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doi:10.2204/iodp.proc.325.103.2011 Transect HYD-01C summarySedimentology and biological assemblagesCoralgal and coralgal-microbialite boundstones are the dominant lithologies recovered along transect HYD-01C (Fig. F125). Just below the modern seafloor between one and three coralgal boundstone and coralgal-microbialite boundstone units occur in most transect HYD-01C holes, except for Holes M0030A and M0030B, in which recovery was extremely low, and Hole M0037A, the most distal and deepest site on the transect (at 122 meters below sea level [mbsl]). The coralgal lithologies, spanning one or two sections depending on the hole, contain little or no microbialite and range from <1 m thickness in Hole M0032A to 8 m in Hole M0031A. These coralgal lithologies consistently overlie coralgal-microbialite units in Holes M0031A–M0033A, M0035A, M0036A, M0038A, and M0039A. In Hole M0034A, a 2 m thick coral boundstone underlies an 18 m thick coralgal-microbialite unit, whereas in Hole M0036A, the coralgal boundstone is interbedded with 6 m of unconsolidated sediment units. The main corals in the coralgal units are massive Isopora with lesser amounts of massive Porites, submassive to massive Montipora, and branching Acropora. The coralgal-microbialite units are dominated volumetrically by microbialites, and these boundstones are the thickest lithologies in every hole except Hole M0037A. They range from 10 m thick in Hole M0031A to ~30 m thick in Hole M0033A. They contain diverse coral assemblages dominated by massive Isopora, branching Acropora, and Seriatopora, but also locally abundant massive Porites and Faviidae. In six of the nine holes along transect HYD-01C, unconsolidated sediment from <1 to 19 m thick underlies the upper coralgal-microbialite boundstone units and is composed of bioclastic lime sand to pebbles containing mollusks, larger foraminifera, Halimeda, fragments of corals and red algae, bryozoans, echinoderms, and sea urchin spines. In Hole M0034A, the unconsolidated unit is overlain by a coralgal lithology, whereas in Hole M0036A, the unconsolidated unit is bracketed by coralgal units. These unconsolidated sediments were probably partly disturbed by coring operations. A thin (<3 m) skeletal packstone to grainstone unit rich in larger foraminifera, calcareous algae, and/or a dark coralgal worm tube boundstone is interbedded with, or underlies, the unconsolidated sediment unit in Holes M0031A–M0033A, M0035A, M0036A, M0038A, and M0039A. A similar unconsolidated unit also forms the base of the recovered sequences in Holes M0031A and M0036A. Hole M0037A, the most distal and deepest site at 122 mbsl along transect HYD-01C, has a different lithologic composition and succession, with almost uninterrupted unconsolidated sediments extending from the seafloor to the base of the hole. The uppermost 12 m of unconsolidated lime sands to pebbles overlie a thin (10 cm) interval of grainstone rich in foraminifera, coralline algae, and coral fragments that in turn overlies 8 m of lime sand rich in larger foraminifera and mollusks. Although there is clear evidence of downhole contamination in the upper part of each section, these deposits appear to be undisturbed and, therefore, are probably in situ, with minimal disturbance from downhole contamination. Table T4 documents all larger foraminifera described in this transect in association with hole, run, and depth (below seafloor). Physical propertiesPartial recovery was achieved at holes drilled on transect HYD-01C. With regard to physical property measurements (summarized in Table T2), cores were only partially saturated and often underfilled, impacting on the data coverage and quality. Borehole depths are as follows:
In general, recovery was low and recovered intervals were often disturbed by drilling or partially unsaturated because of the unlithified to semilithified nature of the cored formations. Density and porosityDensity and porosity vary similarly in all of the boreholes drilled across transect HYD-01C. Discrete sample porosity ranges from 20% to 58% because of significant variability in the pore systems (e.g., moldic, vuggy growth framework and intergranular) (Fig. F126). Bulk densities of discrete samples vary between 1.75 and 2.48 g/cm3. Densities measured on whole cores with the multisensor core logger (MSCL) are sometimes <2 g/cm3. This is likely owing to the partial saturation of the cores and also to the fact that the majority of the core comprises unconsolidated fragments. There is a classic linear relationship between the porosity (ϕ) and bulk density (ρ = ρs[1 – ϕ] + ρwϕ) of discrete samples (Fig. F127). Grain density varies between 2.71 and 2.85 g/cm3 and may correspond to a value between the density of calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). P-wave velocityA plot of velocity (from discrete samples) versus porosity (from discrete samples) for all samples from this transect shows an inverse relationship (Fig. F128) between P-wave velocity and porosity. MSCL data range from 1500.34 to 1937.94 m/s, much lower values than discrete measurements on core plugs. The scale dependency of petrophysical measurements, along with the (inevitable) difference in “selective” sampling of core as opposed to bulk MSCL measurements, is evident: for a given density value, discrete measurements have higher P-wave velocity values than MSCL measurements. At the high end of the range in velocity for a given porosity, these differences can be interpreted as the added effect of pore characteristics like pore shape and connectivity and textural properties of the coral and microbialite units. Differences on the low end of the range in velocity for a given porosity may originate from lack of burial compaction and/or pronounced diagenesis. Magnetic susceptibilityMagnetic susceptibility data collected at this transect are difficult to interpret because of gaps in the dataset as a result of core recovery issues. However, the majority of data falls between –5 × 10–5 and 5 × 10–5 SI across all of the holes, with occasional clear magnetic susceptibility highs defined by smooth curves. Electrical resistivityReliable resistivity measurements were difficult to obtain using the MSCL because of the presence of loose sediments or partially saturated cores. When resistivity was measured on unconsolidated or sandy sediments, low values were found (Hole M0037A, 1–2 m CSF-A, where resistivity is between 1 and 2 Ωm). Relatively high resistivities were found when measuring more consolidated sediments (Hole M0034A, 12–14 m CSF-A, where resistivity is between 10 and 30 Ωm [coral framework and microbialite]). A more detailed study of electrical properties of the cores would require measurements with fully saturated discrete samples. Color reflectanceThe values of color reflectance spectrophotometry were calculated for each borehole as discrete measurements. The main parameters measured are the total reflectance (L*) and the color indexes a* (green to red, green being negative and red positive) and b* (blue to yellow, blue being negative and yellow positive). The a*/b* ratio was also calculated for all boreholes, as it can be used as a better proxy to identify changes in characteristics of the sediment than the independent values of a* and b*. Measurements were taken in the most uniform zones in a unit. This is shown by the data, in the sense that massive corals sampled at several points present a consistent pattern of color. In these situations, the data obtained show a main value with a small deviation for the three parameters (L*, a*, and b*). In the locations where Tubipora sp. was found, a strong signal in the red spectrum (a*) was found. In most of the boreholes, slightly higher values of reflectance are present just below the seafloor where modern reef sediment was recovered. In transect HYD-01C, Holes M0031A–M0033A are located in similar water depths and can be correlated. No relevant trends were found in these cores, but the reflectance for all of these boreholes shows similar values. This is also true of Holes M0035A and M0036A, which are also in comparable water depths. As the final sea level data are not yet available, the cores were all presented as meters core depth below seafloor (m CSF-A). Discrete measurements of reflectance values for all boreholes in transect HYD-01C are represented in Figure F129. Boreholes are represented in order from coast (shallow) to offshore (deep). PaleomagnetismTransect HYD-01C comprises 11 holes, collected from 6 sites, located on the northern side of Hydrographer’s Passage. Materials recovered 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 mean of the values from these sites appears to indicate that they are primarily related to diamagnetic materials. The lower peaks recorded in the magnetic susceptibility results (diamagnetic material) are difficult to correlate to a common geological or paleomagnetic feature. However, the majority of the positive peaks in magnetic susceptibility, which indicate the presence of ferromagnetic and/or paramagnetic material, are located at particular depths: 3–4, 7–8, 17–20, and 27–30 mbsf. This feature appears to be associated with lithological variations, in particular the occurrence of sandy layers, which indicate an important variation in the growth regime of the Great Barrier Reef and potentially mark significant paleoclimatic changes. Further rock magnetic studies on these layers may provide information relating to the nature and processes that produce these lithological variations, whereas environmental magnetic studies could reinforce the climatic origin of these layers and provide information on the amount, composition, and grain size of the magnetic component retained in the sediments. GeochemistryA total of 16 interstitial water samples from transect HYD-01C were obtained from Holes M0031A (1), M0033A (2), M0035A (1), M0036A (3), M0037A (6), and M0039A (3). Samples were analyzed for cation and anion concentrations (Table T5). Parameters including pH, alkalinity, and concentrations of ammonium were measured during the offshore phase of the expedition, whereas the major cation and anion measurements were undertaken 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. ChronologyAt transect HYD-01C (Fig. F125), the 50 m reef target was drilled in Hole M0034A (Site 3) and has been dated to 10 cal y BP at the top of the hole (Core 325-M0034A-2R; 52 mbsl) and 11 cal y BP at 68 mbsl. Therefore, this site seems to contain a record of the middle of the last deglaciation. Holes M0031A–M0033A (Site 6) drilled into a feature at 90 mbsl and returned ages of 13–14 cal y BP near the tops of the cores (325-M0031A-2R, 325-M0032A-1R, and 325-M0033A-3R). These three holes all penetrate to greater depths where older ages were returned (25 cal y BP, Core 325-M0031A-16R; 20 cal y BP, Core 325-M0032A-8R; 31 cal y BP, Core 325-M0033A-15R) from depths between 106 and 127 mbsl. Hole M0032A also returned a pre-Last Glacial Maximum (LGM) age of 61 cal y BP from a depth of 121 mbsl (Core 325-M0032A-18R). Therefore, holes at Site 6 recovered material from the LGM interval (and also earlier), as well as captured part of the last deglaciation. Holes M0035A and M0036A (Site 11), farther offshore, drilled a feature at 100 mbsl and returned an age of 15 cal y BP from near the top of one of the cores (325-M0035A-3R, 103 mbsl) and ages of 18–17 cal y BP from deeper in the holes (18 cal y BP, Core 325-M0035A-12R; 17 cal y BP, Core 325-M0036A-8R; 17 cal y BP, Core 325-M0036A-11R) between 109 and 116 mbsl. The bases of the holes at Site 11 returned ages of 21 cal y BP from 124 to 127 mbsl (Cores 325-M0036A-18R and 325-M0035A-20R). Therefore, Site 11 appears to record the end of the LGM as well as the early portion of the deglaciation. Holes M0038A and M0039A (Site 8) recovered material of similar age, 13–19 cal y BP, to that at Site 11 but from a feature at 107 mbsl. Here, the tops of the holes (Cores 325-M0038A-1R and 325-M0039A-2R) at 107 and 108 mbsl produced ages of 14–13 cal y BP, and the bottom of Hole M0039A recovered material dating to 19 cal y BP. Therefore, Site 8 also recorded the end of the LGM as well as the early portion of the last deglaciation. Hole M0037A, at the deepest site on transect HYD-01C (Site 9), produced ages of 8, 16, and 13 cal y BP (Cores 325-M0037A-2R, 4R, and 7R, respectively), but these are not in stratigraphic order. Therefore, this site probably accumulated material during the last deglaciation, and the sequence is likely to be composed of reworked material. Downhole measurementsDownhole geophysical logs provide continuous information on physical, chemical, textural, and structural properties of geological formations penetrated by a borehole. In intervals of low or disturbed core recovery, downhole geophysical logs provide the only way to characterize the borehole section. This is especially true when recovery is poor and when comparable measurements or observations cannot be obtained from core, as downhole geophysical logging allows precise depth positioning of core pieces by visual (borehole images) and/or petrophysical correlation. Borehole geophysical instrumentsThe suite of slimline logging tools deployed at transect HYD-01C is as follows:
Two boreholes were logged at this transect (Holes M0031A and M0036A). However, both holes were API diameter, and hence approximately double the diameter of an ideal HQ (“logging”) hole. Following completion of coring in Hole M0031A, the ASGR through-pipe measurement was run as standard, thus guaranteeing at least one continuous measurement unrelated to open-hole borehole conditions. All the above-mentioned downhole tools were deployed in Hole M0036A. However, because of rapid hole infill as borehole conditions became increasingly hostile, the measurable depth for open-hole logging in Hole M0036A diminished throughout the logging operation. In order to record ultra-high-resolution geophysical downhole logging data, the acquisition was done from the rooster box, which was heave compensated. Preliminary resultsWireline logging operations on transect HYD-01C provided two sets of comparable through-pipe gamma ray data. Very little open-hole data were acquired in Hole M0036A because of hole instability. However, it is possible to discern three major logging units at these two sites solely considering the gamma ray data collected through API pipe (Fig. F130).
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