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doi:10.2204/iodp.sp.325.2009

Proposed drill sites

Site location

The GBR is the largest epicontinental reef system currently existing on this planet, extending 2000 km in a northwest–southeast direction along the northeast coast of Queensland (Davies et al., 1989) (Fig. F2). The origin of this morphologically and biologically important sedimentary system is poorly constrained; recently, an age of <500,000 y has been assigned to the initiation of the GBR system (International Consortium, 2001).

The northern, central, and southern GBR define ideal sites for the evaluation of sea level changes in the period 20,000 to 8,000 cal. y BP. The reefs on the shelf edge east of Cooktown form the semicontinuous outer barrier of the northern GBR. In this area, as well as in the far northern GBR, the reef is narrow with ribbon reefs on its eastern edge, extensive coastal fringing reefs, and patch reefs; in the south, it broadens with patch reefs separated by open water or narrow channels. In the outer shelf east-north-east of Townsville, modern reefs form a line of pinnacles in front of the main reef edge and lateral growth on the windward margin. South of 15°30′S, the reefs are generally ≥30 km offshore and reach 100 km at 22°30′S. Farther south the shelf widens considerably, reaching widths >200 km. East of Mackay the modern reefs form a complex, double series of flood-tide deltaic reefs (i.e., Pompey Complex) (Hopley, 2006). The coastal lagoon between the main body of the GBR and the mainland has a maximum depth of 145 m but rarely exceeds 60 m (Wolanski, 1982).

Previous scientific investigations on the Great Barrier Reef

Previous sedimentological and geophysical studies have identified a succession of subsea morphologic structures interpreted as drowned reefs at depths of 100, 90, 60–50, and 35–40 m (Carter and Johnson, 1986; Harris and Davies, 1989; Hopley, 2006; Beaman et al., 2007), especially in the four key areas:

  1. Cooktown shelf and slope (Ribbon Reef);

  2. Cairns shelf and slope (Grafton Passage, Flora Passage, and Noggin Pass);

  3. Townsville shelf and slope (Bowl and Viper Reef); and

  4. Mackay shelf and slope (Hydrographer's Passage).

A series of drowned linear reefs and lagoons occupy specific depths over at least a 30 km stretch on the outer continental shelf in the vicinity of Hydrographer's Passage in the southern GBR region (Fig. F2). Based on the RV Southern Surveyor cruise in September–October 2007 (Webster et al., 2008a, 2008b), the proponents have identified five primary drill sites from three of these key regions on the Cooktown, the Cairns, and the Mackay shelf edge (Fig. F2).

Studies on the GBR (McKenzie et al., 1993; Davies and Peerdeman, 1998) concentrating on the shelf edge southeast of Townsville and east of Cooktown have defined the morphologic shape of the outer reef–upper continental slope and the geological origin of the GBR itself. Based on high-resolution seismic profiles in the fore-reef section in front of the GBR, Feary et al. (in McKenzie et al., 1993) recognized three seismic megasequences (0–490, 490–555, and below 555 ms, respectively) that define a clearly aggradational upper sequence, a transitional middle sequence, and a progradational lower sequence. Ocean Drilling Program (ODP) drilling in 1991 (Leg 133) defined the origin of the GBR as very young (i.e., the GBR was initiated during marine isotope Stages 9–11) (McKenzie et al., 1993; Davies and Peerdeman, 1998).

A new phase of drilling in 1995 in Boulder Reef (15°23.944′S, 145°26.182′E) and Ribbon Reef 5 (15°22.40′S, 145°47.149′E) areas using a reef-mounted jack-up platform further enhanced this story, proving that the GBR is some 100 m thick, resting on a subreef subtropical red algal facies which in turn overlies a deepwater temperate grainstone facies (Davies and Peerdeman, 1998; International Consortium, 2001). Detailed stratigraphic and facies analysis of the 90 and 210 m long drill cores also shows the upper part of the platform to be composed of repeated cycles of transgressional coolwater coralline-dominated carbonates topped by shallow-water highstand coral reefs (Webster and Davies, 2003; Braga and Aguirre, 2004). The Holocene reef, however, does not show this couplet, as it is coral dominated from its inception at 8,000 cal. y BP. Strontium isotope and magnetostratigraphic data from the base of the Pleistocene coral reef sequence has confirmed that the origin of the GBR is very young, <500,000 y (International Consortium, 2001; Webster and Davies, 2003; Braithwaite et al., 2004).

Available site survey data at the Great Barrier Reef

The proposed drill sites on the GBR are distributed in three distinct regions (Fig. F2): offshore Cooktown (Ribbon Reef 5 and 3), offshore Cairns (Noggin Pass), and McKay shelf (Hydrographer's Passage).

From previous site survey data (described in detail in the June 2007 Preliminary Report to the Environmental Protection and Safety Panel (EPSP) and synthesized recently by Beaman et al., 2007), it was clear that a succession of barrier reefs occupy the outer shelf at water depths between 40 and 100 m with terrace features at ~80–110 m depth along much of the GBR. None of these structures has been adequately investigated, yet they have the potential to provide unique and critical information about the nature of sea level and climatic change offshore eastern Australia and important information about their role as habitats and substrates for present day biological communities. With the exception of the Ribbon Reef 5 region, only limited systematic high-resolution swath bathymetry mapping, imaging, or sampling has ever been attempted.

Recently, the proponents led a site survey cruise to gather the most comprehensive data set ever collected from the GBR shelf edge (Webster et al., 2008b). During the cruise on the Southern Surveyor, the remaining site survey information needed for IODP drilling operations in the GBR was acquired. Four study sites (Ribbon Reef, Noggin Pass, Viper Reef, and Hydrographer's Passage) were mapped along the Queensland margin where the approximate location of submerged reefs is known (Fig. F2). The data types acquired were

  • EM300 swath bathymetry and backscatter;

  • Subbottom sparker and Topas PS18 seismic reflection profiles;

  • High-resolution underwater stereoscopic images and high-resolution multibeam bathymetry, acquired onboard a state-of-the-art Autonomous Underwater Vehicle (AUV);

  • Continuous measurements establishing present day oceanographic conditions on the shelf edge, using the AUV's onboard Seabird Conductivity, Temperature, and Depth profiler (CTD); and

  • Dredged rock samples from the tops of the shelf edge reefs, acquired using a standard rock dredge and a Smith-McIntyre sediment grab.

These data were used to define specific drill targets for IODP GBR drilling operations (see "Coring strategy" and "Site summaries" for proposed transect locations and available site survey information).

Summary of 2007 GBR site survey data for revised sites

Offshore Cooktown: Ribbon Reef 5 and 3 (RIB-01C and RIB-02A)

EM300 swath mapping of the Ribbon Reefs survey area covered 1609.87 km2. The Ribbon Reef 5 area was also surveyed by Webster and colleagues in 2005 using a Reson 8101 (240 kHz) swath mapping system and Datasonics CAP-6600 Chirp 3.5 kHz subbottom profiler, and these data (Beaman et al., 2007) have previously been submitted to the IODP Site Survey Database (SSDB). Based on a detailed examination of all available site survey data (multibeam, backscatter, seismic profiles, AUV imagery, and bottom samples), we propose to drill two transects of holes across the most well developed fossil reef features, one off Ribbon Reef 5 (RIB-01C) and another off Ribbon Reef 3 (RIB-02A) (see Fig. F2 for general location and Figs. F3 and F5 for detailed maps).

Site survey data seaward of modern Ribbon Reef 5 illustrate the succession of morphological features that define location RIB-01C (Figs. F3, F4):

  1. A modern reef front talus zone, which extends from the base of the spurs and grooves at between 15 and 20 m to 47 m.

  2. A gently sloping terrace ~150 m wide between 50 and 55 m that is also characterized by prominent subbottom reflectors.

  3. A well-developed submerged reef observed at 50 m lying parallel to the shelf break. This defines the 50 m reef feature.

  4. A gently sloping terrace ~140 m wide at a depth of 65 to 75 m.

  5. The main shelf-break at 75 m, characterized by a subtle raised rim 1–2 m high that defines the 70 m reef feature.

  6. The moderately sloping upper slope that descends from the main shelf-break at 75 m. Below 350 m the upper slope is deeply incised by well-developed canyons that extend down into the Queensland Trough in water depths >2 km.

Site survey data seaward of the modern Ribbon Reef 3 illustrate the succession of morphological features that define location RIB-02A (Figs. F5, F6):

  1. A modern reef front talus zone, which extends to 44 m.

  2. A well-developed reef detected on the shelf lying parallel to the shelf-break at 47 m that defines the 50 m reef feature.

  3. A gently sloping terrace ~330 m wide in 55 to 80 m water depth, with prominent subbottom reflectors visible.

  4. A submerged reef observed at 80 m that is characterized by a subtle raised rim 1–2 m high and defines the 70 m reef feature.

  5. The main shelf-break at 105 m that is characterized by a 3–5 m raised rim that forms the 100 m reef feature. Below 500 m the upper slope is deeply incised by canyons that extend down into the Queensland Trough.

Offshore Cairns: Noggin Pass (NOG-01B)

EM300 swath mapping of the Noggin Pass survey area covered 1243.27 km2. The available site survey data seaward of the modern Noggin Reef illustrate the succession of morphological features that define location NOG-01B (Figs. F7, F8):

  1. A double fronted barrier reef 250 and 90 m wide and separated by a lagoon 80 m wide; the barrier reefs occur at a depth of 44 to 42 m.

  2. A lagoon, up to 250 m wide, at a depth of 57 to 54 m with reef pinnacles some 50 m wide and rising to a depth of 56 m. The lagoon is characterized by prominent subbottom reflectors and is fronted by a discontinuous barrier, which tops at 55 m, that represents the 50 m reef feature.

  3. A gently sloping terrace ~450 m wide in 60 to 80 m water depth. A break in slope at the edge of this terrace marks the 80 m reef feature.

  4. A gently sloping terrace ~140 m wide in 91 to 99 m water depth. A distinct break in slope at the edge of this terrace marks the 100 m reef feature.

  5. A narrow 50 m terrace at 108 m and the main shelf-break that forms the 110 m reef feature.

  6. A gentle upper slope characterized by >100 m of fore-reef slope sediments. Below ~250 m water depth the upper slope is deeply incised by a well-developed canyon system that extends to 1400 m in the Queensland Trough.

Mackay shelf: Hydrographer's Passage (HYD-01C and HYD-02A)

EM300 swath mapping of the Hydrographer's Pass survey area covered 810.68 km2. Based on a detailed examination of all available site survey data, we propose to drill two transects of holes across the most well developed fossil reef features, one in the northwest (HYD-01C) and the other in the southeast (HYD-02A) (see Fig. F2 for general location and Figs. F9 and F11 for detailed maps).

Site survey data from the northwest of Hydrographer's Pass illustrate the succession of morphological features that define location HYD-01C (Figs. F9, F10):

  1. A double fronted barrier reef 200 and 100 m wide and separated by a lagoon 2 km wide and up 70 m deep; the barrier reefs occur at a depth of 51 to 55 m. In some regions the lagoon is characterized by prominent subbottom reflectors, as well as 50 m wide patch reefs rising to 55 m.

  2. A steep sloping 500 m wide terrace with a sharp break in slope marking the 80 m reef feature.

  3. A complex 1 km wide lagoon and reef terrace system. The lagoon, at a depth of 85–87 m, is up to 600 m wide with numerous prominent subbottom reflectors. Seaward, the lagoon grades into a 400 m relatively flat terrace and a sharp break in slope that marks the 90 m reef feature.

  4. A 700 m wide lagoon and reef pinnacle system. The lagoon is 300 m wide at a depth of 95 m and is characterized by prominent subbottom reflectors. The lagoon grades seaward into a dense system of patch reefs or pinnacles ~30–40 m across that range in depth between 95 and 97 m.

  5. A major break in slope that defines the 100 m reef feature and has a series of smaller seaward pinnacles and terraces interpreted as reefs at depths of 110 and 120 m.

  6. A gentle upper slope characterized by >160 m of fore-reef slope sediments.

Site survey data from the southeast of Hydrographer's Pass illustrate the succession of morphological features that define location HYD-02A (Figs. F11, F12):

  1. A submerged reef shoal 600 m wide at 31 m with well-developed landward and seaward terraces at 40 m.

  2. A double fronted barrier reef 250 and 150 m wide and separated by a lagoon 2.5 km wide and up to 55 m deep; the barrier reefs occur at a depth of 54 to 56 m. The lagoon is characterized by numerous prominent subbottom reflectors and partially buried patch reefs. The patch reefs become more dense seaward between 60 and 70 m.

  3. A steep slope 400 m seaward of the 50 m reef is defined by a series of smaller pinnacles and terraces interpreted as distinct reefs at depths of 65, 70, and 80 m.

  4. A complex 2.3 km wide lagoon along with a reef pinnacle and terrace system between 90 and 100 m. The lagoon is 2 km wide and up to 102 m deep with prominent subbottom reflectors that are only interrupted by significant breaks in slope that define the 90 and 100 m reef features.

  5. The major break in slope, which occurs at 103 m and defines the 100 m reef feature, with a series of smaller pinnacles and terraces interpreted as reef features at a depth of 110 m.

  6. A prominent 70 m wide reef terrace observed at 126–128 m. The feature is interpreted to be the LGM reef.

  7. A gentle upper slope characterized by >160 m of fore-reef slope sediments.

Coring strategy

Barbados offshore drilling (Fairbanks, 1989) has demonstrated that the reef sequence corresponding to the last deglaciation developed on slopes and forms discontinuous successive terraces of various lateral extent and stratigraphic thickness. Therefore, to recover the whole post glacial reef sequence successive reef terraces that occur seaward of the living barrier reef must be drilled.

Our detailed analyses of the combined GBR site survey data sets have demonstrated the occurrence of successive reef features at various depths between 130 and 25 m which correspond to drilling targets. Thus, at each site, it is proposed to realize a transect of several offshore drill holes in order to recover the entire post glacial reef sequence (see below). Initial results obtained during Expedition 310 confirm that this drilling strategy is sound (Camoin, Iryu, McInroy, et al., 2007).

Totals for all sites are as follows:

Total sediment penetration = 1480 m,

Total substratum penetration = 360 m, and

Total penetration = 1840 m.

In this scenario, "sediment" refers to the reef structure and "substratum" the substrate on which the reefs are sitting (e.g., older reef, prereef sediments). Totals for individual sites are found in Tables T1, T2, T3, T4, and T5.

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