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doi:10.2204/iodp.proc.325.104.2011

Hole M0042A

Operations

Site 2, Hole M0042A

The Greatship Maya arrived at Site 2 at 2330 h on 6 March 2010 and began to settle on dynamic positioning. The seabed template was secured in the moonpool, repairs to the camera cable continued, and the API pipe was run to just above the seabed. At 0145 h on 7 March, the downpipe camera was deployed. The first standard rotary corer (ALN) core was recovered at 0330 h, and operations continued until 0500 h, when repairs to a latch head dog stopped coring for 55 min. After ALN Run 5 was recovered at 0715 h, it was decided that the API was spudded into the seabed firmly enough to switch to HQ coring. By 1020 h, the seabed template was secured in the moonpool with steel wires, the API pipe was set in the elevators, and the HQ rods were ready to be run. The first HQ core (Run 6) was recovered at 1240 h. However, the API pipe was sinking in the hole (1.5 m), presumably because of soft sediments, so hole stabilization using a thicker mud continued until 1330 h. During Run 7, the bit became blocked, requiring flushing. However, the API pipe started sinking again, now resting on the elevators, and it was believed that swabbing the hole was causing further instability. The decision was made to trip the HQ rods and return to API coring until a harder formation was encountered in which to set the API pipe.

At 1500 h, a sample was recovered from the HQ bottom-hole assembly and curated as a wash sample. Between 1540 and 1700 h, the API pipe was reconnected and the elevator removed. ALN coring restarted at 1710 h and continued until 0705 h on 8 March, when the hole was terminated at 46.4 mbsf with an average recovery of 23.6%.

Downhole logging was conducted following coring operations, and mobilization of the drill floor and logging equipment continued until 1010 h. Between 1010 and 1150 h, the through-pipe gamma log was run. On completion, the API pipe was tripped to 7 mbsf, and conditioning of the hole continued until 1340 h. The seabed template was then lowered onto the seabed, and the dual induction logging sonde was deployed at 1410 h. Following this, the acoustic borehole image, spectral gamma, magnetic susceptibility, sonic, optical borehole image, and caliper sondes were deployed at 1530, 1710, 1845, 2000, 2125, and 2320 h, respectively. Some hole caving was noted, with the tools logging from 45.07, 43.98, 39.93, 38.49, 33.23, 38.84, and 38.58 wireline log matched depth below seafloor (WMSF), respectively. By 0030 h on 9 March, all downhole logging had been completed and demobilization of the equipment and securing of the drill floor for transit had begun. At 0130 h, the seabed template was recovered, and by 0345 h, all API pipe had been tripped. The seabed transponder was onboard and secured by 0400 h, and the vessel began the 6.5 km transit seaward to Site 12, Hole M0044A.

Over the course of coring and logging this hole, three separate remotely operated vehicle dives were conducted, viewing the drill string, the small entry hole, and the dispersion of drill cuttings.

Sedimentology and biological assemblages

Hole M0042A is divided into 10 lithostratigraphic units.

Unit 1: Section 325-M0042A-1R-1: modern lime sand and algal bindstone

The uppermost Unit 1, consisting only of Section 325-M0042A-1R-1, is composed of algal bindstone and bioclastic packstone fragments, with lime sand containing fragments of Halimeda, mollusks, larger foraminifera, and coralline algae. Coralline algae form an open framework of thin plants. Fragments of coralline algal bindstone with worm tube and brownish stains in the uppermost 4 cm of Section 325-M0042A-1R-1 probably represent the modern seafloor. Interval 325-M0042A-1R-1, 14–19 cm, contains larger benthic foraminifera in muddy coarse sands.

The only coral is a fragment of a solitary Fungiidae in the uppermost 5 cm of the section.

Unit 2: Sections 325-M0042A-1R-CC through 3R-1: coralgal boundstone

Unit 2, spanning Sections 325-M0042A-1R-CC through 3R-1, contains pieces of coralgal boundstone (Fig. F26). The boundstone consists mainly of massive corals covered with coralline algae and minor microbialite crusts and has some internal sediments and fragments of mollusks and bryozoans. All components are bioeroded locally. The coralline algal crust in interval 325-M0042A-2R-1, 5–7 cm, has a “reddish” surface.

Large corals are mainly submassive to massive Faviidae (Favia(?) and Platygyra(?)) and one branching Acropora. Fragments include Acropora, Cyphastrea, Porites, Montipora(?), and various Faviidae.

Unit 3: Sections 325-M0042A-3R-CC through 5R-CC: coralgal-microbialite boundstone

Unit 3, spanning Sections 325-M0042A-3R-CC through 5R-CC, consists of coralgal-microbialite boundstone. Coralline algae alternate with microbialite crusts and appear to trap bioclasts. Coralline algae occur as thick crusts on top of corals or as irregular, contorted structures intergrown with microbialites. Microbialites can be several centimeters thick and are usually dark colored (Fig. F27) with a succession from laminated textures at the base to digitate forms at the top. Some internal bioclastic sediments, containing Halimeda and mollusks, occur throughout Unit 3. All components of the boundstone are bioeroded by bivalves and sponges. The geopetal filling of an intraskeletal void at Section 325-M0042A-4R-1, 40 cm, is probably microbialite.

The upper part of Unit 3 is dominated by medium to robustly branching Acropora (Fig. F28) and encrusting to submassive Porites. Associated corals are Seriatopora, Tubipora musica, encrusting Porites, and Faviidae. The only corals toward the base of Unit 3 are fragments of Porites and Montipora.

Unit 4: Sections 325-M0042A-8W-1 to 11R-1, 20 cm: unconsolidated sediment

Unit 4, spanning Section 325-M0042A-8W-1 through interval 325-M0042A-11R-1, 1–20 cm, consists of unconsolidated lime granules and pebbles. Major components are coral fragments (as large as 9 cm), coralgal-microbialite boundstone fragments, Halimeda, larger foraminifera, mollusks, and coralline algae. Coralline algae occur both as crusts in boundstone fragments and as loose branching plants. Fragments from the overlying boundstones and fresh mollusk shells clearly indicate downhole contamination.

Larger corals are mainly massive Isopora and Montipora, with some branching to massive Porites. Diverse fragments include branching Pocilloporidae (Pocillopora, Seriatopora, and Stylophora), branching Acropora, and some pieces of Montipora, Isopora, Platygyra, Porites, Montastrea(?), and Platygyra.

Unit 5: Sections 325-M0042A-11R-1, 20 cm, through 13R-CC: grainstone

Unit 5, spanning Sections 325-M0042A-11R-1, 20 cm, through 13R-CC, is a yellowish grainstone to rudstone rich in larger foraminifera, Halimeda, mollusks, and coral fragments (Fig. F29). Granules and pebbles from downhole contamination are present only in Section 325-M0042A-12R-1. The grainstone in Sections 325-M0042A-13R-1 and 13R-CC is highly fragmented (shattered by drilling). Muddy coarse sands from interval 325-M0042A-13R-1, 26–31 cm, contain sparse abraded Heterostegina and Amphistegina.

The few large corals are submassive Porites. Diverse fragments include Pocillopora, Seriatopora, Stylophora, Acropora, Porites, and possibly Montipora.

Unit 6: Sections 325-M0042A-14R-1 through 16R-CC: unconsolidated sediment

Unit 6, spanning Sections 325-M0042A-14R-1 through 16R-CC, consists of unconsolidated lime granules and pebbles. Major components are coral fragments, coralgal-microbialite boundstone fragments, mollusks, and larger foraminifera. Fragments of overlying boundstones and other particles indicate downhole contamination.

There are no large corals. Numerous small fragments include Acroporidae, Pocilloporidae, Porites, and Porites(?) or Montipora(?).

Unit 7: Sections 325-M0042A-17R-1 through 325-M0042A-19R-1: grainstone with rhodoliths

Unit 7, spanning Sections 325-M0042A-17R-1 through 19R-1, consists of yellowish grainstone to rudstone rich in rhodoliths (i.e., nodules composed mainly of coralline algae). Rhodoliths are small (<3 cm) to very small (<1 cm) and subelliptical. Larger foraminifera, mollusks, and coral fragments are also common. There is widespread dissolution of clasts, creating moldic porosity.

The only corals are small to tiny fragments, mostly unidentifiable, that include Pocilloporidae, Porites, and possibly Montipora. The nucleus of one rhodolith is an acroporid. A piece of octocoral spiculite near exists the base of the unit.

Unit 8: Sections 325-M0042A-20R-1 to 21R-1, 10 cm: unconsolidated sediment

Unit 8, spanning Sections 325-M0042A-20R-1 to 21R-1, 10 cm, consists of unconsolidated lime granules and pebbles. Major components are coral fragments, coralgal-microbialite boundstone fragments, mollusks, and larger foraminifera. Fragments of overlying boundstones and fresh mollusk shells clearly indicate downhole contamination.

Corals consist of fragments that include Acropora, Isopora, Montipora(?), Seriatopora, and possibly other Pocilloporidae.

Unit 9: Sections 325-M0042A-21R, 1 cm, through Section 22R-1: gray rudstone

Unit 9, spanning Sections 325-M0042A-21R-1, 10 cm, through 22R-1, consists of fragments of gray-colored rudstone rich in larger foraminifera, Halimeda, and coral and bivalve fragments. Aragonitic components are partially dissolved (Fig. F30).

Recognizable corals are mainly molds of Acropora, Isopora, and Seriatopora. Fragments are mainly Pocilloporidae and Acroporidae.

Unit 10: Sections 325-M0042A-22R-CC through 29R-CC: rudstone with brown stains

The lowermost Unit 10, spanning Sections 325-M0042A-22R-CC through 29R-CC, consists of yellowish rudstone to grainstone with large rhodoliths and coral fragments (Fig. F31). Halimeda segments, larger foraminifera, and mollusks are other major components. Rhodoliths are as wide as 8 cm in maximum diameter (Figs. F32, F33), and some have coral fragments as nuclei. Aragonite clasts are largely dissolved (Fig. F34). The deposit has brown staining locally (Fig. F35) and contains root remains (rhizoliths) in interval 325-M0042A-25R-1, 55–57 cm, indicating subaerial exposure (Fig. F36). A brown carbonate crust covers some fragments in Section 325-M0042A-22R-CC, at the top of Unit 10, and may represent a calcrete or speleothem developed on the original rudstone. Interval 325-M0042A-29R-CC, 0–5 cm, contains larger foraminifera in muddy, very coarse sands.

Because of extensive alteration and dissolution, most corals are not identifiable (Fig. F37). Nuclei of some rhodoliths include Acroporidae, Poritidae, and Pocilloporidae.

Physical properties

A total of 10.94 m of core was successfully recovered from Hole M0042A, which was cored to a total depth of 46.4 m DSF-A (23.58% recovery). Physical property data are summarized in Table T2.

Density and porosity

In Hole M0042A, bulk density measured on whole cores varies from 1.03 to 2.49 g/cm³ (Fig. F38). Core recovery is such that it was not possible to acquire data for three quarters of the hole. This lack of data, combined with the majority of the cores being <0.2 m in length and composed of rubbly material, results in poor data quality for Hole M0042A. It is impossible to comment on trends downhole. However, it was possible to measure 17 samples from the hole for discrete moisture and density analysis (Fig. F39). Discretely sampled bulk density data varies from 1.51 to 2.45 g/cm³, whereas porosity values in the hole range from 16% to 72%. These results for porosity and bulk density are difficult to compare with lithology. There is boundstone between 0 and 7.5 m CSF-A and principally lime pebbles and lime granules between 7.5 m and 45 m CSF-A. In terms of grain density, values range from 2.70 to 2.81 g/cm³ (Fig. F39). Owing to the questionable quality of the bulk density data in this hole, it is difficult to draw comparisons between the two data sets.

P-wave velocity

Multisensor core logger P-wave velocity measurements range from 1510.50 to 1865.90 m/s (Fig. F38). The lower values of the range could be due to poor core quality (see “Physical properties” in the “Methods” chapter). Three discrete P-wave velocity measurements were taken on samples from Hole M0042A cores, with values ranging from 3772 to 4223 m/s (mean value for resaturated core). There are no clear downhole trends with this data set (Fig. F40A) and there is no obvious relationship between bulk density and discrete P-wave velocity measurements (Fig. F40B).

Magnetic susceptibility

Core recovery in Hole M0042A was such that the cores are generally short; therefore, identification of any downhole trends is difficult. Magnetic susceptibility values range from –12.42 × 10^–5 to 38.73 × 10^–5 SI (Fig. F38).

Electrical resistivity

Similar to the other data sets, resistivity data have suffered because of short core length and discontinuous recovery downhole, resulting in an erratic profile for resistivity downhole. Electrical resistivity measured on whole cores varies from 0.76 to 40.36 Ωm (Fig. F38).

Digital line scans and color reflectance

All cores from Hole M0042A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Hole M0042A was the deepest hole, recovering the oldest material in transect HYD-02A. Color reflectance in Hole M0042A varies between 52.1% and 93.02% L* (Fig. F41). Variations in color reflectance parameters show some dispersion for individual sections, especially in the uppermost 20 m, corresponding with lime lithologies. From 20 to 30 m CSF-A, the values obtained were in the same range with a very small dispersion in individual sections due to the presence of larger fragments of corals and rudstone. From 34 to 45 m CSF-A, values were highly variable within sections, ranging from 65% to 85% with some outliers. The extremely high value present at 40.38 m CSF-A is probably due to the presence of liquid water and drilling mud contamination. The low reflectance values measured at the base of the hole are due to the presence of shattered rudstone. No significant variations in color indexes were noticed. Values of a* were mainly positive (red color), and values of b* were always positive (yellow color). The a*/b* ratio did not indicate additional information in these cases because of the small variation in the values of a* and b*.

Paleomagnetism

Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on all samples taken from the working half of the recovered core (Fig. F42). Very low negative (diamagnetic) susceptibilities occur throughout the entire core, ranging from –3.07 × 10^–8 to –0.90 × 10^–8 m³/kg with an arithmetic mean value of –1.03 × 10^–8 m³/kg. Three positive values at 4.04, 22.09, and 40.42 mbsf were recorded, with magnetic susceptibilities of 0.95 × 10^–8, 0.58 × 10^–8, and 0.07 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals.

Chronology

Two calibrated radiocarbon ages (10 cal y BP, Core 325-M0042A-1R; 13 k. y. cal. BP, Core 10R) (Fig. F43) and one U-Th age (169 cal y BP, Core 24R) (see 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. Therefore, the upper portions of this hole have recorded the middle part of the deglaciation, whereas the lower cores seem to have recovered material from the Pleistocene. Specifically, this hole contains material from marine isotope Stage 6, but there are five cores below the 169 cal y BP section that may contain older material and 13 cores between the 169 and 13 cal y BP sections that may record younger periods of the Pleistocene.

Downhole measurements

Downhole logging operations were completed in Hole M0042A to a maximum depth of 45.69 m WMSF (seafloor picked from the log data) with the ANTARES Spectral Natural Gamma Probe (ASGR) sonde (see “Downhole logging” in the “Methods” chapter for detailed tool information). Recovery in Hole M0042A was just over 23.58%; therefore, the continuous downhole measurements provide a useful tool to fill in data gaps. Downhole logging was conducted in an API hole, the diameter of which is beyond the maximum working size for the acoustic and optical tools (ABI40 and OBI40). However, the full tool suite (with the exception of the IDRONAUT) was run downhole. Chronologically the tools deployed were

1. ASGR 256 (through API pipe),

2. DIL45 (conductivity in open hole),

3. ABI40 (API hole diameter too great to obtain acoustic images),

4. ASGR 256 (spectral gamma ray in open hole),

5. EM51 (magnetic susceptibility in open hole),

6. Sonic (centralizers undersized so that the tool would fit through the API bit; therefore, tool not optimally centralized in the hole and so errors were introduced into the measurements),

7. OBI40 (optical tool; API hole diameter too great and mud caking prevented good image collection), and

8. Caliper (borehole diameter).

Hole stability for open-hole logging was relatively good. The open-hole stage of logging began with 44.17 m WMSF available hole and ended with the caliper tool (CAL3) (last tool run) logging 37.76 m WMSF of open hole.

From the logging data, four main logging units were identified in Hole M0042A (Fig. F44):

1. Unit I (0–13.96 m WMSF; Cores 325-M0042A-1R through 10R) is characterized by low total gamma ray (TGR) counts (through-pipe and open hole) and high conductivity (457 mmho). Magnetic susceptibility is very low, fluctuating around 0.6 mSI (maximum = ~0.65 mSI; minimum = ~ 0.575 mSI). Velocities measured in the unit are all just above 1500 m/s and are therefore similar to that of seawater. Borehole diameter was extremely large in this unit (as wide as 40 cm), which may have been caused by movement of the bottom of the API bottom-hole assembly and/or may reflect the nature of the borehole formation. A number of lithostratigraphic units have been described in the depth range of this logging unit. Lithologies observed from shallow to deep are lime sand and algal bindstone (Section 325-M0042A-1R-1), coralgal boundstone (Sections 1R-CC through 3R-1), coralgal-microbialite boundstone (Sections 3R-CC through 5R-CC), and unconsolidated sediment (lime granules and pebbles).

2. Unit II (13.96–24.26 m WMSF; Cores 325-M0042A-11R through 17R) is defined by TGR counts measured in open hole between 14 and 32 cps, conductivity values of ~350 mmho (minimum = ~155.79 mmho; maximum = ~425.78), magnetic susceptibility between 0.6 and 0.63 mSI, and borehole diameter in gauge. Sonic velocity values are unreliable in this portion of the hole, with values lower than that of seawater (1500 m/s). Within this defined logging unit, lithostratigraphic units vary from grainstone (Sections 325-M0042A-11R-1 through 13R-CC) to unconsolidated sediment (lime granules and pebbles; Sections 14R-1 through 16R-CC) to grainstone with rhodoliths. These lithologic changes are reflected most in the conductivity data.

3. Unit III (24.26–35.36 m WMSF; Cores 325-M0042A-18R through 24R) is characterized by increasing values of TGR counts downhole, with values starting at 20 cps and increasing to ~56 cps. Conductivity undulates around ~450 mmho throughout the unit; however, P-wave velocity values seem to differentiate between two units, the upper having velocities of ~1700 m/s and the lower with velocities of ~1900 m/s. Caliper data show that the hole remains in gauge throughout. Lithostratigraphic units observed over this logging unit interval are grainstone with rhodoliths (Sections 325-M0042A-18R-1 through 19R-1), unconsolidated sediment (lime granules and pebbles; Sections 20R-1 through 21R-1), gray rudstone (Sections 21R-1 through 22R-1), and rudstone with brown staining (Sections 22R-CC through 24R-1). It is possible that the velocity log is differentiating between the unconsolidated unit and the gray rudstone beneath it.

4. Unit IV (35.36–45.47 m WMSF; Cores 325-M0042A-25R through 29R) represents a zone of reduced TGR counts with values of ~25 cps (where open-hole data are present); additionally, conductivity exhibits a decrease to 100 mmho at the very top of Unit IV and then a gradual increase to a maximum of ~682 mmho at ~40.6 m WMSF. Magnetic susceptibility fluctuates between a minimum value of 0.57 and 0.62 mSI. The caliper tool registers the hole as in gauge. The sole lithology found within this logging unit is rudstone with brown staining (Sections 325-M0042A-25R-1 through 29R-CC). Conductivity data suggest that there may be an increase in pore space concentration and or pore space size and connectivity as depth increases to ~40.6 m WMSF.

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