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doi:10.2204/iodp.proc.325.103.2011

Hole M0034A

Operations

Site 3, Hole M0034A

The Greatship Maya was on station and settled on dynamic positioning (DP) at 1825 h on 18 February 2010 (Table T1). Repairs to the roughneck slips continued while discussions about the best way to hang the HQ pipe were conducted. Between 1925 and 2100 h, the moonpool door was closed. This process took longer than anticipated because of damage sustained during the initial transit from Townsville. At 1900 h, API pipe started being run to just above the seabed. A downpipe camera survey was conducted, and by 2305 h, preparations to run the first core barrel were made. The first standard rotary corer (ALN) core was recovered at 0020 h on 19 February, and coring continued for another six runs until 0515 h, when preparations began to change to HQ coring. Running the HQ rods took from 0900 to 1047 h using the API pipe as an outer casing. Flushing the hole began and continued for 25 min.

The first HQ core (Run 8) was recovered at 1150 h, and HQ coring continued for another four runs until 1500 h, reaching a depth of 18.6 mbsf. The bit became blocked during Run 12 and it took 25 min to free it. Flushing then continued until 1525 h. However, the currents had been increasing along with wind speed over the course of coring, and at 1525 h the HQ rods twisted off ~2 m below the drill deck. At 1530 h, the vessel was moved to straighten the API pipe, and fishing for the end of the broken HQ pipe began using a casing recovery tap. The recovery tool broke off the rod at 1710 h and was manually recovered using a pup joint. At 1915 h, an attempt was made to hammer a tap into the broken joint. At 2235 h, an additional API pipe was added, connecting to the joint above the moonpool door, and was pulled back to the slips. Flushing and reaming continued in an attempt to lower the API pipe slightly in the hole, but the hole was by this point caving and periodically forcing the API pipe upward. By 0235 h on 20 February, the fishing operation could restart, as the top of the API pipe was safely set in the slips at the drill floor. However, another API pipe was added at 0310 h, as the pipe was sinking in a softer lithology.

The HQ pipe was tagged at 0345 h, and recovery began. This was completed at 0734 h, when preparations began to change back to API coring using the ALN core barrel. The hole was open-holed with the API pipe back down to 18.6 mbsf. When the ALN barrel was initially deployed, mud pressure spiked and the barrel was recovered without advancing. Two washers were removed from the inner barrel bearing head to increase the shoe/bit air gap, and the barrel was redeployed, with the first core being recovered at 1005 h. Run 15 had poor recovery, so a washer was added to the inner barrel bearing head again to reduce the effect of flushing. However, this caused the pressure to spike again, so the barrel was tripped and a new impregnated bit was put on. Between 1235 and 1310 h, operations halted because repairs were being made on the mud pumps. Four further attempts were then made to core, all of which resulted in no advance and overpressurizing of the drill string. An infill sample from an unknown depth was recovered on one occasion and was curated as Run 16.

At 1518 h, the Greatship Maya lost Differential Global Positioning System (DGPS) corrections and the vessel moved 23 m off station in 56 m water depth. The GPS was restored at 1520 h and DPGS at 1524 h. Although the vessel was manually moved back onto station within 10 min, the API pipe was bent, requiring a full trip. Once the API pipe was on deck (1810 h), three pipes were found to be bent and unusable. Checks were run with the ALN and bottom-hole assembly in the slips, using both liners and metal splits, and no overpressurizing was noted.

Discussions took place between the Master of the Greatship Maya, Chief Engineer, Bluestone Party Chief, European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO) Operations Superintendant, European Geological Survey Surveyor, Chief Mate, and Chief Electrical Engineer as to why the GPS and DGPS signal had been lost. The Master contacted shore-based GC Rieber personnel for advice and stated that no further drilling operations were to take place until the problem had been rectified, thereby terminating Hole M0034A at 23.1 mbsf with an average recovery of 29.1%. Another GPS drop-out occurred at 2000 h for 10 min. A passing vessel was contacted, and they had not suffered any loss of signal.

During the early morning of 21 February, the Greatship Maya suffered three power failures due to the bow thrusters ramping up to 100% without warning and tripping the generator because of overdemand. At 0945 h, it was decided that a secondary, independent positioning system was required before further drilling could take place. This would hold the vessel on DP should a further GPS/DGPS signal drop-out occur. Permission was sought from the Great Barrier Reef Marine Park Authority (GBRMPA) to use the 3 m × 3 m seabed template with a HiPap beacon installed on it. The Environmental Management Plan had previously only allowed a 1 m × 1 m template to be used, which was being pushed off location in the strong currents experienced at Hydrographer’s Passage. Notice of permission granted was received from GBRMPA by 1005 h.

It was decided, however, that no coring operations could take place until the secondary power failure issue was rectified. Engine and DP data were downloaded and sent to GC Rieber Singapore, who passed the information onto ConverTeam, who were responsible for the electrical and DP installation. By 2300 h, the vessel crew had identified the problem, run a simulation to ascertain that this was a correct identification, and rectified the issue. At 2330 h, the Master informed the drill floor operations team that he was satisfied that changes to the thruster settings had rectified the problem and that the vessel could move to Site 11, Hole M0035A.

Sedimentology and biological assemblages

Hole M0034A is divided into four lithostratigraphic units.

Unit 1: interval 325-M0034A-1R-1, 0–20 cm: modern coralgal boundstone

The uppermost Unit 1, consisting only of interval 325-M0034A-1R-1, 0–20 cm, is a modern sediment composed of lime coralgal boundstone pebbles associated with fine-grained lime sand. Some pebbles have brown staining. Pebble-sized bioclasts of mollusk shell and bryozoans are present. Well-preserved specimens of the larger foraminifera Amphistegina, Heterostegina, Operculina, and fragments of Homotrema are common in interval 325-M0034A-1R-1, 6–11 cm. Only one, possibly hermatypic (zooxanthellate), coral in this unit was identifiable.

Unit 2: Sections 325-M0034A-1R-1, 20 cm, to 12R-1, 14 cm: coralgal-microbialite boundstone

Unit 2, spanning Sections 325-M0034A-1R-1, 20 cm, to 12R-1, 14 cm, consists of coralgal-microbialite boundstone. Microbialites are almost absent from the upper part of this unit, from Sections 325-M0034A-1R-1, 20 cm, to 4R-1, 20 cm, but they become common to abundant in the lower Sections 4R-1, 20 cm, to 12R-2, 14 cm. The boundstone consists mainly of massive corals (Fig. F49) covered with thin (and, much less commonly, thick) crusts of nongeniculate coralline algae that, in turn, are encrusted by thick microbialite envelopes (Figs. F50, F51). Growth forms of nongeniculate coralline algae are exclusively encrusting. Microbialites are buff colored and generally weakly laminated. Bioclasts of Halimeda segments and, less commonly, molluscan shell fragments are enclosed in the microbialites (Fig. F52). Corals are slightly bioeroded, with borings partly to completely filled with fine sand to silt-sized calcareous internal sediment (Fig. F53).

The uppermost part of Unit 2 consists of a large massive Porites colony (Fig. F54); below this colony, the dominant corals are massive to robustly branching Isopora colonies (Figs. F52, F55). Associated corals are Goniopora(?) (Fig. F56), Acropora with medium-thickness branches, massive Faviidae (Fig. F57), branching Pocilloporidae, and encrusting Porites and Montipora. Most fragments are Isopora but they also include Montipora, Porites, Acropora, Seriatopora, Pocillopora(?), Goniopora(?), and Faviidae. Many fragments are both bioeroded and abraded, suggesting some postdepositional transport.

Unit 3: Sections 325-M0034A-13R-1 through 14R-CC: packstone

Unit 3, spanning Sections 325-M0034A-13R-1 through 14R-CC, consists mainly of massive clasts and, less commonly, branching corals. There are no nongeniculate coralline algae, but there are small (as large as pebble-sized) lithoclasts of gray packstone.

The most common coral is Isopora with fragments of Isopora, Seriatopora, and Acropora(?).

Unit 4: Section 325-M0034A-16W-CC: unconsolidated sediments

The lowermost Unit 4, consisting only of Section 325-M0034A-16W-CC, is composed of gravelly sediment. This sediment appears to be derived from the seafloor and borehole wall, and it contains abundant bioclasts of Halimeda, fragmented specimens of the benthic foraminifer Heterostegina (scarce), mollusks, and corals, as well as undifferentiated limestone clasts. There are no visible corals or fragments.

Physical properties

Hole M0034A was drilled to 23.10 m DSF-A, with a total of 6.71 m of core recovered (29.05% recovery). Table T2 summarizes all physical property data acquired from this transect, including means and standard deviations for each data type in each borehole.

Density and porosity

Gamma density values from whole-core multisensor core logger measurements range from 1.01 to 2.57 g/cm³ (Fig. F58). Bulk density was also measured on 10 discrete samples. Between 2 and 20 m CSF-A, the bulk density of discrete samples presents an increasing trend with depth from 1.96 to 2.48 g/cm³ and porosity values present a decreasing trend from 46% to 17% downsection (Fig. F59). This is the only borehole where we find a quasimonotonic variation in porosity and density versus depth. However, it is difficult to relate this finding to the described lithostratigraphy.

P-wave velocity

MSCL P-wave measurements on whole cores offshore were not successful. However, three discrete samples were collected and measured from Hole M0034A onshore (Fig. F60A). P-wave values from discrete samples range from 3508 to 4471 m/s (mean resaturated P-wave). From these three measurements, one can infer an increase in P-wave with depth. However, because of the gap in recovery, there is a large section of unknown values (Fig. F60A). Bulk density and P-wave values show similar trends when measured on discrete samples. However, it is different from Hole M0033A in that as bulk density decreases, P-wave increases. (Fig. F60B).

Magnetic susceptibility

Magnetic susceptibility values in Hole M0034A range from –2.23 × 10^–5 to 1.35 × 10^–5 SI (Fig. F58). Note that Section 325-M0034A-11R-1 was remeasured as part of the quality assurance/quality control plan (see “Physical properties” in the “Methods” chapter). The remeasured value indicates that the apparently higher magnetic susceptibility of this core was an artifact of an error with the 80 mm loop, meaning that the trend is true but the absolute values are overestimated in the original measurement. There are no downhole trends evident from the dataset.

Electrical resistivity

Resistivity ranges from low values of 1.50 Ωm to higher values of 34.92 Ωm (Fig. F58). Resistivity values are highly spread over most of this range from Sections 325-M0034A-8R-1 through 12R-1.

Digital line scans and color reflectance

All cores from Hole M0034A were measured using the digital line scan system with all data recorded at a resolution of 150 pixel/cm as both images and RGB values. All appropriate cores were scanned for color reflectance. Color reflectance in Hole M0034A L* values vary between 41.82% and 82.77% but over a shorter range (50%–80%) if the outliers are removed (Fig. F61). Measurements of color reflectance parameters a* and b* had small variations in the average value per section. The ratio a*/b* clearly separates two outliers with higher values in the red color scale, which is likely due to the presence of red fine branching Tubipora musica.

Paleomagnetism

Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on samples taken from the working half of the recovered core (Fig. F62). Positive low susceptibility values were recorded uniformly throughout the samples, from the top of the core to the total depth. Positive values ranged from 0.30 × 10^–8 to 1.98 × 10^–8 m³/kg with a mean value of 0.53 × 10^–8 m³/kg. There are also three samples characterized by negative (diamagnetic) susceptibilities located at 17.12, 18.64, and 20.14 mbsf with values of –0.16 × 10^–8, –0.81 × 10^–8, and –0.76 × 10^–8 m³/kg, respectively.

Chronology

Two calibrated radiocarbon ages (10 cal y BP, Core 325-M0034A-1R; 11 cal y BP, Core 7R) (Fig. F63) and one U-Th age (11 cal y BP, Core 13R) (Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is unaffected by corrections for initial ²³⁰Th, adding to the confidence in this age interpretation. This hole recovered material from the middle portion of the deglaciation.

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