2 – Geomorphology of Coral Reefs with Special Reference to the Great Barrier Reef
13
Northern deltaic reefs. The northernmost 96 km of
shelf edge of the GBR south of the Gulf of Papua consists of short (up to 4 km) narrow reefs parallel to the
shelf edge, separated by passages up to 200 m wide.
Water depths at the shelf edge are only 30 m to 40 m
but on the ocean side is a steep drop-off down to
700 m. Complex delta-like lobes have formed on the
inner western side of each passage, deposited as sedimentary structures in response to strong flood tide
currents. These have formed the foundation for reef
growth.
Ribbon reefs (Fig. 2.5E). These form a classic shelf
edge barrier reef system between 11°S and 17°S (about
700 km). Individual reefs are up to 28 km long, separated by narrow passages most of which are less than
1 km wide. Currents through the passes are weaker
than opposite Torres Strait and although the ends of the
reefs curve inwards there are no deltaic structures.
However, tidal jets carrying nutrient rich water from
the front of the reefs (that have a steep drop-off down
to 1000 m) surge through the channels and maintain
the extensive Halimeda bioherms described above. The
outside of Ribbon 5 has been explored by submersible
and shows dead reefs at 50 m and 70 m above an
almost vertical limestone cliff down to 200 m. The
Pleistocene is at 15 m on Ribbon 5 and 19 m on
Yonge Reef. Holocene growth commenced about
8000 years ago and the ribbon reefs appear to have
reached modern sea level between 6000 and 5000 yrs
BP. Because of the high energy conditions, the ribbon
reefs display a strong zonation.
The submerged reefs of the central GBR. South of
Cairns, where the shelf widens, reefs are set back from
the shelf edge, which is in 70 m to 80 m of water, beyond which is a more gently inclined slope reaching
only 200 m within 3 km. Some reefs and shoals are
found in this area. Where examined, all outer shelf
reefs show the Pleistocene antecedent surface at depths
below 20 m, and, although colonised quickly after they
were drowned by the rising Holocene sea level (basal
dates greater than 8500 yrs BP), they did not reach
modern sea level until 4000 yrs BP, even though they
maintained vertical accretion rates of 6 m ka
1 . The
most significant feature of this 800 km section of the
GBR is the presence of up to three lines of submerged
reefs rising from water depths down to 100 m. In
places these reefs may be as much as 70 m deep but
elsewhere approach the surface though many remain
unmapped. These features are composite, formed during many transgressive and regressive episodes. Many
appear to be dead, perhaps drowned by rapid sea level
rise, sediment shedding from upslope or decline in
water quality when the shelf margin was first drowned.
However, some dense stands of coral have been seen
on ROV dives. This area of the shelf has experienced
hydro-isostatic subsidence but the impingement of
the active Halifax Basin onto the shelf may be the
most important influence on the distribution of the
submerged reefs.
The Pompey Complex. The unique reefs of the
Pompey Complex extend over a shelf distance of
140 km and include some of the largest reefs of the
GBR, many between 50 km
2 and 100 km
2 . The entire
reef tract, which parallels the shelf edge, is between 10
and 15 km wide. However, the reefs themselves are
not on the shelf edge, which lies a further 20 km seaward, separated from the Pompey Reefs by a series of
probably fault controlled steps, most prominently at
70 m and 80 m, the edges of which are the location
of further submerged reefs some of which come to
within 10 m of the surface. Narrow channels cut
through the Pompey Reefs reaching depths of over
100 m, up to 40 m deeper than the shelf on either side.
These reefs are on the outer edge of the widest part of
the GBR shelf, in the area of highest tidal range that
reaches 10 m on the mainland and exceeds 4 m within
the reefs. Tidal currents greater than 4 m s
1 have been
recorded and are responsible for both the scouring of
the channels and the deltaic reefs at both ends of them.
Unlike the northern deltaic reefs, water depths have
been sufficiently shallow for ebb tide deltas to develop
on the seaward side of the reefs as well as the western
side, producing extremely complex morphology. Intricate lagoons, with blue hole sinkholes (to 90 m
deep) formed at low sea levels also typify the Pompey
Reefs.
The Swain Reefs. These form a contrasting area of
numerous small planar and lagoonal reefs to the south
13
Northern deltaic reefs. The northernmost 96 km of
shelf edge of the GBR south of the Gulf of Papua consists of short (up to 4 km) narrow reefs parallel to the
shelf edge, separated by passages up to 200 m wide.
Water depths at the shelf edge are only 30 m to 40 m
but on the ocean side is a steep drop-off down to
700 m. Complex delta-like lobes have formed on the
inner western side of each passage, deposited as sedimentary structures in response to strong flood tide
currents. These have formed the foundation for reef
growth.
Ribbon reefs (Fig. 2.5E). These form a classic shelf
edge barrier reef system between 11°S and 17°S (about
700 km). Individual reefs are up to 28 km long, separated by narrow passages most of which are less than
1 km wide. Currents through the passes are weaker
than opposite Torres Strait and although the ends of the
reefs curve inwards there are no deltaic structures.
However, tidal jets carrying nutrient rich water from
the front of the reefs (that have a steep drop-off down
to 1000 m) surge through the channels and maintain
the extensive Halimeda bioherms described above. The
outside of Ribbon 5 has been explored by submersible
and shows dead reefs at 50 m and 70 m above an
almost vertical limestone cliff down to 200 m. The
Pleistocene is at 15 m on Ribbon 5 and 19 m on
Yonge Reef. Holocene growth commenced about
8000 years ago and the ribbon reefs appear to have
reached modern sea level between 6000 and 5000 yrs
BP. Because of the high energy conditions, the ribbon
reefs display a strong zonation.
The submerged reefs of the central GBR. South of
Cairns, where the shelf widens, reefs are set back from
the shelf edge, which is in 70 m to 80 m of water, beyond which is a more gently inclined slope reaching
only 200 m within 3 km. Some reefs and shoals are
found in this area. Where examined, all outer shelf
reefs show the Pleistocene antecedent surface at depths
below 20 m, and, although colonised quickly after they
were drowned by the rising Holocene sea level (basal
dates greater than 8500 yrs BP), they did not reach
modern sea level until 4000 yrs BP, even though they
maintained vertical accretion rates of 6 m ka
1 . The
most significant feature of this 800 km section of the
GBR is the presence of up to three lines of submerged
reefs rising from water depths down to 100 m. In
places these reefs may be as much as 70 m deep but
elsewhere approach the surface though many remain
unmapped. These features are composite, formed during many transgressive and regressive episodes. Many
appear to be dead, perhaps drowned by rapid sea level
rise, sediment shedding from upslope or decline in
water quality when the shelf margin was first drowned.
However, some dense stands of coral have been seen
on ROV dives. This area of the shelf has experienced
hydro-isostatic subsidence but the impingement of
the active Halifax Basin onto the shelf may be the
most important influence on the distribution of the
submerged reefs.
The Pompey Complex. The unique reefs of the
Pompey Complex extend over a shelf distance of
140 km and include some of the largest reefs of the
GBR, many between 50 km
2 and 100 km
2 . The entire
reef tract, which parallels the shelf edge, is between 10
and 15 km wide. However, the reefs themselves are
not on the shelf edge, which lies a further 20 km seaward, separated from the Pompey Reefs by a series of
probably fault controlled steps, most prominently at
70 m and 80 m, the edges of which are the location
of further submerged reefs some of which come to
within 10 m of the surface. Narrow channels cut
through the Pompey Reefs reaching depths of over
100 m, up to 40 m deeper than the shelf on either side.
These reefs are on the outer edge of the widest part of
the GBR shelf, in the area of highest tidal range that
reaches 10 m on the mainland and exceeds 4 m within
the reefs. Tidal currents greater than 4 m s
1 have been
recorded and are responsible for both the scouring of
the channels and the deltaic reefs at both ends of them.
Unlike the northern deltaic reefs, water depths have
been sufficiently shallow for ebb tide deltas to develop
on the seaward side of the reefs as well as the western
side, producing extremely complex morphology. Intricate lagoons, with blue hole sinkholes (to 90 m
deep) formed at low sea levels also typify the Pompey
Reefs.
The Swain Reefs. These form a contrasting area of
numerous small planar and lagoonal reefs to the south