Previous   |    Next

doi:10.2204/iodp.proc.325.106.2011

Hole M0053A

Operations

Site 6, Hole M0053A

API pipe started being run in Hole M0053A at 1400 h on 27 March 2010 with the intent to use the HQ coring system. Operations halted between 1510 and 1530 h to make repairs to the elevator. Between 1725 and 1800 h, operations stopped because of a squall passing through the area. Between 1800 and 1900 h, the dies on the roughneck were checked prior to running the bumper sub and lowering the seabed template to ~10 m below the sea surface. At 2010 h, the API string was lowered to the seabed and the bumper sub heave was monitored. The heave was not sufficient, possibly because of soft sediment, so a 3 m pup was added. The bumper sub appeared to compensate, and at 2130 h the seabed template was lowered to the seabed. However, at 2210 h, the bumper sub stopped compensating. At 2300 h, the decision was taken to abandon HQ coring and return to API pipe.

The seabed template was recovered between 2300 and 0030 h on 28 March and the bumper sub between 0030 and 0110 h. Additional API pipe was run between 0110 and 0120 h. The first extended nose corer core was recovered at 0210 h (Table T1) (all future runs, with the exception of Run 26, used the standard rotary corer core barrel). Between 0520 and 0550 h on 28 March, operations stopped while the latch head assembly was changed, and between 1330 and 1350 h, coring operations again stopped because of heavy rain obscuring the driller’s view of the derrick. Coring then continued for another 17 runs until the hole was terminated at 0250 h on 29 March at 37.3 mbsf, with an average recovery of 32.7%.

At 0300 h on 29 March, the API pipe was tripped to just above the seabed, and a downpipe camera survey was conducted. This operation was completed by 0335 h, with the camera being left in the pipe for the vessel move 66 m to the next site. The seabed transponder was recovered at 0355 h, and the vessel began the move to Site 7 (Hole M0054A) at 0410 h.

Sedimentology and biological assemblages

Hole M0053A is divided into 10 lithostratigraphic units.

Unit 1, interval 325-M0053A-1X-1, 0–13 cm: unconsolidated muddy Halimeda sand

The uppermost Unit 1, consisting only of the uppermost interval 325-M0053A-1X-1, 0–13 cm, is composed of unconsolidated muddy Halimeda sand (Fig. F14). Bioclasts include corals, mollusks, larger foraminifera, and encrusting foraminifera (including Homotrema rubrum). Some bioclasts have brown or black staining.

The only large coral is a long Pocillopora branch. Fragments include Seriatopora, Pocillopora, Montipora(?), Tubipora musica, and Heliopora coerulea.

Unit 2: interval 325-M0053A-1X-1, 13–106 cm: coralgal boundstone with Halimeda

Unit 2, spanning interval 325-M0053A-1X-1, 13–106 cm, consists of coralgal boundstone associated with Halimeda sand (Fig. F15). The boundstone is broken into angular to subangular lithoclasts up to pebble size. Massive corals are dominant. Nongeniculate coralline algae exist only as thin encrusting forms. Spaces around boundstone clasts are filled with Halimeda sand with bioclasts of corals, mollusks, and larger foraminifera. Some bioclasts are stained black. Well-preserved specimens of Amphistegina, Alveolinella, Heterostegina, and Soritinae are common in muddy gravels from interval 325-M0053A-1X-1, 60–65 cm.

Larger corals are submassive to massive Montipora(?), Goniopora(?), Porites, Faviidae, and Tubipora musica.

Unit 3: Sections 325-M0053A-1X-CC through 2R-CC: lime sand and granules

Unit 3, spanning Sections 325-M0053A-1X-CC through 2R-CC, consists of lime sand and granules, probably produced by drilling disturbance. The sand and granules appear to have originated in the coralgal boundstones above (Unit 2) or below (Unit 4) this unit. Bioclasts of corals, mollusks, and Halimeda are present.

All corals are small fragments of Seriatopora, Porites, Isopora, Acroporidae, and Agariciidae.

Unit 4: Sections 325-M0053A-3R-CC through 10R-CC: coralgal boundstone

Unit 4, spanning Sections 325-M0053A-3R-CC through 10R-CC, is a coralgal boundstone consisting of corals with thin to thick crusts of nongeniculate coralline algae (Figs. F16, F17). Bioclasts of mollusks and Halimeda are present. Corals are bioeroded, with borings and constructional voids partly filled with unconsolidated silt-sized to fine-grained lime sand.

Corals are dominated by massive Isopora (Fig. F18) and medium-thickness branching (possibly corymbose?) Acropora, Faviidae (possibly Leptoria(?)) (Fig. F19), and some Seriatopora. Associated corals include Pocillopora, encrusting Montipora, Porites(?), Faviidae, Fungiidae, and Tubipora musica. Fragments include Isopora, Seriatopora, Pocillopora, Montipora(?), Porites(?), Tubipora musica, and Acroporidae.

Unit 5: Sections 325-M0053A-11R-1 through 17R-1: coralgal-microbialite boundstone

Unit 5, spanning Sections 325-M0053A-11R-1 to 17R-1, 21 cm, consists mainly of microbialites. Corals are covered by thin to thick nongeniculate coralline algal crusts which are, in turn, encrusted by microbialites (Fig. F20). Nongeniculate coralline algae volumetrically fill less space than microbialites and corals. Microbialites have laminated fabrics, and depressions in the microbialites are filled with bioclastic packstone rich in Halimeda segments (Figs. F21, F22). Some corals are bioeroded. Borings and constructional voids in the microbialite are occasionally filled with unconsolidated very fine sand-sized internal sediment.

Dominant corals in the abundant and diverse coral assemblage are massive Isopora and medium-thickness branching (possibly corymbose?) Acropora. The main associated corals are Montipora, fine branching and corymbose Acropora, Pocilloporidae, Faviidae, and Tubipora musica. Fragments include Acropora, Seriatopora, Montipora, Porites, Faviidae, and Tubipora musica.

Unit 6: Sections 325-M0053A-17R-CC through 21R-1: microbialite boundstone

Unit 6, spanning Sections 325-M0053A-17R-CC through 21R-1, is dominated by microbialite (Fig. F23) in which corals and nongeniculate coralline algae are much less common than in the previous coralgal-microbialite boundstone (Unit 5). Some microbiolite pieces have depressions filled with bioclastic packstone containing Halimeda segments (Figs. F24, F25). Some constructional voids are filled with unconsolidated very fine sand-sized internal sediment.

Corals are less abundant than in the overlying Unit 5. Most are branching Acropora, Porites, Montipora(?), Seriatopora, massive Platygyra (Fig. F26), and Agariciidae. The main associated corals are Acropora, Seriatopora, submassive Cyphastrea, and Tubipora musica. Fragments include Acropora, Porites(?), Montipora(?), and Tubipora musica.

Unit 7: Sections 325-M0053A-21R-CC through 23R-CC: coralgal-microbialite boundstone

Unit 7, spanning Sections 325-M0053A-21R-CC through 23R-CC, consists of pebble-sized or smaller angular to subangular lithoclasts of coralgal-microbialite boundstone with coarse lime sand. Bioclasts (mostly mollusks and corals) are rare in the sand and likely to be derived from the boundstone.

The only large coral is a submassive Montipora. Other corals are fragments of Porites(?), branching Acropora, Cyphastrea, Pavona, and Porites or Montipora.

Unit 8: Sections 325-M0053A-24R-CC through 25R-CC: unconsolidated sand and granules

Unit 8, spanning Sections 325-M0053A-24R-CC through 25R-CC, consists of unconsolidated sand and granules. Bioclasts include corals, nongeniculate coralline algae (branching species), Halimeda, larger foraminifera, and echinoids. Lithoclasts of gray bioclastic packstone are present. Occasional well-preserved or fragmented specimens of Amphistegina are present in muddy coarse sands from interval 325-M0053A-24R-CC, 6–11 cm.

The only corals are small fragments, including branching Seriatopora, Acropora, and Tubipora musica, and pieces of Montipora or Porites.

Unit 9: Sections 325-M0053A-26X-CC through 29R-CC: packstone and lime sand

Unit 9, spanning Sections 325-M0053A-26X-CC to 29R-CC consists of unconsolidated lime sands with variously sized bioclasts. Intervals 325-M0053A-26X-CC through 27R-CC, 1–33 cm, are composed of consolidated bioclastic packstone and unconsolidated lime sand (Figs. F27, F28). The packstone consists of bioclasts of Halimeda and mollusks, but most bioclasts are too small to be identified. The lower part of Unit 9 (interval 325-M0053A-27R-CC, 33 cm, to Section 29R-CC, 2 cm), consists of unconsolidated lime sand with gravel-sized bioclasts of corals, Halimeda, and less abundant larger foraminifera and mollusks. Some coral branches are covered with thin encrusting nongeniculate coralline algae. Occasional well-preserved or fragmented specimens of Amphistegina are present in gravelly coarse-grained sands from interval 325-M0053A-27R-1, 40–45 cm.

Small coral fragments are restricted to Sections 325-M0053A-26X-CC and 27R-1, as are fine-branching Acropora and Pocilloporidae, with a few pieces of Montipora or Porites.

Unit 10: Sections 325-M0053A-30R-1 through 33R-CC: dark gray sand with corals

The lowermost Unit 10, spanning Sections 325-M0053A-30R-1 through 33R-CC, consists of coarse to very coarse lime sand with coral clasts and larger foraminifera (Fig. F29). The lime sand is partly lithified in interval 325-M0053A-31R-10, 1–6 cm, where grayish lithoclasts may be derived from older limestones with gray to black staining. Large Operculina tests occur in interval 325-M0053A-33R-1, 0–5 cm. Medium to coarse sands from interval 325-M0053A-30R-1, 15–20 cm, contain abundant abraded or fragmented specimens of Baculogypsina, Calcarina, Amphistegina, and Operculina. Fragmented specimens of Operculina, Elphidiidae, Amphistegina, and Baculogypsina are common in medium to coarse sands from interval 325-M0053A-32R-1, 20–25 cm. Fragmented specimens of Operculina, Amphistegina, Alveolinella, and Baculogypsina are common in gravelly medium to coarse sands from interval 325-M0053A-33R-CC, 5–10 cm.

The only large coral is a massive Galaxea in Core 325-M0053A-31R (Fig. F30). Fragments include Seriatopora, foliaceous Pectiniidae(?), and an ahermatypic coral.

Physical properties

Hole M0053A was cored to a total depth (TD) of 37.30 m DSF-A, of which 12.18 m was successfully recovered (32.65% recovery). Physical property measurements taken on this core are summarized in Table T2.

Density and porosity

Multisensor core logger bulk density varies from 1.01 to 2.34 g/cm³ in cores from Hole M0053A (Fig. F31). Cores recovered from this hole have an average length of 0.34 m and are frequently biscuited or rubbly, making it difficult to comment on downhole trends. The only exception to this is Section 325-M0053A-1R-1, which shows an increase in bulk density downsection. Eighteen discrete samples were taken from this hole for moisture and density measurements (Fig. F32).

Grain density varies between 2.72 and 2.80 g/cm³, and bulk density varies between 1.94 and 2.38 g/cm³ (Fig. F32). Results for porosity and bulk density are difficult to compare with lithology. There is principally coralgal and coralgal-microbialite boundstone between 4 and 23 m CSF-A, with porosity varying between 21% and 48%. The three discrete samples taken between 25 m CSF-A and TD have high porosity (lime sand zone with a porosity ranging from 36% to 47%).

P-wave velocity

P-wave velocity measurements taken on whole cores offshore yielded data in the uppermost 1 m and bottommost 7 m of the hole. Values range from 1525 to 1830 m/s (Fig. F31). As with other holes, those values close to the velocity of seawater should be treated with caution because data quality may have been compromised by poor core quality. The data set is patchy, and it is therefore impossible to comment on trends. However, four discrete samples were measured using the P-wave logger. Values from these dominantly microbialite boundstone samples range from 3434 to 3973 m/s (mean values for resaturated samples) (Fig. F33). A plot of moisture and density bulk density versus discrete P-wave velocity indicates that velocity increases with increasing sample density (Fig. F33B).

Magnetic susceptibility

The range of magnetic susceptibility values for Hole M0053A is –1.64 × 10^–5 to 69.26 × 10^–5 SI (Fig. F31). Magnetic susceptibility is elevated, compared to the rest of the hole, in an interval from ~16 to 22 m CSF-A, with values in the range of 5 × 10^–5 to 20 × 10^–5 SI. This interval roughly corresponds to the presence of an a interval of microbialite boundstone. There is also a magnetic susceptibility peak at 5.31 m CSF-A (69.26 × 10^–5 SI).

Electrical resistivity

Noncontact electrical resistivity measured on whole cores in Hole M0053A is variable, ranging from 0.52 to 31.28 Ωm (Fig. F31). Similar to the other multisensor core logger measurements, it is difficult to assess whether any genuine downhole trends occur in this hole because of the sporadic occurrence of data (a consequence of recovery and core length).

Digital line scans and color reflectance

Cores from Hole M0053A were digitally scanned and, where appropriate, cores were measured for color reflectance. Hole M0053A, in general, yielded highly dispersed values in each section with some variations in the range of reflectance values with hole depth (Fig. F34). Color reflectance in Hole M0053A varies between 35.96% and 79.95% for L*. The presence of heterogeneous material in this hole makes it difficult to relate the color reflectance measurements to the sediment without analyzing each measurement individually. The dispersion of the reflectance measurements in the uppermost 1 m of the hole is due to variable composition in this area of lime sand mud, boundstone, and lime pebbles. The range of color reflectance in this top part of the hole covers a wide range from 35.96% to 69.24%. The presence of a massive coral and a coralgal boundstone at ~3 m CSF-A creates a less dispersed pattern for the reflectance measurements (L*). From 4 to 23 m CSF-A, the alternation of heterogeneous lithologies within the coralgal, coralgal-microbialite, and microbialite boundstone units creates a highly scattered data set. A slightly negative downhole trend is also observed. A grainstone unit at 26.6 m CSF-A gives a consistent measurement of color reflectance parameters followed by more dispersed values due to the presence of coarse to very coarse lime sand. At 28 m CSF-A, a section of dark gray coarse lime sand gives a consistent measurement for L*. From 28 m CSF-A to TD, this dark gray coarse lime sand and a massive Cyphastea sp. present high dispersion in the reflectance values. No large variations in color indexes are evident. Values of a* are low but mainly positive (red color), and values of b* are always positive (yellow color). The a*/b* ratio indicates a variation in trend between the uppermost (0–25 m CSF-A) and lower (25–37 m CSF-A) parts of the hole.

Thermal conductivity

For Hole M0053A, three measurement points were taken, giving thermal conductivity values ranging from 0.94 to 1.15 W/(m·K) (Fig. F35).

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. F36). Positive susceptibilities range from 0.01 × 10^–8 to 14.69 × 10^–8 m³/kg with an arithmetic mean of 3.61 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals. Higher values of magnetic susceptibility are concentrated largely in the stratigraphic interval between 13 and ~22 mbsf. In addition, negative susceptibilities concentrated in two main intervals (0–9 mbsf and 23 mbsf to TD) (diamagnetic) were recorded with an arithmetic mean of –0.46 × 10^–8 m³/kg.

Chronology

Two calibrated radiocarbon ages (17 cal y BP, Core 325-M0053A-3R; 20 cal y BP, Core 9R) (Fig. F37) and one U-Th age (24 cal y BP, Core 25R) (see Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is only slightly affected by corrections for initial ²³⁰Th (the seawater correction makes the age 0.4 k.y. younger). This hole recovered material from the Last Glacial Maximum, as well as the early deglacial. This hole is noteworthy in that it appears to have substantial recovery from the Last Glacial Maximum, with Cores 325-M0053A-10R through 25R being bracketed by 20 and 24 cal y BP ages.

Top of page   |    Previous   |    Next