Previous   |    Next

doi:10.2204/iodp.proc.325.106.2011

Hole M0058A

Operations

Site 8, Hole M0058A

The Greatship Maya reached Site 8 at 1950 h on 4 April 2010, and the seabed transponder was deployed prior to the vessel settling on station over Hole M0058A by 2005 h. Between 2010 and 2240 h, repairs on the roughneck and roughneck hydraulics were undertaken prior to running API pipe. The first run started at 0220 h on 5 April and coring continued for 15 runs; all runs were made with the extended nose corer with the exception of Run 5, which was made with the standard rotary corer (Table T1). Coring operations were going smoothly until problems with the power packs surging and cutting out caused operations to stop between 1035 to 1245 h and again between 1355 and 1425 h. The hole was terminated after Run 15 at 41.4 mbsf, with an average recovery of 82%.

At 1515 h, the power pack stalled, delaying tripping of the API pipe. All API pipe was on deck by 1845 h, and the drill floor and template were secured and ready for transit by 2105 h. At 2115 h, the Greatship Maya came off dynamic positioning, and it departed Site 8 (Hole M0058A) at 2130 h.

Transit to Townsville

The Greatship Maya departed the final site of transect NOG-01B at 2130 h on 5 April 2010 and arrived at the port of Townsville, Australia, at 1430 h on 6 April. Demobilization of the vessel and clearances of the containers by AQIS and Customs officials continued until 1130 h on 7 April, when all European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO) personnel and science party members departed the vessel.

Sedimentology and biological assemblages

Hole M0058A is divided into seven lithostratigraphic units.

Unit 1: Sections 325-M0058A-1X-1, to 3X-1, 141 cm: mud

The uppermost Unit 1, spanning Section 325-M0058A-1X-1 to Core 3X-1, 141 cm, consists of homogeneous green (6/10Y) mud without visible layering or signs of bioturbation (Fig. F139). Planktonic (?) and small benthic foraminifera are common. Sponge spicules are rare. The boundary between Unit 1 and the underlying fine to medium sands of Unit 2 is irregular and may be an erosional surface (Fig. F140). There are no corals in this unit (or in any other unit of Hole M0058A).

Unit 2: Sections 325-M0058A-3X-1, 141 cm, to 4X-1, 107 cm: fine to medium sand with larger foraminifera

Unit 2, spanning Sections 325-M0058A-3X-1,141 cm, to 4X-1, 107 cm, consists of fine to medium sand with abundant larger foraminifera. Planktonic foraminifera are absent. There is no layering and no signs of bioturbation. Interval 325-M0058A-4X-1, 0–10 cm, is an interlayer of mud containing small benthic foraminifera and, rarely, larger foraminifera (Operculina). Bioclasts are common in interval 325-M0058A-4X-1, 0–107 cm, and include bryozoans, coralline algae, serpulids, echinoids, Halimeda, mollusks, and larger foraminifera. Cemented lithoclasts of fine to medium sand occur in the lower part of Unit 2 (interval 325-M0058A-4X-1, 56–107 cm). One lithoclast is covered with nongeniculate coralline algae and serpulids. There are no corals.

Unit 3: Sections 325-M0058A-4X-1, 107 cm, through 6X-CC: fine to medium sand with granule-sized bioclasts and grainstones

Unit 3, spanning Sections 325-M0058A-4X-1, 107 cm, to 6X-CC, 1 cm, consists of fine to medium sand with granule- to pebble-sized bioclasts and pebble- to cobble-sized lithoclasts of brown grainstone (Fig. F141). Loose bioclasts in the sand include fragments of mollusks, bryozoa, coralline algae, echinoids, larger foraminifera, and serpulids. No planktonic foraminifera are visible. The grainstone is composed of fragments of Halimeda, coralline algae, larger foraminifera, and mollusk shells. Moldic voids in the grainstone may indicate partial dissolution of originally aragonitic components. There are no corals.

Unit 4: Sections 325-M0058A-7X-1 to 11X-2, 19 cm: mud

Unit 4, spanning Sections 325-M0058A-7X-1 to 11X-2, 19 cm, consists of homogeneous green (6/5GY-6/10Y-5/Y10) mud (Figs. F142, F143) with no signs of layering or bioturbation. Small benthic foraminifera occur throughout this unit and are very abundant in the uppermost interval 325-M0058A-7X-1, 0–1 cm. Planktonic foraminifera are present in Sections 325-M0058A-7X-3 through 8R-CC. In Section 325-M0058A-8X-2, there are well-preserved gastropod shells in two horizons: Sections 325-M0058A-8X-2, 30 cm, and 325-M0058A-8X-3, 8 cm. There are no corals.

Unit 5: Sections 325-M0058A-11X-2, 19 cm, to 11X-3, 48 cm: bioturbated mud

Unit 5, spanning Sections 325-M0058A-11X-2, 19 cm, to 11X-3, 48 cm, consists of dark green (4/10Y) bioturbated mud (Fig. F144). There is no visible layering. Small benthic foraminifera occur throughout this unit. No planktonic foraminifera are visible. There are no corals.

Unit 6: Sections 325-M0058A-11X-3, 48 cm, to 13X-1, 7 cm: fine to medium sand

Unit 6, spanning Sections 325-M0058A-11X-3, 48 cm, to 13X-1, 7 cm, consists of fine to medium dark green (4/10Y) sand containing bioclasts, with an interlayer of mud in Section 325-M0058A-12X-CC (Fig. F145). Larger foraminifera Operculina and small benthic foraminifera are common. Whereas other bioclasts are rare, they include planktonic foraminifera (usually with black-stained tests), mollusks, bryozoans, echinoids, and fragments of packstone. There are no corals.

Unit 7: Sections 325-M0058A-13X-1, 7 cm, through 15X-2: mud

The lowermost Unit 7, spanning Sections 325-M0058A-13X-1, 7 cm, through 15X-2, consists of green (6/10GY) mud. Benthic foraminifera are common throughout this unit. Planktonic foraminifera are rare in Core 325-M0058A-13X and absent from Cores 14X and 15X. Mollusk shells are rare. In the uppermost part of Unit 7, intervals 325-M0058A-13X-1, 7–73 cm, and 13X-1, 121–145 cm, have distinct 1–3 cm diameter burrows filled with fine to medium sand (Fig. F146). There are no corals.

Physical properties

Hole M0058A was cored to a total depth of 41.40 m DSF-A, of which 33.94 m were successfully recovered. This equates to 81.98% recovery, which is the best recovery for the holes in Expedition 325. Petrophysical data are summarized in Table T2.

Density and porosity

Multisensor core logger bulk density varies from 1.60 to 2.32 g/cm³ in cores from Hole M0058A (Fig. F147). Unlike most other holes, the quality of the gamma density data for Hole M0058A is excellent as a consequence of good core quality (well-filled liners and saturated cores). Overall, bulk density increases downhole in the top 10 m CSF-A, coinciding with the upper lithostratigraphic mud and a lower fine to medium sand unit. Beyond this, bulk density is relatively constant. The corresponding range of bulk densities given by discrete samples is 1.57 to 2.62 g/cm³, and the two downhole plots are in very close agreement (Fig. F148).

The cores are mainly composed of fine to medium lime muds. Porosity of the discrete samples ranges from very low (6%) to high (68%). The higher porosity values can be attributed to the presence of clays within the samples. Grain density varies between 2.70 and 3.73 g/cm³.

P-wave velocity

Hole M0058A is the only hole that gives a complete record of P-wave velocity from whole-core multisensor core logger measurements (Fig. F147). Data values range from 1504 to 1739 m/s with two intervals identified as having higher P-wave velocity: ~5.6–7.2 m CSF-A and ~28.8–32.4 m CSF-A. The relatively low values for P-wave velocity are suggestive of underconsolidated material. A total of 35 discrete P-wave measurements were taken on samples from Hole M0058A. Many of these yielded poor data as a consequence of the sample being underconsolidated. Samples that gave good P-wave signals resulted in velocities ranging from 1508 to 2281 m/s (mean initial value) (Fig. F149A). One outlier value of 3673 m/s (mean resaturated value [not included in the data table]) corresponds to the only lithified sample (brown grainstone) measured in this hole (Sample 325-M0058A-4X-1, 97–99 cm), at 10.56 m CSF-A. In discrete samples from Hole M0058A, P-wave velocity plotted against bulk density identifies three groups (Fig. F149B).

Magnetic susceptibility

As already mentioned, Hole M0058A has the best core recovery and core quality of all the holes and, as such, yields some of the best magnetic susceptibility data. Values range from –0.22 × 10^–5 to 52.74 × 10^–5 SI, with zones of lower magnetic susceptibility giving values as high as ~8 × 10^–5 SI (Fig. F147). Three clear zones of elevated magnetic susceptibility exist from ~6.5 to 10 m CSF-A (maximum value = 26.55 × 10^–5 SI), ~26 to 32.5 m CSF-A (maximum value = 35.73 × 10^–5 SI), and ~35.5 to 36.5 m CSF-A (29.74 × 10^–5 SI). Preliminary comparison of this data set with the kappabridge data set (see “Paleomagnetism”) suggest that there is very good agreement between the two.

Electrical resistivity

Electrical resistivity values measured on whole cores from Hole M0058A are within the limited range of 0.50 to 1.68 Ωm (Fig. F147), dominantly around 0.7 Ωm. Typically, resistivity values are affected by lithology, pore fluid, and salinity, as well as core liner saturation. However, in this hole there are no clear changes in resistivity coincident with lithostratigraphic unit boundaries. Instead, values are generally constant downhole and the data set is relatively complete (a testament to the good core quality).

Digital line scans and color reflectance

Hole M0058A had the best recovery of all holes cored during Expedition 325. All cores were digitally scanned and, where appropriate, measured for color reflectance (Fig. F150). From 0 to 6.94 m CSF-A, the homogeneous green mud unit creates a slightly negative downhole trend in reflectance. Results are very consistent with depth until the bottom of this interval, where there is a larger data range owing to the presence of a fine to medium sand layer. An interval of fine to medium sand was measured from 8.75 to 10.23 m CSF-A and shows greater scatter in the reflectance data. Sections from 14.68 m CSF-A to total depth are mainly composed of a green mud unit showing a slight decrease in reflectance to 17.01 m CSF-A, followed by an increase in L* values to ~25.5 m CSF-A, where reflectance decreases again until it reaches a layer of medium fine sand at 28.85 m CSF-A (Fig. F150). This sand unit is easily detectable by higher scatter in the reflectance data set (28.85–31.22 m CSF-A). From 32.54 m CSF-A, another section of green mud unit gives more homogeneous reflectance values. A small sandy mud layer can be identified by the higher dispersion of the measurements, reaching peak reflectance at 35.75 m CSF-A. Another green mud unit from 35.48 to 38.28 m CSF-A shows a slight decrease in reflectance. Data scatter again shows the presence of sand at the bottom of the hole. Some trends observed in the L* data are also identifiable from a* or b* values because of a change in sediment color. A clear example of this correlation is the positive trend in a* from ~25.75 to 30 m CSF-A, which correlates with the medium fine sand layer at this depth. The values of the a*/b* ratio also show a similarity in trends with reflectance values, but the curves are smoother than for L*.

Thermal conductivity

A total of 29 measurements, one per section, were undertaken on full cores from Hole M0058A. Thermal conductivity values range from 0.967 to 1.237 W/(m·K). Variations in thermal conductivity with depth show no trend line (Fig. F151).

Data integration

Good agreement is observed between color reflectance, magnetic susceptibility data obtained with the MSCL, and the paleomagnetic discrete measurements (Fig. F152). These three measurements indicate the presence of potential cycles at the same depths, which are reflected in changes in the magnetic susceptibility or reflectance (L*) of the sediments. Hole M0058A is located at the fore reef slope; hence, changes in the coloration and magnetic susceptibility of the sediment might relate to variations in the contribution of calcium carbonate and terrigenous sediments from the adjacent shelf as sea levels oscillated over the last several glacial–interglacial timescales.

Paleomagnetism

Measurements of low-field or mass-specific magnetic susceptibility (χ) were performed on samples taken from the working half of the recovered core using both 1 cm³ and standard paleomagnetic box samples (Fig. F153). Positive magnetic susceptibilities occur throughout the core, ranging from 4.50 × 10^–8 to 277.78 × 10^–8 m³/kg, with an arithmetic mean value of 38.24 × 10^–8 m³/kg. Figure F153 shows two zones of high susceptibility with respect to the base values. The first zone is located between 8.63 and ~9.35 mbsf and the second between ~27 to 32.60 mbsf, with a maximum value of 277.78 × 10^–8 m³/kg, as mentioned above. These positive susceptibilities indicate the presence of ferromagnetic minerals.

Chronology

This hole has one radiocarbon date from Core 325-M0058A-4X that is older than the limit of the radiocarbon method (Fig. F154). Although this hole may have younger material in Cores 325-M0058A-1X through 4X, the rest of this hole is either older than 50 cal y BP or the material dated comprises reworked carbonate.

Top of page   |    Previous   |    Next