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doi:10.2204/iodp.proc.347.108.2015

Stratigraphic correlation

Four holes were drilled at Site M0064: Holes M0064A (41.5 mbsf), M0064B (10.2 mbsf), M0064C (45.10 mbsf), and M0064D (41.20 mbsf). The meters composite depth scale for Site M0064 was based on correlation of magnetic susceptibility and natural gamma between holes (Fig. F15). At this site, the offset and overlap between adjacent holes was monitored (using data from every other core) by measuring Fast-track magnetic susceptibility (see “Physical properties”) during the drilling process. These data proved to be an efficient tool to monitor and adjust the drilling process to maximize composite core recovery. Sediment cores were also logged with a standard MSCL to enable more precise hole-to-hole correlation and to construct a composite section for Site M0064 (Fig. F15). Before analysis, all magnetic susceptibility data were cleaned, removing any outliers from the measurements from the top of each section. The correlation was checked against scanned core slab images and lithologic descriptions. The depth offsets that define the composite section for Site M0064 are given in Table T6 (affine table).

Correlation between the magnetic susceptibility data in Holes M0064A, M0064B, M0064C, and M0064D is good, enabling the production of a continuous splice record for this site (Table T7). The accuracy of the correlation was visually checked from scanned core slab images using Corelyzer software. At Site M0064, correlation was straightforward to 11.54 mcd (Section 347-M0064C-3H-3, 41 cm). The lowermost part of Hole M0064C was appended in the splice record. No compression or expansion corrections were applied to the data, so the offset within each core was equal for all points. Thus, it is possible that some features are not similarly aligned between holes.

Seismic units

Seismic sequence boundary-sediment core-MSCL log (magnetic susceptibility) correlations are shown in Figure F16. Correlation is based on the integration of seismic data and lithostratigraphy (see “Lithostratigraphy”). Two-way traveltime values were calculated for each lithostratigraphic unit boundary using sound velocity values measured during the OSP (see “Physical properties”; Table T8). Lithostratigraphic unit boundaries were examined at these calculated two-way traveltime values to define the extent of agreement between seismic boundaries and actual lithologic and/or physical property disconformable surfaces. Uncertainties in the time-depth function and the effects of gas-saturated sediments could have resulted in inconsistencies between seismic features, sedimentological observations from cores, and MSCL logs.

Seismic Unit I

• Two-way traveltime: 0.0829 ms
• Lithology: very dark greenish gray diatom-bearing clay (lithostratigraphic Unit I)
• Depths: 0–0.33 mbsf (M0064A), 0–1.34 mbsf (M0064D)

Unit I corresponds to a thin, relatively transparent unit in the uppermost part of the seismic profile.

Seismic Unit II

• Two-way traveltime: 0.0835 ms
• Lithology: greenish gray to very dark greenish gray sandy silt
• Depths: 0.33–0.6 mbsf (M0064A), 1.34–1.75 mbsf (M0064D)

Unit II coincides with the uppermost strong reflector visible in the seismic profile. Magnetic susceptibility values are low in both Units I and II.

Seismic Unit III

• Two-way traveltime: 0.0938 ms
• Lithology: dark grayish brown laminated clay and silty clay with dispersed clasts (lithostratigraphic Subunits IIIa and IIIb)
• Depths: 0.6–8.53 mbsf (M0064A), 0–4.6 mbsf (M0064B), 0–7.29 mbsf (M0064C), 1.75–9.3 mbsf (M0064D)

Seismic Unit III shows a slightly irregular internal structure and a strong reflector at its base. Magnetic susceptibility values are relatively low and comparatively stable throughout this unit. However, they increase rapidly downcore at the lower boundary of Unit III.

Seismic Unit IV

Seismic Subunit IVa

• Two-way traveltime: 0.1116 ms
• Lithology: stratified muddy diamicton and sandy gravel (lithostratigraphic Subunit IVa)
• Depths: 8.53–26.5 mbsf (M0064A), 4.6–10.2 mbsf (M0064B), 7.29–25.7 mbsf (M0064C), 9.3–25.4 mbsf (M0064D)

Subunit IVa correlates relatively well with strong irregular reflectors visible in the seismic profile. Magnetic susceptibility values increase rapidly at the upper boundary of Subunit IVa and remain, with some variability, relatively high. The seismic profile shows strong parallel structures in the lowermost part of this unit, suggesting a possible unconformity or erosional contact between Subunits IVa and IVb.

Seismic Subunit IVb

• Two-way traveltime: 0.1241 ms
• Lithology: primarily massive diamicton, possible shear fabrics (lithostratigraphic Subunit IVb)
• Depths: 26.5–36.35 mbsf (M0064A), 25.7–36.7 mbsf (M0064C), 25.4–41.20 mbsf (M0064D)

Magnetic susceptibility values are slightly lower in Subunit IVb than in Subunit IVa. The seismic profile of Subunit IVb shows faint irregular structures that are slightly stronger and clearer than in the following subunit.

Seismic Subunit IVc

• Two-way traveltime: 0.1332 ms
• Lithology: stratified sandy clayey silt with dispersed clasts and diamicton (lithostratigraphic Subunit IVc)
• Depths: 36.35–41.5 mbsf (M0064A), 36.7–45.0 mbsf (M0064C)

This unit is followed by sedimentary bedrock based on the seismic profile.

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