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doi:10.2204/iodp.proc.347.107.2015

Stratigraphic correlation

Five holes were drilled at Site M0063: Holes M0063A (115.81 mbsf), M0063B (29.00 mbsf), M0063C (96.4 mbsf), M0063D (86.8 mbsf), and M0063E (92.8 mbsf). Hole M0063E was a microbiology hole and was sampled intensively down to ~70 mbsf. The meters composite depth (mcd) scale for Site M0063 was based on correlation of magnetic susceptibility and natural gamma between holes. Cored material was heavily expanding to a depth of ~60 mbsf, resulting in loss of sediment in Hole M0063A. Holes M0063B, M0063C, M0063D, and M0063E were drilled with a modified strategy (see “Operations”), giving core material more space to expand inside the core liner. This approach minimized sediment loss but also resulted in disturbances at the top of each core. In MSCL data, the disturbed core tops were seen as strong noise; this could later be verified from split cores (Fig. F27).

Because of expansion, the original meters below seafloor scale was partially overlapping between cores (Fig. F27), with the base of one core appearing to sit below the top of the core beneath it. Before correlation, all magnetic susceptibility data were cleaned for the top 10–80 cm of each section, removing any measured outliers. In Hole M0063E, a heavily sampled microbiology hole, only Fast-track magnetic susceptibility data were available from the whole length, in addition to natural gamma and normal MSCL measurements from unsampled core parts. The correlation was checked against scanned core slab images and sedimentological interpretations. Some prominent sedimentological features were difficult to align, so in these cases correlation was based more on MSCL and natural gamma data. Within the uppermost 30 mbsf, correlation was possible within 0.5 m error margin. From 30 to 60 mbsf, it was possible to achieve 1–0.5 m accuracy and from 60 to 90 mbsf 0.5 m accuracy. The depth offsets that define the composite section for Site M0063 are given in Table T14 (affine table).

This was the first site where compression of the original meters below seafloor scale in ratio to sediment expansion was tested, according to the following equation:

where the real length of core is equal to the length drilled and T_depth = total depth. This adjustment did not appear to significantly improve accuracy, probably because expansion was not linear throughout the hole. Comparison of downhole log gamma and natural gamma from Hole M0063A (Fig. F28) indicates that although the upper part of individual cores were disturbed, there did not seem to be large gaps in core recovery.

The uppermost 20 mbsf was somewhat disturbed. Between ~22 and 42 mbsf, expansion/sediment loss was at its greatest (30%–40%). Another possibly disturbed interval started around 84 mbsf and extended down to ~100 mbsf. Because of the potential for relatively large error margins due to nonlinear expansion, no splice was constructed at Site M0063.

No correction was therefore made for expansion or compression to the data, so the offset within each core was equal for all points. Therefore, 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 F29. 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 T15). Lithostratigraphic units/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 properties disconformable surfaces. Uncertainties in the time-depth function and effects of gas-saturated sediments could have resulted in moderate inconsistencies between seismic features, sedimentological observations from the cores, and the MSCL logs.

Seismic Unit I

• Two-way traveltime: 631 ms
• Lithology: organic-rich clays (lithostratigraphic Subunits Ia–Id)
• Depth: 0–25 mbsf

The lithostratigraphic Unit I upper surface (two-way traveltime = 611 ms) does not coincide with the seismic profile. This could be due to small temperature (and salinity) differences in the water column; the calculation was based on the assumption that the water temperature is +4°C. The lower boundary of Unit I seems to coincide with a more clearly defined reflector within a transparent, homogeneous unit. Therefore, the low velocity value (1200 m/s) for the uppermost organic-rich clay seems to be valid.

Seismic Unit II

• Two-way traveltime: 644 ms
• Lithology: gray clay, upper part iron sulfide laminations and massive lower part (Subunits IIa and IIb)
• Depth: 25–34 mbsf

Seismic Unit III

• Two-way traveltime: 654 ms
• Lithology: mixed clay sediments with contorted lamina (lithostratigraphic Subunits IIIa and IIIb)
• Depth: 34–41 mbsf

Seismic Unit IV

• Two-way traveltime: 663 ms
• Lithology: iron sulfide laminated gray clay which changes gradually into varved clays (lithostratigraphic Subunits IVa and IVb)
• Depth: 41–48 mbsf

Seismic Unit V

• Two-way traveltime: 670 ms
• Lithology: contorted convolute bedded clay (lithostratigraphic Unit V)
• Depth: 48–53 mbsf

Seismic Unit VI

• Two-way traveltime: 725 ms
• Lithology: varved clay downward-increasing silt and sand content and with dispersed pebbles (lithostratigraphic Unit VI)
• Depth: 53–92 mbsf

The Unit VI lower boundary coincides with a boundary between different seismic units. In places, seismic Unit VI has a layered/parallel inner structure; in other places it seems more homogeneous.

Seismic Unit VII

• Lithology: sandy diamict
• Depth: 92–93 mbsf

For Unit VII there is no information about the lower boundary. Based on seismic profiles, it seems that unit contains silt and sand material.

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