A major outcome was the development of a series of
interconnected three-branch rift systems with the Townsville and Queensland Troughs and the Osprey Embayment
representing failed arms of such a system (Mutter and
Karner, 1980). These troughs are the most important structural elements associated with the formation of the Great
Barrier Reef Province.
Causes of architecture
The causes of the architecture described above are rifting,
drifting, subsidence, sea-level/climate change, and collision. These are briefly summarized as follows:
Rifting
Late Cretaceous extension formed the basins described
above, all of which are rift basins proximal to carbonate
platforms. Of particular importance is the QueenslandTownsville-Cato rift systems, but also of importance if
less developed is the rift system extending south from
the Queensland trough through the Whitsunday basin
to the Capricorn basin separating the Queensland shelf
from the Marion and Queensland plateaux. The rifting
process has clearly influenced the form of platform development in both, a general way through influencing the
shape of the platforms and providing a substrate for shallow water deposition, but also in a specific way through
the influence of fault blocks as sites for early reef development (Figures 2 and 5a, b).
Subsidence
The effect of subsidence is mainly to produce space. It can
also have the effect of termination. Quantitative subsidence data for northeast Australia in four wells, Anchor
Cay 1, DSDP Site 209, Capricorn 1A, and Aquarius 1
are shown in Figure 6. This data indicates that northeast
Australia has not subsided solely as a result of uniform
post-rift thermal cooling; rapid subsidence pulses have
occurred at different times, and such pulses have clearly
initiated transgressions. In the northern region at Anchor
Cay, subsidence increased to 50 m/million years in the
Miocene and increased again to 149 m/million years in
the Pliocene. On the Queensland Plateau, at ODP site
209 subsidences doubled after the middle Miocene. In
the Capricorn Basin, major subsidence of 75 m/million
years occurred in the middle Oligocene and again in the
Pliocene with uplift in the intervening early Pliocene. In
the north and south, subsidence pulses occurred in the
Miocene and the Pliocene. Seismic data from the central
to northern shelf (Townville to north of Cairns) shows
a mid-shelf hinge of subsidence, delineating the point
from which sediment packages thicken eastward. The
northern shelf north of Cairns lies dominantly west of this
hinge line; in the central region, most reef development
occurs adjacent to the hinge line whereas southwards the
Pompey and Swain reefs occur well to the east of the hinge
line. There is fundamental control of reef growth by basement subsidence.
Drift
Hotspot (Duncan, 1981; Wellman, 1983) and magnetostratigraphic studies (Idnurm, 1985, 1986) define the basis
for reconstruction of Indian-Australian plate motion
through the Cenozoic (Figure 7a). Since the late Eocene
when northeast Australia was located between 29
and
44
S, the continent has moved almost directly northward
to its present position between 9
and 24
S. This movement would have resulted in profound climatic changes
along the northeast Australian margin. Simplistically
(Figure 7), the northern tip of northeast Australia would
have entered the tropics only 25 Ma ago whilst the southern part of the Great Barrier Reef would have entered the
tropics only in the last 1 Ma with substantial impact on
the likely sedimentary facies that could have formed.
However, a more detailed analysis of the effects of plate
motions on paleo-oceanography (see Davies et al., 1989,
p. 65) gives a much more precise analysis (Figure 7b) from
which the following conclusions are drawn:
1. In the earliest Eocene, temperatures were very briefly
warm enough for coral reef growth. Chaproniere
(1984) reported large foraminifera from a dredge haul
off the northwest Queensland Plateau, which he concluded grew in waters with temperatures of 18–27
C.
2. Sea surface temperatures from the middle Eocene to
the middle Early Miocene (44–19 Ma) over the whole
northeast Australian margin were temperate.
3. During the early Miocene sea surface temperatures off
northeast Australia were marginal for coral reef development. In modern parlance, they were subtropical,
comparable to southern Queensland today.
4. A rapid rise in sea surface temperatures occurred in the
early mid-Miocene. Conditions were warm enough for
coral reef growth in the northern and central GBR.
5. During the upper Miocene, tropical conditions persisted
in the north and center but warming slowed in the south
and conditions were not conducive for coral reef
growth. Subtropical conditions prevailed in the south.
6. During the Pliocene and Pleistocene, the entire northeast Australian margin was situated within the tropics.
The consequences of these apparently simple conclusions
are substantial and fundamental for northeast Australia:
a. Any tropical platform must be underlain by a temperate
platform and that initial rifting of the margin will contain temperate facies sediments.
b. The consequence of the drifting model of sediment
development is that reefs grew first in the north and,
progressively, later in the south depending on SST.
c. Tropical facies development will thin southward and
overlie a progressively southerly thickening subtropical to temperate section. Facies diachroneity is, therefore, a fundamental factor of a stratigraphy dominated
by plate motion through climatic belts.
The conclusions can be tested by comparing present day
facies distributions along the east Australian margin with
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
513
interconnected three-branch rift systems with the Townsville and Queensland Troughs and the Osprey Embayment
representing failed arms of such a system (Mutter and
Karner, 1980). These troughs are the most important structural elements associated with the formation of the Great
Barrier Reef Province.
Causes of architecture
The causes of the architecture described above are rifting,
drifting, subsidence, sea-level/climate change, and collision. These are briefly summarized as follows:
Rifting
Late Cretaceous extension formed the basins described
above, all of which are rift basins proximal to carbonate
platforms. Of particular importance is the QueenslandTownsville-Cato rift systems, but also of importance if
less developed is the rift system extending south from
the Queensland trough through the Whitsunday basin
to the Capricorn basin separating the Queensland shelf
from the Marion and Queensland plateaux. The rifting
process has clearly influenced the form of platform development in both, a general way through influencing the
shape of the platforms and providing a substrate for shallow water deposition, but also in a specific way through
the influence of fault blocks as sites for early reef development (Figures 2 and 5a, b).
Subsidence
The effect of subsidence is mainly to produce space. It can
also have the effect of termination. Quantitative subsidence data for northeast Australia in four wells, Anchor
Cay 1, DSDP Site 209, Capricorn 1A, and Aquarius 1
are shown in Figure 6. This data indicates that northeast
Australia has not subsided solely as a result of uniform
post-rift thermal cooling; rapid subsidence pulses have
occurred at different times, and such pulses have clearly
initiated transgressions. In the northern region at Anchor
Cay, subsidence increased to 50 m/million years in the
Miocene and increased again to 149 m/million years in
the Pliocene. On the Queensland Plateau, at ODP site
209 subsidences doubled after the middle Miocene. In
the Capricorn Basin, major subsidence of 75 m/million
years occurred in the middle Oligocene and again in the
Pliocene with uplift in the intervening early Pliocene. In
the north and south, subsidence pulses occurred in the
Miocene and the Pliocene. Seismic data from the central
to northern shelf (Townville to north of Cairns) shows
a mid-shelf hinge of subsidence, delineating the point
from which sediment packages thicken eastward. The
northern shelf north of Cairns lies dominantly west of this
hinge line; in the central region, most reef development
occurs adjacent to the hinge line whereas southwards the
Pompey and Swain reefs occur well to the east of the hinge
line. There is fundamental control of reef growth by basement subsidence.
Drift
Hotspot (Duncan, 1981; Wellman, 1983) and magnetostratigraphic studies (Idnurm, 1985, 1986) define the basis
for reconstruction of Indian-Australian plate motion
through the Cenozoic (Figure 7a). Since the late Eocene
when northeast Australia was located between 29
and
44
S, the continent has moved almost directly northward
to its present position between 9
and 24
S. This movement would have resulted in profound climatic changes
along the northeast Australian margin. Simplistically
(Figure 7), the northern tip of northeast Australia would
have entered the tropics only 25 Ma ago whilst the southern part of the Great Barrier Reef would have entered the
tropics only in the last 1 Ma with substantial impact on
the likely sedimentary facies that could have formed.
However, a more detailed analysis of the effects of plate
motions on paleo-oceanography (see Davies et al., 1989,
p. 65) gives a much more precise analysis (Figure 7b) from
which the following conclusions are drawn:
1. In the earliest Eocene, temperatures were very briefly
warm enough for coral reef growth. Chaproniere
(1984) reported large foraminifera from a dredge haul
off the northwest Queensland Plateau, which he concluded grew in waters with temperatures of 18–27
C.
2. Sea surface temperatures from the middle Eocene to
the middle Early Miocene (44–19 Ma) over the whole
northeast Australian margin were temperate.
3. During the early Miocene sea surface temperatures off
northeast Australia were marginal for coral reef development. In modern parlance, they were subtropical,
comparable to southern Queensland today.
4. A rapid rise in sea surface temperatures occurred in the
early mid-Miocene. Conditions were warm enough for
coral reef growth in the northern and central GBR.
5. During the upper Miocene, tropical conditions persisted
in the north and center but warming slowed in the south
and conditions were not conducive for coral reef
growth. Subtropical conditions prevailed in the south.
6. During the Pliocene and Pleistocene, the entire northeast Australian margin was situated within the tropics.
The consequences of these apparently simple conclusions
are substantial and fundamental for northeast Australia:
a. Any tropical platform must be underlain by a temperate
platform and that initial rifting of the margin will contain temperate facies sediments.
b. The consequence of the drifting model of sediment
development is that reefs grew first in the north and,
progressively, later in the south depending on SST.
c. Tropical facies development will thin southward and
overlie a progressively southerly thickening subtropical to temperate section. Facies diachroneity is, therefore, a fundamental factor of a stratigraphy dominated
by plate motion through climatic belts.
The conclusions can be tested by comparing present day
facies distributions along the east Australian margin with
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
513