy gravels at depths of 40 - 80 m.
The climate is strongly seasonal, with a summer-monsoonal climate, reflected in the shelf
hydrodynamic regime. In summer months (November-March), the weather is punctuated by
occasional cyclones, during which shelf waters are influenced by the discharge of muddy river
plumes and by wind-driven strong northward along-shelf flows (see also Case Study 9.2).
Major sediment transport events on the shelf are thus generally controlled by cyclones (Case
Table 10.1), the most severe (and least frequent) of which generate most sediment transport
and may rework the combined results of all intervening (and less severe) cyclones, so that the
resulting sedimentary record is dominated by a few cyclone beds. The stratigraphical record
of cyclones is varied in nature, patchy and spasmodic, and mostly only available for the period
since
c
. 5.5 kyr BP, the mid-Holocene sea-level highstand. Beach ridges on the Cairns coastal
plain have an average interval of
c
. 280 years between the emplacement of successive ridges
( Jones 1985). Equivalent figures for chenier ridges to the north and south are 177-280 years
(Nott & Hayne 2001; Case Fig. 10.1B).
On the Cairns inner shelf, nearshore sediment cores display one or more sharp-based, fining
upward, shell hash and sand to mud beds, interpreted as representing storm deposits (Carter
et al. 2002). Radiocarbon dates from shells indicate ages of 3100, 2980 and 2830 yr BP for
the three beds, indicating
′
100-year periods (120 yr and 150 yr) between successive major
cyclones. On the inner shelf south of Cairns, modern storm beds (Cyclone Winifred, 1986,
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