2 – Geomorphology of Coral Reefs with Special Reference to the Great Barrier Reef
11
(iii) Preexisting positive sedimentary structures such
as alluvial fans (Magnetic and Great Palm Is),
transgressionary sedimentary accumulations (Pioneer Bay, Orpheus Island), leeside island sandspits
(Rattlesnake Island), deltaic gravels (Myall Reef,
Cape Tribulation) and boulder spits (Iris Point,
Orpheus Island).
Various modes of reef growth have been recognised,
including the growth of offshore structure and subsequent backfilling to form the reef flat (e.g. Hayman Island
and Yam Island in Torres Strait) or the development of a
nearshore reef subsequently attached to the mainland by
shoreline progradation. However, the most common
form is progradation, usually episodically, due to sea
level fall or other environmental factors, from shoreline
outwards to reef front.
Mid-shelf reefs
Except on the narrower shelf north of Cairns, the midshelf reefs lie outside the hydro-isostatic zero isobase
and therefore do not record the higher mid-Holocene
sea level, and many did not reach present sea level until after the highstand. They can be described via an
evolutionary classification (Figs 2.4, 2.5A–E), commencing with all these reefs growing over older Pleistocene
reefal foundations at depths ranging from 5 to more
than 25 m. The younger stages generally grow from
the deepest foundations and include:
A Juvenile Stage consisting of:
(i) the drowned Pleistocene reef with possibly only
Halimeda growth;
(ii) submerged shoal reefs, usually with coral growth
over the higher parts of the older reef foundation,
and
(iii) irregular reef patches, formed when coral growth
first reaches sea level.
A Mature Stage consisting of:
(i) crescentic reefs formed when reef patches coalesce
into more extensive reef flat on the windward
margins, with a hardline reef front;
(ii) lagoonal reefs that form as the outer reef flat extends around the margins enclosing a lagoon that
is slowly infilled by patch reef growth and by
transport of sediment from the windward margins
into the lagoon.
A final Senile Stage consisting of:
(iii) planar reefs, with the lagoon infilled and reef flat
extending across the entire reef. Sediment and seagrass beds or mangroves may dominate the reef
flat and coral cays are common on such reefs, including low wooded islands.
Two dominant factors determine the stage a midshelf reef has reached. The first is the depth to the
Pleistocene antecedent reef (that influences the depth
of lagoon to be infilled), with reefs growing from shallow foundations most quickly reaching modern sea
level and progressing to a mature or senile stage. The
second is the size of the reef, as this determines the ratio of highly productive perimeter to the volume of lagoon to be infilled. Larger reefs, once they reach sea
level, progress much more slowly. It was shown earlier
that the depth of Pleistocene foundations on the GBR
was not random, with shallow depths in the north and
far south, a cross shelf gradient in the southern-central
GBR, and generally depths greater than 20 m on the
central GBR shelf. Reef types reflect this variation with
submerged reefs, patches and crescentic reefs dominating the central GBR and most planar reefs found north
of Cairns or south of Mackay.
Outer shelf reefs
Most reefs on the shoulder of the continental shelf (with
the exception of the detached reefs in the north) are linear, parallel to the shelf edge and have originated as
fringing reefs at earlier low sea levels, though they are
certainly multicyclic. Morphology of the shelf edge and
tidal range are the greatest influences on reef type.
Northern detached reefs. These reefs appear to grow
on isolated pinnacles of fragmented continental crust.
Ribbon-like reefs occur on the windward side of some,
with large Halimeda meadows to the lee. On Raine
Island, the top of the Pleistocene is 11 m below reef flat
level, with the reef flat developing about 6000 years
ago, and the cay (a significant turtle nesting site) initially accumulating about 4500 years ago.
11
(iii) Preexisting positive sedimentary structures such
as alluvial fans (Magnetic and Great Palm Is),
transgressionary sedimentary accumulations (Pioneer Bay, Orpheus Island), leeside island sandspits
(Rattlesnake Island), deltaic gravels (Myall Reef,
Cape Tribulation) and boulder spits (Iris Point,
Orpheus Island).
Various modes of reef growth have been recognised,
including the growth of offshore structure and subsequent backfilling to form the reef flat (e.g. Hayman Island
and Yam Island in Torres Strait) or the development of a
nearshore reef subsequently attached to the mainland by
shoreline progradation. However, the most common
form is progradation, usually episodically, due to sea
level fall or other environmental factors, from shoreline
outwards to reef front.
Mid-shelf reefs
Except on the narrower shelf north of Cairns, the midshelf reefs lie outside the hydro-isostatic zero isobase
and therefore do not record the higher mid-Holocene
sea level, and many did not reach present sea level until after the highstand. They can be described via an
evolutionary classification (Figs 2.4, 2.5A–E), commencing with all these reefs growing over older Pleistocene
reefal foundations at depths ranging from 5 to more
than 25 m. The younger stages generally grow from
the deepest foundations and include:
A Juvenile Stage consisting of:
(i) the drowned Pleistocene reef with possibly only
Halimeda growth;
(ii) submerged shoal reefs, usually with coral growth
over the higher parts of the older reef foundation,
and
(iii) irregular reef patches, formed when coral growth
first reaches sea level.
A Mature Stage consisting of:
(i) crescentic reefs formed when reef patches coalesce
into more extensive reef flat on the windward
margins, with a hardline reef front;
(ii) lagoonal reefs that form as the outer reef flat extends around the margins enclosing a lagoon that
is slowly infilled by patch reef growth and by
transport of sediment from the windward margins
into the lagoon.
A final Senile Stage consisting of:
(iii) planar reefs, with the lagoon infilled and reef flat
extending across the entire reef. Sediment and seagrass beds or mangroves may dominate the reef
flat and coral cays are common on such reefs, including low wooded islands.
Two dominant factors determine the stage a midshelf reef has reached. The first is the depth to the
Pleistocene antecedent reef (that influences the depth
of lagoon to be infilled), with reefs growing from shallow foundations most quickly reaching modern sea
level and progressing to a mature or senile stage. The
second is the size of the reef, as this determines the ratio of highly productive perimeter to the volume of lagoon to be infilled. Larger reefs, once they reach sea
level, progress much more slowly. It was shown earlier
that the depth of Pleistocene foundations on the GBR
was not random, with shallow depths in the north and
far south, a cross shelf gradient in the southern-central
GBR, and generally depths greater than 20 m on the
central GBR shelf. Reef types reflect this variation with
submerged reefs, patches and crescentic reefs dominating the central GBR and most planar reefs found north
of Cairns or south of Mackay.
Outer shelf reefs
Most reefs on the shoulder of the continental shelf (with
the exception of the detached reefs in the north) are linear, parallel to the shelf edge and have originated as
fringing reefs at earlier low sea levels, though they are
certainly multicyclic. Morphology of the shelf edge and
tidal range are the greatest influences on reef type.
Northern detached reefs. These reefs appear to grow
on isolated pinnacles of fragmented continental crust.
Ribbon-like reefs occur on the windward side of some,
with large Halimeda meadows to the lee. On Raine
Island, the top of the Pleistocene is 11 m below reef flat
level, with the reef flat developing about 6000 years
ago, and the cay (a significant turtle nesting site) initially accumulating about 4500 years ago.