Ribbon Reef No. 5
Drilled in 1995, the detailed description of the corals, sediments (Webster and Davies, 2003) and coralline algae
(Braga and Aguirre, 1997) in a core from Ribbon Reef 5
define the basis for understanding the establishment of
the Great Barrier Reef. The litho-logs (Figure 11a) divide
into three parts:
0–96 m – Six reef horizons comprised of two coral assemblages (1) robust branching Acropora (humilis, robusta,
palifera, Stylophora (pistillata) and Pocillopora
(verruclosa, damicornis) with associated massive
Goniastrea and Platygyra and (2) Massive Porites (lutea,
solida) with associated encrusting Porites and Montipora.
Boundaries between reef horizons are identified by combinations of prominent horizons of pale brown sediment
containing areas of calcitized plant cells, radiometric dating, negative shifts in 018 values, exposure horizons, color
changes from pink to brown, changes in algal associations
and shifts in sediment facies (Webster and Davies, 2003).
Algal associations throughout reflect shallow water tropical environments.
96–158 m – A rhodolith sequence dominated by four
rhodolith grainstone horizons, coralline algal grainstones/
packstones and two in situ coral framework horizons.
The two in situ coral framework horizons represent the
oldest reef horizons encountered in the Ribbon 5 core
and separate the rhodolith horizon as into an upper and
lower section. Changes from coral to rhodolith-dominated
horizons are reflected in changes in the coralline algal
associations and in one instance at 130 m by a sharp irregular surface and a substantial shift in the 018 record. The
algal associations define lower and upper subtropical environments with tropical reef horizons sandwiched between,
i.e., the earliest reef growth occured on a substrate of subtropical rhodolith grainstones.
158–210 m – 52 m of alternating grainstones/packstones
and packstones/wachestones. The former are comprised
of poorly sorted (rudaceous (4 mm) to medium sand)
fragmentary corals, encrusting and branching corallines,
bryozoans, foraminifers (Amphistegina, Iterostegina,
encrusting forams), molluscs, serpulids, echinoids, and
gastropods: fresh water diagenesis throughout is indicated
by interparticle spaces infilled by thick calcite rims,
druses, and syntaxial overgrowths while mouldic porosity
is developed within molluscs, corals, and especially
Halimeda. The latter are well sorted, sometimes graded
but finer grained (medium to coarse sand) with similar clasts
but with up to 50% lime mud dispersed unevenly; little fresh
water diagenesis evident. Boundaries between the
grainstones/packstones and packstones/wachestones are
sharp. Corals are subordinate to coralline algae, of which
there are two assemblages, a shallower water Lithophyllum
assemblage and a deeper water Mesophyllum assemblage;
both occur today on the warm temperate to subtropical
mid- to outer shelf of southern Queensland, south of the
Great Barrier Reef (Braga and Aguirre, 1997). The sediments are typical of the same region. The wachestones/
packstones are defined as outer shelf (þ60 m water depth)
highstand deposits while the grainstones are defined as
mid- to inner shelf regressive or lowstand deposits. The relatively sharp boundary between the facies types is, therefore, “event driven” by oscillating sea levels. The fresh
water diagenesis is due to exposure in the lowstand
effecting the grainstones but not the highstand
wachestones/packstones. They are unlikely to represent
the debris flows and turbidites postulated by Braithwaite
et al. (2004).
Ocean drilling program site 820
Drilled in 1991, Site 820 forms part of a suite of holes
(819, 820, and 821) drilled immediately in front of the
Great Barrier Reef in Grafton Passage, east of Cairns
(Davies et al., 1991). While Site 821 is considered the
most complete sedimentological section, it unfortunately
suffers from intense fresh water diagenesis and Site 819
has a major hiatus in the upper part of the section. Site
820 is, therefore, considered to be by far the most complete section and the basis for the interpretations shown
in Figure 11c (Davies and Peerderman, 1998). Three sedimentologic units are recognized:
Unit A 0–65 m composed of wachestones separated by
8 m thick sandy bioclastic packstones composed of planktonic and benthic forams, mollusks, pteropods, minor
echinoids, corals, corallines, bryozoans, rhodoliths and
Halimeda. High and low sedimentation rates reflect alternating high and low sea-level oscillations. Seismic imaging shows aggradation with clear erosional geometries
reflecting similar sea-level oscillations.
Unit B 65–145 m composed of wachestones and thin
packstone units overlying wachestones and thick
packstone units. A major reduction in sedimentation rates
coincides with the boundary between Units AþB while
alternating high and low rates in Unit B are thought to
reflect oscillating sea levels. Seismic imaging shows the
upper wachestones and thin packstones to be aggrading
packages and the lower wachestones and thick packstones
as prograding packages, the boundary occurring at around
110 m in the core.
Unit C – between 145 and 220 m dominated by sandy
packstone beds and minor wachestones. An unconformity
may occur in this sequence while seismic imaging defines
prograding sediment geometries.
In all of the above, there are good agreements between
stratigraphic, sedimentologic, and seismic data. However,
it is when this data is tied to the isotope data (Figure 11c)
that it starts to make sense in terms of the origin of the
Great Barrier Reef. Three principal isotopic sequences
are recognized:
(1) Between 0 and 75 m (Unit A above) the signal is dominated by low frequency, high amplitude variation
defined by Peerdeman et al. (1993) as related to the
orbital eccentricity frequency. This isotopic sequence
extends from isotope stage 1–8 and, therefore, has
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
521
Drilled in 1995, the detailed description of the corals, sediments (Webster and Davies, 2003) and coralline algae
(Braga and Aguirre, 1997) in a core from Ribbon Reef 5
define the basis for understanding the establishment of
the Great Barrier Reef. The litho-logs (Figure 11a) divide
into three parts:
0–96 m – Six reef horizons comprised of two coral assemblages (1) robust branching Acropora (humilis, robusta,
palifera, Stylophora (pistillata) and Pocillopora
(verruclosa, damicornis) with associated massive
Goniastrea and Platygyra and (2) Massive Porites (lutea,
solida) with associated encrusting Porites and Montipora.
Boundaries between reef horizons are identified by combinations of prominent horizons of pale brown sediment
containing areas of calcitized plant cells, radiometric dating, negative shifts in 018 values, exposure horizons, color
changes from pink to brown, changes in algal associations
and shifts in sediment facies (Webster and Davies, 2003).
Algal associations throughout reflect shallow water tropical environments.
96–158 m – A rhodolith sequence dominated by four
rhodolith grainstone horizons, coralline algal grainstones/
packstones and two in situ coral framework horizons.
The two in situ coral framework horizons represent the
oldest reef horizons encountered in the Ribbon 5 core
and separate the rhodolith horizon as into an upper and
lower section. Changes from coral to rhodolith-dominated
horizons are reflected in changes in the coralline algal
associations and in one instance at 130 m by a sharp irregular surface and a substantial shift in the 018 record. The
algal associations define lower and upper subtropical environments with tropical reef horizons sandwiched between,
i.e., the earliest reef growth occured on a substrate of subtropical rhodolith grainstones.
158–210 m – 52 m of alternating grainstones/packstones
and packstones/wachestones. The former are comprised
of poorly sorted (rudaceous (4 mm) to medium sand)
fragmentary corals, encrusting and branching corallines,
bryozoans, foraminifers (Amphistegina, Iterostegina,
encrusting forams), molluscs, serpulids, echinoids, and
gastropods: fresh water diagenesis throughout is indicated
by interparticle spaces infilled by thick calcite rims,
druses, and syntaxial overgrowths while mouldic porosity
is developed within molluscs, corals, and especially
Halimeda. The latter are well sorted, sometimes graded
but finer grained (medium to coarse sand) with similar clasts
but with up to 50% lime mud dispersed unevenly; little fresh
water diagenesis evident. Boundaries between the
grainstones/packstones and packstones/wachestones are
sharp. Corals are subordinate to coralline algae, of which
there are two assemblages, a shallower water Lithophyllum
assemblage and a deeper water Mesophyllum assemblage;
both occur today on the warm temperate to subtropical
mid- to outer shelf of southern Queensland, south of the
Great Barrier Reef (Braga and Aguirre, 1997). The sediments are typical of the same region. The wachestones/
packstones are defined as outer shelf (þ60 m water depth)
highstand deposits while the grainstones are defined as
mid- to inner shelf regressive or lowstand deposits. The relatively sharp boundary between the facies types is, therefore, “event driven” by oscillating sea levels. The fresh
water diagenesis is due to exposure in the lowstand
effecting the grainstones but not the highstand
wachestones/packstones. They are unlikely to represent
the debris flows and turbidites postulated by Braithwaite
et al. (2004).
Ocean drilling program site 820
Drilled in 1991, Site 820 forms part of a suite of holes
(819, 820, and 821) drilled immediately in front of the
Great Barrier Reef in Grafton Passage, east of Cairns
(Davies et al., 1991). While Site 821 is considered the
most complete sedimentological section, it unfortunately
suffers from intense fresh water diagenesis and Site 819
has a major hiatus in the upper part of the section. Site
820 is, therefore, considered to be by far the most complete section and the basis for the interpretations shown
in Figure 11c (Davies and Peerderman, 1998). Three sedimentologic units are recognized:
Unit A 0–65 m composed of wachestones separated by
8 m thick sandy bioclastic packstones composed of planktonic and benthic forams, mollusks, pteropods, minor
echinoids, corals, corallines, bryozoans, rhodoliths and
Halimeda. High and low sedimentation rates reflect alternating high and low sea-level oscillations. Seismic imaging shows aggradation with clear erosional geometries
reflecting similar sea-level oscillations.
Unit B 65–145 m composed of wachestones and thin
packstone units overlying wachestones and thick
packstone units. A major reduction in sedimentation rates
coincides with the boundary between Units AþB while
alternating high and low rates in Unit B are thought to
reflect oscillating sea levels. Seismic imaging shows the
upper wachestones and thin packstones to be aggrading
packages and the lower wachestones and thick packstones
as prograding packages, the boundary occurring at around
110 m in the core.
Unit C – between 145 and 220 m dominated by sandy
packstone beds and minor wachestones. An unconformity
may occur in this sequence while seismic imaging defines
prograding sediment geometries.
In all of the above, there are good agreements between
stratigraphic, sedimentologic, and seismic data. However,
it is when this data is tied to the isotope data (Figure 11c)
that it starts to make sense in terms of the origin of the
Great Barrier Reef. Three principal isotopic sequences
are recognized:
(1) Between 0 and 75 m (Unit A above) the signal is dominated by low frequency, high amplitude variation
defined by Peerdeman et al. (1993) as related to the
orbital eccentricity frequency. This isotopic sequence
extends from isotope stage 1–8 and, therefore, has
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
521