Princess Charlotte Bay. Area B (Figure 3) occurs between
Cape Bedford and Hinchinbrook Island (16
and 18
S),
where the shelf varies in width from 50 to 75 km, and is
bathymetrically relatively simple. Reefs generally occur
as fringing reefs along the coast and as large platform
and ribbon reefs on the mid- to outer shelf; these are separated from the coast by a channel or inner lagoon with
water depths of around 35 m. The outer shelf ribbon reefs
form a discontinuous barrier particularly in the northern
part of the area but becomes gradually more discontinuous
to the south, ceasing to exist as a line of reefs to the south
of Cairns, at latitude 18S but continuing as line shoals on
the shelf edge averaging 60 m water depth. Area
C (Figure 3) occurs between Hinchinbrook Island and
the Whitsunday Islands (18
and 20
S) where the shelf
takes a clear change in direction, changing from north–
south to north–west to south–east. The width of the shelf
also widens considerably to an average of 90–125 km.
The inner 50–80 km of the shelf is reefless except for
fringing reefs attached to high islands but eastward, the
mid- to outer shelf slopes gently from 45 to 75 m, at which
depth a marked change in slope occurs, reflecting the inner
edge of an outer shelf terrace some 20–30 km in width.
The Slashers, John Brewer, Myrmidon, Pith, Wheeler,
Keeper, Bowl, Stanley, and Viper reefs occupy this outer
shelf terrace. A drowned barrier reef complex occupies
the outer shelf of the 75 m terrace and extends to the southeast for 200 km. Area D (Figure 3) occurs between the
Whitsunday Islands to the Swain Reefs (20
to 22
S),
the shelf continuing its northwest to southeast orientation
away from the coast giving rise to a dramatic increase in
the width of the shelf to over 300 km in the south. The
inner two thirds of the shelf are reefless except for small
fringing reefs around continental islands; eastward, however, the outer shelf is a plethora of gigantic reefs forming
the Pompey complex and smaller more dispersed reefs
forming the Swains complex. Area E (Figure 3) occurs
to the south of the Capricorn Channel (22
and 24
S)
where the shelf is less than 100 km wide, with the Capricorn and Bunker reefs occupying only a narrow zone on
the mid-shelf. Well-known reefs in the region include
Heron, Wreck, One Tree, Fitzroy, Fairfax, and Lady
Musgrave.
Small drowned reefs occupy the shelf edge in
80–100 m of water (Davies et al., 2004). In addition to
the reef ecosystem, Halimeda bioherms and biostromes
comprise an almost equally important ecosystem in
the northern and central, southern Great Barrier Reef
(See Halimeda Bioherms, this volume).
The rift basins
In addition to the platforms, the architecture of the northeast Australian margin is dominated by a series of rift
basins (Figure 1a and b) which formed as a result of fragmentation of a northeastern extension of the Tasman fold
belt (Gardner, 1979; Ewing et al., 1970; Falvey, 1972;
Falvey and Taylor, 1974; Taylor, 1975; Mutter, 1977;
Taylor and Falvey, 1977; Mutter and Karner, 1980). The
Pandora and Bligh Troughs and the Osprey Embayment
occur in the north; the Queensland and Townsville
Troughs in the central region and the Cato trough occupies
the eastern and southern sections of the GBR. Relatively,
smaller rift basins are also known to underlie the shelf of
the Great Barrier Reef – the Papuan platform, the Halifax
basin, the Whitsunday basin, and the Capricorn basin
(Figure 1a and b). In the north the Papuan platform is
a thick (? 4–5 km) Jurassic to Recent basin containing
reefs within a fluvio-deltaic sequence representing the vast
outpourings from the Fly River in New Guinea. The
poorly defined Halifax Basin is a major structure beneath
the continental shelf to the east of Townsville and underlies the central outer GBR: it is a Cretaceous/Tertiary basin
covering some 60,000 km
2 and a maximum thickness in
excess of 4,000 m. It lies at the intersection of the Queensland and Townsville Troughs. The Whitsunday Basin is
a very shallow structure containing little more than
1,000–1,500 m of Cretaceous/Tertiary sediments underlying the region to the west of the Pompey and Swains reefs
and may represent a connection between the Halifax and
Capricorn basins. In the south, the Capricorn basin is
sandwiched between the Bunker and Swains Highs.
Bunker and Swains Highs
The Queensland and Townsville Troughs lie adjacent to
the central/ northern reef region. Directly adjacent to the
Great Barrier Reef (Figures 1a, b and 4a, b), the Queensland Trough occupies the region between the continental
shelf and the Queensland Plateau between 14
S and
17
30S. The floor of the trough slopes northward from
1,000 m at the junction with the Townsville Trough to
around 3,000 m at its junction with the Osprey Embayment. Rather than being a simple graben, the Queensland
trough may have been initiated in an oblique wrench zone
caused by left-lateral movements between Australia and
the Queensland Plateau (Symonds et al., 1984). The
Townsville Trough is a west to east trough separating the
Queensland and Marion Plateaux. It is over 500 km in
length and 150 km wide. At its confluence with the
Queensland Trough, the Townsville Trough is 1,000 m
deep, deepening eastward to 3,000 m at its exit into the
Coral Sea Basin. At this eastern end, at about
Longitude 154
E, a bifurcation sends one branch south
into the Cato Trough and the other winding sinuously
north into the Coral Sea Basin. The Capricorn Basin
started in the late Cretaceous as the result of an eastward
shift of the Maryborough basin rift axis to the east concomitant with spreading in the Cato Trough. The basin
extends in a northwest to southeast direction and is
500 km long by 250 km wide and is bordered by the Bunker and Swains Highs. This early to late Cretaceous rift
phase development preceded continental breakup and the
formation of the Coral Sea Basin by sea floor spreading
in the Paleocene (62–56 million year BP) or latest Cretaceous (Weissel and Watts, 1979; Symonds et al., 1984).
510
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
Cape Bedford and Hinchinbrook Island (16
and 18
S),
where the shelf varies in width from 50 to 75 km, and is
bathymetrically relatively simple. Reefs generally occur
as fringing reefs along the coast and as large platform
and ribbon reefs on the mid- to outer shelf; these are separated from the coast by a channel or inner lagoon with
water depths of around 35 m. The outer shelf ribbon reefs
form a discontinuous barrier particularly in the northern
part of the area but becomes gradually more discontinuous
to the south, ceasing to exist as a line of reefs to the south
of Cairns, at latitude 18S but continuing as line shoals on
the shelf edge averaging 60 m water depth. Area
C (Figure 3) occurs between Hinchinbrook Island and
the Whitsunday Islands (18
and 20
S) where the shelf
takes a clear change in direction, changing from north–
south to north–west to south–east. The width of the shelf
also widens considerably to an average of 90–125 km.
The inner 50–80 km of the shelf is reefless except for
fringing reefs attached to high islands but eastward, the
mid- to outer shelf slopes gently from 45 to 75 m, at which
depth a marked change in slope occurs, reflecting the inner
edge of an outer shelf terrace some 20–30 km in width.
The Slashers, John Brewer, Myrmidon, Pith, Wheeler,
Keeper, Bowl, Stanley, and Viper reefs occupy this outer
shelf terrace. A drowned barrier reef complex occupies
the outer shelf of the 75 m terrace and extends to the southeast for 200 km. Area D (Figure 3) occurs between the
Whitsunday Islands to the Swain Reefs (20
to 22
S),
the shelf continuing its northwest to southeast orientation
away from the coast giving rise to a dramatic increase in
the width of the shelf to over 300 km in the south. The
inner two thirds of the shelf are reefless except for small
fringing reefs around continental islands; eastward, however, the outer shelf is a plethora of gigantic reefs forming
the Pompey complex and smaller more dispersed reefs
forming the Swains complex. Area E (Figure 3) occurs
to the south of the Capricorn Channel (22
and 24
S)
where the shelf is less than 100 km wide, with the Capricorn and Bunker reefs occupying only a narrow zone on
the mid-shelf. Well-known reefs in the region include
Heron, Wreck, One Tree, Fitzroy, Fairfax, and Lady
Musgrave.
Small drowned reefs occupy the shelf edge in
80–100 m of water (Davies et al., 2004). In addition to
the reef ecosystem, Halimeda bioherms and biostromes
comprise an almost equally important ecosystem in
the northern and central, southern Great Barrier Reef
(See Halimeda Bioherms, this volume).
The rift basins
In addition to the platforms, the architecture of the northeast Australian margin is dominated by a series of rift
basins (Figure 1a and b) which formed as a result of fragmentation of a northeastern extension of the Tasman fold
belt (Gardner, 1979; Ewing et al., 1970; Falvey, 1972;
Falvey and Taylor, 1974; Taylor, 1975; Mutter, 1977;
Taylor and Falvey, 1977; Mutter and Karner, 1980). The
Pandora and Bligh Troughs and the Osprey Embayment
occur in the north; the Queensland and Townsville
Troughs in the central region and the Cato trough occupies
the eastern and southern sections of the GBR. Relatively,
smaller rift basins are also known to underlie the shelf of
the Great Barrier Reef – the Papuan platform, the Halifax
basin, the Whitsunday basin, and the Capricorn basin
(Figure 1a and b). In the north the Papuan platform is
a thick (? 4–5 km) Jurassic to Recent basin containing
reefs within a fluvio-deltaic sequence representing the vast
outpourings from the Fly River in New Guinea. The
poorly defined Halifax Basin is a major structure beneath
the continental shelf to the east of Townsville and underlies the central outer GBR: it is a Cretaceous/Tertiary basin
covering some 60,000 km
2 and a maximum thickness in
excess of 4,000 m. It lies at the intersection of the Queensland and Townsville Troughs. The Whitsunday Basin is
a very shallow structure containing little more than
1,000–1,500 m of Cretaceous/Tertiary sediments underlying the region to the west of the Pompey and Swains reefs
and may represent a connection between the Halifax and
Capricorn basins. In the south, the Capricorn basin is
sandwiched between the Bunker and Swains Highs.
Bunker and Swains Highs
The Queensland and Townsville Troughs lie adjacent to
the central/ northern reef region. Directly adjacent to the
Great Barrier Reef (Figures 1a, b and 4a, b), the Queensland Trough occupies the region between the continental
shelf and the Queensland Plateau between 14
S and
17
30S. The floor of the trough slopes northward from
1,000 m at the junction with the Townsville Trough to
around 3,000 m at its junction with the Osprey Embayment. Rather than being a simple graben, the Queensland
trough may have been initiated in an oblique wrench zone
caused by left-lateral movements between Australia and
the Queensland Plateau (Symonds et al., 1984). The
Townsville Trough is a west to east trough separating the
Queensland and Marion Plateaux. It is over 500 km in
length and 150 km wide. At its confluence with the
Queensland Trough, the Townsville Trough is 1,000 m
deep, deepening eastward to 3,000 m at its exit into the
Coral Sea Basin. At this eastern end, at about
Longitude 154
E, a bifurcation sends one branch south
into the Cato Trough and the other winding sinuously
north into the Coral Sea Basin. The Capricorn Basin
started in the late Cretaceous as the result of an eastward
shift of the Maryborough basin rift axis to the east concomitant with spreading in the Cato Trough. The basin
extends in a northwest to southeast direction and is
500 km long by 250 km wide and is bordered by the Bunker and Swains Highs. This early to late Cretaceous rift
phase development preceded continental breakup and the
formation of the Coral Sea Basin by sea floor spreading
in the Paleocene (62–56 million year BP) or latest Cretaceous (Weissel and Watts, 1979; Symonds et al., 1984).
510
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT