Geoscience Reference
In-Depth Information
Reef front
Reef crest /
flat
Lagoon
Deposition of successive
ridges composed of coral
rubble removed from
reef front and reef crest
sites
Overturning of
coral bommies
Boulder emplacement
on reef flats
Off-reef transport of
fine sediments - settle
in deeper water
Reworking/transport
of shallow lagoon sands
Scouring of
shoreline sands
Accumulation of overwash
sand/rubble lobes in the
lee of reef flat
Fragmentation of shallow
water branched corals - rubble
moved both landward and offshore
Deeper water corals dislodged by
material flushed down the reef front
Talus deposits of
coral rubble at
base of reef front
Fig. 9.18
Schematic diagram illustrating the main erosional (bold italics) and depositional (italics) processes associated with cyclone
impacts on coral reefs.
the Queensland coast from the north-east, drive
along-shelf currents that transport sediment
northwards along the shelf, resulting in marked
cross-shelf variations in Holocene sediment
accumulation (Case Study 9.2).
Along mangrove-colonized shorelines, the main
sedimentological impacts of cyclonic disturbance
result from shoreline erosion caused by storm
surges. In Florida, some 15 m of shoreline ero-
sion occurred following Hurricane Andrew in
Case study 9.2 Cyclone-controlled sediment distribution on the Great Barrier Reef shelf
(the 'cyclone pump' model)
The Great Barrier Reef (GBR) shelf is the largest modern tropical mixed carbonate-siliciclastic
shelf system, extending around 2000 km along the eastern and north-eastern margins of
Australia, and varying in width from around 200 km in the south to 50 km in the north. It is
characterized by extensive reef development along its seaward margin, but also by sediment
deposition on the shelf. In the central regions, the shelf can be partitioned into three distinct
shore-parallel sediment zones. The inner shelf (0 -22 m depth) is dominated by terrigenous
sands and muds (5-10 m thick) and includes the development of intermittent detrital coral
build-ups (Smithers & Larcombe 2003). Under fair-weather conditions, sediment transport is
dominated by the northwards, along-shelf current-driven movement of sediments resuspended
by wave action in nearshore areas, and bedload transport occurs only in the shallow (
5m)
shoreface zone. The middle-shelf (22 - 44 m depth) is a zone of sediment starvation and is
characterized by only a thin veneer (
<
1 m thick) of Holocene shelly, muddy sands. This zone
receives little or no terrigenous sediment and there is limited sediment transport under fair-
weather conditions. The outer-shelf (40 -80 m depth) is dominated by a tract of reef develop-
ment and by associated carbonate-dominated detrital sediments. In fair weather little or no
terrigenous sediment reaches the outer reefs (Case Fig. 9.2a).
Cyclones are a common disturbance on the central GBR (two to three cyclones occur
each year at latitude 20°S) and therefore represent a major control on shelf sedimentation.
Cyclones influence sediment input to the GBR by:
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