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Transect HYD-02A summary

Sedimentology and biological assemblages

Few common patterns link lithologic successions in the eight holes along transect HYD-02A (Fig. F88). The following highlights describe some features along transect HYD-02A, focusing on shared features rather than on exceptions:

  • In several holes, the upper sedimentary unit consists of unconsolidated to lithified modern or subrecent seafloor sediment that is coarser grained in the shallower holes (M0042A and M0044A) and finer grained in the deeper ones (Holes M0040A and M0041A).

  • Boundstones occur immediately below the modern sediments in all eight holes along the transect. Their thickness averages 9–10 m in the deepest two holes (M0040A and M0041A) and the shallowest hole (M0042A), and increases to 25 m in the two holes at intermediate depths (Holes M0047A and M0043A).

  • In every hole that penetrated below the boundstone, there is a unit of unconsolidated material, usually lime sand, in which Halimeda is one of the main components. Recovered thicknesses of this material ranged from 5 to 10 m.

  • The two holes (M0042A and M0043A) that penetrated below the unconsolidated interval encountered a packstone/grainstone unit, <1 m thick, that overlies unconsolidated sand in Hole M0043A and overlies alternating intervals of lithified grainstone to rudstone and unconsolidated sands in Hole M0042A. The lithified intervals in Hole M0042A contain clear evidence of subaerial exposure, including calcrete and possible root remains.

Boundstone lithologies contain variable proportions of coral, coralline algae, and microbialite that define several coralgal, coralgal-microbialite, and microbialite boundstones similar to those in the other Great Barrier Reef transects. The major corals in the boundstones are submassive to massive Porites, Montipora, branching Pocilloporidae, branching Acropora, massive Isopora, and submassive to massive Faviidae.

The common patterns of boundstone distribution in most of the holes are

  • Coralgal boundstones, from 4 to 24 m thick, are the uppermost or only boundstone in six of the eight holes (excluding Holes M0040A and M0044A).

  • Microbialite-rich boundstones (coralgal-microbialite or microbialite boundstones) lie beneath the coralgal boundstone or are the only boundstone lithology in Holes M0043A and M0042A.

  • Microbialite boundstone, 4–7 m thick, occurs only in the deepest two holes (M0040A and M0041A).

  • No simple relationship exists between the presence/absence of coralgal and coralgal-microbialite boundstones and the geographic location and/or water depth of holes along transect HYD-02A.

Table T3 documents all the larger foraminifera described in this transect in association with hole, run, and depth (below seafloor).

Physical properties

Recovery at transect HYD-02A sites was partial with an average of ~21%. However, recovery in Holes M0040A and M0041A reached ~50%. Cores were partially saturated and often disturbed, fractured, or contaminated. All of these factors act to degrade the quality of any physical property data collected. Borehole depths for this transect are as follows:

  • Hole M0040A = 126.07 mbsl, 21.50 m DSF-A.
  • Hole M0041A = 126.58 mbsl, 22.10 m DSF-A.
  • Hole M0042A = 50.78 mbsl, 46.40 m DSF-A.
  • Hole M0043A = 102.93 mbsl, 35 m DSF-A.
  • Hole M0044A = 105.25 mbsl, 11.00 m DSF-A.
  • Hole M0045A = 105.25 mbsl, 14.60 m DSF-A.
  • Hole M0046A = 117.49 mbsl, 20.40 m DSF-A.
  • Hole M0047A = 99.12 mbsl, 33.20 m DSF-A.
  • Hole M0048A = 104.57 mbsl, 7.10 m DSF-A.

Plugs and samples taken for discrete P-wave and moisture and density measurements were obtained from both consolidated and unconsolidated core material.

Density and porosity

Bulk density was measured at transect HYD-02A using gamma ray attenuation. Gamma ray attenuation on the multisensor core logger provided an estimate of bulk density (also referred to as gamma density) from whole cores. Discrete moisture and density measurements were also taken with a pycnometer on plugs and/or on rock fragments. This provides grain density, bulk density (in the case of plug samples), and porosity data. As in the previous transect, HYD-01C, one observes a classic linear relationship between the porosity (ϕ) and bulk density (ρ = ρs[1 – ϕ] + ρwϕ) of discrete samples measured for all boreholes at transect HYD-02A (Fig. F89).

Average grain density (ρs) is 2.77 g/cm3. Grain density varies between 2.70 and 2.85 g/cm3 and may correspond to a value between the grain density of calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). Porosity values for all boreholes in this transect can be viewed in Figure F90. Porosity in the transect ranges between 16% and 72%. Similar trends in porosity can be picked out in Holes M0047A and M0043A with a stepwise decrease in porosity between 0 and 12 m CSF-A followed by an increase at ~15 m CSF-A and gradual decrease to ~25 m CSF-A. Holes M0040A and M0041A have almost identical trends in porosity, with 25% porosity (~0–10 m CSF-A) increasing to 53% at the bottom of the drilled holes.

P-wave velocity

A crossplot of velocity versus porosity (both from discrete samples) for all sites shows primarily an inverse relationship (Fig. F91) between P-wave velocity and porosity. Whole-core multisensor core logger data ranges from 1508.59 to 1895.75 m/s. As expected, much lower, possibly erroneous (see “Physical properties” in the “Methods” chapter) P-wave velocity values have been recorded by the multisensor core logger for coralgal boundstone units, unlike the discrete measurements on core plugs, which are probably more reliable.

Magnetic susceptibility

Magnetic susceptibility data are very difficult to interpret for this transect because of limited core recovery in all holes across transect HYD-02A. However, values are generally similar across the holes, with the majority of readings being in the –1 × 10–5 to 1 × 10–5 SI range, which is delineated by short intervals of magnetic susceptibility highs.

Electrical resistivity

Over the entire transect, resistivity is highly variable, with the lowest values (0.56 Ωm) measured in Hole M0040A and highest values (44.84 Ωm) recorded in Hole M0044A. Because of the relatively poor core quality (as mentioned above) and undersaturated cores, very little confidence can be placed in these data.

Color reflectance

In transect HYD-02A, Holes M0048A, M0047A, M0043A, M0044A, and M0046A are located at similar water depths and can be correlated (with <5 m separation between the drilled holes). Holes M0047A and M0043A exhibit similar trends, but Hole M0047A presents smaller dispersion in the values of reflectance per section. This is probably due to the presence of massive corals in Hole M0047A. Hole M0046A was corrected for depth because of its location on a pinnacle. This was taken into account when plotting it against the other boreholes in order to make direct comparisons across the transect. Hole M0046A shows a similar trend in data points to the shallower holes. However, there is a smoother distribution of the reflectance measurements. Holes M0048A and M0044A have similar values; however, because of the lack of measurements with depth in Hole M0048A, no trend can be identified. Recovery in Hole M0045A was so low that color reflectance was not measured.

Holes M0040A and M0041A are located very close to each other in the same water depth. Both boreholes exhibit less dispersion in color reflectance measurements than other boreholes from this transect. Data from these boreholes show a consistent pattern of ~50% L* at 2.5 m CSF-A, a slight increase at 6–8 m CSF-A, and a decrease in reflectance below that, retuning to ~50% at 21 m CSF-A. Color reflectance measurements for all transect HYD-02A boreholes are represented in Figure F92. Boreholes are plotted from shallower to deeper water (left to right) at the same depth scale. For boreholes found at similar depths, similar trends are present in the color reflectance data, which suggests a possible correlation between them.


Transect HYD-02A comprises nine holes located at five sites in the southern part of Hydrographer’s Passage. Recovered materials were dominated by corals and calcareous sediments. The materials show mainly low and/or negative values of low-field and mass-normalized magnetic susceptibility. The arithmetic means of the measurements taken suggest that they are related mainly to diamagnetic materials. Further studies may detect minimal fractions of ferromagnetic material and further define the magnetic properties and geomagnetic behavior of the records obtained thus far.

The lower magnetic susceptibility values related to diamagnetic material are difficult to correlate to a common geological or paleomagnetic feature. However, high magnetic susceptibility values for samples located in the uppermost 1–2 m or between 2 and 5 mbsf are common to the majority the sites in this transect, are most evident in the deep Site 10 location (Holes M0040A and M0041A), and are mainly associated with lithologic variations. Variations between 17 and 20 mbsf and 27 and 30 mbsf can be attributed to the occurrence of sandy layers, which may represent an important variation in the growth regime of the Great Barrier Reef, potentially marking significant paleoclimatic changes.

At Site 10 (Holes M0040A and M0041A), strong magnetic susceptibility spikes at 14–15 and 16–17 mbsf were recorded. Magnetic susceptibility for these layers is strong enough to suggest alternative hypotheses beyond a simple variation in the concentration of magnetic minerals as a result of a paleoclimatic pulse. However, further rock magnetic studies are required to define the nature and processes that produced these susceptibility variations. Environmental magnetic studies will define the climatic origin of these layers and provide further information on the volume, composition, and grain size of the magnetic component retained in these layers.

Drilling contamination appears to have occurred in the uppermost ~10–30 cm of some cores from Holes M0040A and M0041A. The strong signal may be caused by rust from the pipe. Alternatively, the high values of susceptibility may be due to sulfide pieces that were not washed out of the hole during drilling and accumulated at the top of each core.


A total of 20 interstitial water samples from transect HYD-02A were obtained from Holes M0040A (4), M0041A (6), M0042A (2), M0043A (4), M0046A (2), and M0047A (2). Samples were analyzed for cation and anion concentrations (Table T4). Parameters including pH, alkalinity, and concentrations of ammonium were measured during the offshore phase of the expedition, whereas major cations and anions were measured during the Onshore Science Party. All geochemical constituents were determined to be within the normal ranges for marine sediments. Because of the scarcity of interstitial water samples in this transect, interpretations relating to vertical variations could not be made.


The shallowest hole on transect HYD-02A (Fig. F88), Hole M0042A (Site 2), was drilled into a 50 m feature and returned ages of 13–10 cal y BP from 65 to 51 meters below seafloor (mbsl) (Cores 325-M0042A-1R and 10R), thus recording the middle of the last deglaciation. Deeper in Hole M0042A, an age of 169 cal y BP was recovered from 86 mbsl (Core 325-M0042A-24R), indicating the potential for recovering pre-Last Glacial Maximum (LGM) material in deeper cores from this hole. Sites 7–12 drilled a range of features from 98 to 108 mbsl. Where dated, the tops of holes at these sites return ages of 15–14 cal y BP (15 cal y BP, Core 325-M0043A-2R, 106 mbsl; 14 cal y BP, Core 325-M0044A-2R, 109 mbsl; 15 cal y BP, Core 325-M0047A-3R, 102 mbsl; 14 cal y BP, Core 325-M0048A-2R, 101 mbsl), and the lower portions of the two holes at Site 12 that have greater penetration (Holes M0043A and M0047A) returned ages of 20 and 21 cal y BP from 129 and 127 mbsl (Cores 325-M0043A-18R and 325-M0047A-11R), respectively. Therefore, at these sites, the early portion of the deglaciation was recovered, whereas the LGM has been captured in the deeper cores at Site 12. The deepest site on transect HYD-02A, Site 10 (Holes M0040A and M0041A), was drilled into a feature at 127 mbsl. Ages of 17–10 cal y BP have been returned from the tops of the two holes at this site (Cores 325-M0040A-2R, 4R, and 325-M0041A-2R), indicative of accumulation during the last deglaciation. However, the lower portion of Hole M0040A returned an age of 25 cal y BP (Core 325-M0049A-8R) from 138 mbsl, indicating that Site 10 also recovered material representative of the LGM.

Downhole measurements

Downhole geophysical logs provide continuous information on a broad range of formation properties. In this transect, this continuous data set provides important data where core recovery is limited.

Borehole geophysical instruments

The wireline sondes deployed at HYD-02A were as follows:

  • Optical Borehole Televiewer (OBI40),

  • Acoustic Borehole Televiewer (ABI40),

  • Spectral Natural Gamma Probe (ASGR),

  • Induction Conductivity Probe (DIL45)—with medium investigation depth (ILM, 0.57 m) and deep investigation depth (ILD, 0.83 m),

  • Full Waveform Sonic Probe (SONIC),

  • Magnetic susceptibility probe (EM51), and

  • Caliper probe (CAL3)–borehole diameter.

Preliminary results

Wireline logging operations were performed in one API hole (M0042A) on transect HYD-02A. The priority imaging tools (ABI40 and OBI40) were also run to see if good image data could be obtained in an API hole. The standard maximum hole diameter for successful image data acquisition is 15 cm, and API holes tend to have a minimum diameter of ~20 cm, so images were not successfully collected.

Four main logging units were identified from the downhole data from Hole M0042A:

  1. The uppermost Unit I is characterized by low total gamma ray (TGR) counts (through-pipe and open hole), high conductivity, and very low magnetic susceptibility. Borehole diameter is extremely large in Unit I (>40 cm in places), which may be a consequence of the API bottom-hole assembly moving and swabbing the top of the open hole. Four main lithostratigraphic are associated with Unit I: lime sand and algal bindstone, coralgal boundstone, coralgal-microbialite boundstone, and unconsolidated sediment (lime granules and pebbles).

  2. Unit II is associated with a downsection sequence of grainstone to unconsolidated sediment (lime granules and pebbles) to grainstone with rhodoliths units. These lithologic variations are most mirrored by the conductivity data, which exhibit some minor fluctuations downhole. TGR has intermediate values, whereas magnetic susceptibility is extremely low and constant. The borehole diameter is in gauge in this unit.

  3. Unit III is characterized by increasing values of TGR downhole and relatively high conductivity and stable borehole diameter figures. Lithostratigraphic units associated with this logging unit are (top-down) grainstone with rhodoliths, unconsolidated sediment (lime granules and pebbles), and gray rudstone and rudstone with brown staining.

  4. Unit IV represents a zone of reduced TGR counts. A decline in conductivity is present at the very top of Unit IV, but overall values gradually increase to the base of the hole. Magnetic susceptibility remains very low and only fluctuates slightly, and the caliper registers the hole to be in gauge. Only one lithostratigraphic (rudstone with brown staining) is associated with this logging unit.