2 – Geomorphology of Coral Reefs with Special Reference to the Great Barrier Reef
9
count separate reefs in complex parts of the GBR and to
fully recognise submerged shoals, a figure of 2900 reefs
is generally accepted for the Marine Park. For Torres
Strait there is an additional 37 000 km
2 of shelf area and
a further 750 reefs covering an area of about 6000 km
2 .
N THE MOST RECENT EVOLUTION
OF THE GBR
The early establishment of the GBR is discussed in
Chapter 3 where the importance of Pleistocene sea level
changes is indicated. The last time sea level was at or
above its present level was about 125 000 years ago at
which time the GBR was at least as extensive as it is
today. Subsequently, a series of interstadial high sea
levels between 60 000 and 70 000 yrs BP(Before Present)
and only 15 m to 25 m below present level were capable
of adding further growth to the reefs of the outer shelf.
Sea level reached its lowest point of at least 125 m
around 20 000 years ago. At this time the whole of the
GBR, including the shoulder of the continental shelf,
was exposed and rivers such as the Burdekin flowed
across it to the shelf edge.
The modern GBR developed as the postglacial marine transgression flooded the continental shelf. This took
place about 10 000 years ago although the first flooding
may not have favoured coral growth as Pleistocene soils
and regolith were reworked, producing turbid, eutrophic
conditions especially in the lee of the ribbon reefs where
extensive meadows of Halimeda were the first carbonate
structures to develop. The Halimeda bioherms, or mounds,
up to 18.5 m thick are found inside the ribbon reefs from
Cooktown to Torres Strait. They have been maintained to
the present day by the jetting of deep nutrient rich water
through the passes that exist between the ribbon reefs.
By 9000 years ago sea level was about 20 m below
the present position and the older Pleistocene reefs
were being inundated. The oldest radiocarbon ages
from the Holocene reefs are just over 9000 years and,
once inundated, recolonisation and vertical accretion
was rapid. The bulk of the Holocene reef was laid down
between 8500 and 5500 yrs BP at rates of up to 13 m ka
1
(mean about 6 m ka
1
). Effectively the transgression
was complete by 6500 yrs BP, but shelf warping as the
result of hydro-isostasy (depression of the outer shelf
and compensatory upwarping of the inner shelf due to
weight of the water) produced some cross shelf contrasts. Thus, whilst modern sea level had been achieved
by 6500 yrs BP on the inner shelf (documented by raised
reefs of up to 1.5 m on inner fringing reefs and the inner
shelf low wooded island reefs of the northern GBR),
the age becomes progressively younger towards the
outer shelf where no emergence took place.
There is also evidence of tectonic shelf-warping in the
central GBR where the Halifax Basin of the Coral Sea impinges upon the shelf. Down-warping is suggested by
the extensive line of submerged reefs on the outer shelf
south of Cairns. There are also regional patterns in the
depths down to the Pleistocene foundations of the modern reefs. Although some variation may be expected due
to original morphology or subsequent erosion, the pattern as shown by reef drilling also supports the downwarping on the outer half of the continental shelf. In
Torres Strait and the far northern region south to Cooktown there is a gradual increase in minimal depth from
–5 m in the north to between 15 m and 17 m in the
south. South of Cairns the depth to the Pleistocene foundation increases by more than 5 m and everywhere,
except Britomart Reef (where it is at 19.8 m), is at depths
greater than 20 m. On a cross shelf transect just south of
Mackay the variation is from 0 m at Digby Island, between 8.0 m and 13 m on mid-shelf reefs and 17.5 m
beneath Cockatoo Reef in the Pompey Complex.
N REEF TYPES OF THE GBR
Charles Darwin’s reef classification that shows fringing
reefs around subsiding volcanic islands that evolve
into barrier reefs and finally atolls is applicable only to
open ocean ‘hot spot’ volcanoes and reefs and is not
appropriate for the relatively stable, continental shelf
of the GBR that has no atolls. Nor is there any great
latitudinal contrast on the GBR except for that related
to shelf width. Instead cross shelf environmental gradients divide the GBR into three zones:
Fringing and nearshore reefs
Some 545 fringing reefs with recognisable reef flats
and 213 incipient fringing reefs (shore attached reefs
lacking reef flats) have been recognised in the GBR
9
count separate reefs in complex parts of the GBR and to
fully recognise submerged shoals, a figure of 2900 reefs
is generally accepted for the Marine Park. For Torres
Strait there is an additional 37 000 km
2 of shelf area and
a further 750 reefs covering an area of about 6000 km
2 .
N THE MOST RECENT EVOLUTION
OF THE GBR
The early establishment of the GBR is discussed in
Chapter 3 where the importance of Pleistocene sea level
changes is indicated. The last time sea level was at or
above its present level was about 125 000 years ago at
which time the GBR was at least as extensive as it is
today. Subsequently, a series of interstadial high sea
levels between 60 000 and 70 000 yrs BP(Before Present)
and only 15 m to 25 m below present level were capable
of adding further growth to the reefs of the outer shelf.
Sea level reached its lowest point of at least 125 m
around 20 000 years ago. At this time the whole of the
GBR, including the shoulder of the continental shelf,
was exposed and rivers such as the Burdekin flowed
across it to the shelf edge.
The modern GBR developed as the postglacial marine transgression flooded the continental shelf. This took
place about 10 000 years ago although the first flooding
may not have favoured coral growth as Pleistocene soils
and regolith were reworked, producing turbid, eutrophic
conditions especially in the lee of the ribbon reefs where
extensive meadows of Halimeda were the first carbonate
structures to develop. The Halimeda bioherms, or mounds,
up to 18.5 m thick are found inside the ribbon reefs from
Cooktown to Torres Strait. They have been maintained to
the present day by the jetting of deep nutrient rich water
through the passes that exist between the ribbon reefs.
By 9000 years ago sea level was about 20 m below
the present position and the older Pleistocene reefs
were being inundated. The oldest radiocarbon ages
from the Holocene reefs are just over 9000 years and,
once inundated, recolonisation and vertical accretion
was rapid. The bulk of the Holocene reef was laid down
between 8500 and 5500 yrs BP at rates of up to 13 m ka
1
(mean about 6 m ka
1
). Effectively the transgression
was complete by 6500 yrs BP, but shelf warping as the
result of hydro-isostasy (depression of the outer shelf
and compensatory upwarping of the inner shelf due to
weight of the water) produced some cross shelf contrasts. Thus, whilst modern sea level had been achieved
by 6500 yrs BP on the inner shelf (documented by raised
reefs of up to 1.5 m on inner fringing reefs and the inner
shelf low wooded island reefs of the northern GBR),
the age becomes progressively younger towards the
outer shelf where no emergence took place.
There is also evidence of tectonic shelf-warping in the
central GBR where the Halifax Basin of the Coral Sea impinges upon the shelf. Down-warping is suggested by
the extensive line of submerged reefs on the outer shelf
south of Cairns. There are also regional patterns in the
depths down to the Pleistocene foundations of the modern reefs. Although some variation may be expected due
to original morphology or subsequent erosion, the pattern as shown by reef drilling also supports the downwarping on the outer half of the continental shelf. In
Torres Strait and the far northern region south to Cooktown there is a gradual increase in minimal depth from
–5 m in the north to between 15 m and 17 m in the
south. South of Cairns the depth to the Pleistocene foundation increases by more than 5 m and everywhere,
except Britomart Reef (where it is at 19.8 m), is at depths
greater than 20 m. On a cross shelf transect just south of
Mackay the variation is from 0 m at Digby Island, between 8.0 m and 13 m on mid-shelf reefs and 17.5 m
beneath Cockatoo Reef in the Pompey Complex.
N REEF TYPES OF THE GBR
Charles Darwin’s reef classification that shows fringing
reefs around subsiding volcanic islands that evolve
into barrier reefs and finally atolls is applicable only to
open ocean ‘hot spot’ volcanoes and reefs and is not
appropriate for the relatively stable, continental shelf
of the GBR that has no atolls. Nor is there any great
latitudinal contrast on the GBR except for that related
to shelf width. Instead cross shelf environmental gradients divide the GBR into three zones:
Fringing and nearshore reefs
Some 545 fringing reefs with recognisable reef flats
and 213 incipient fringing reefs (shore attached reefs
lacking reef flats) have been recognised in the GBR