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doi:10.2204/iodp.proc.347.105.2015

Stratigraphic correlation

At Site M0061, three holes were drilled: M0061A (25.2 mbsf), M0061B (28.7 mbsf), and M0061C (23.1 mbsf). The meters composite depth scale for Site M0061 is based on the correlation of magnetic susceptibility between holes (Fig. F21). Shipboard Fast-track magnetic susceptibility measurements obtained from every other core were used to determine the offset between adjacent holes (see “Physical properties”). These data provided an efficient tool to monitor and adjust the drilling process in order to maintain an adequate core overlap. All core material was also logged with a standard MSCL to enable more precise hole-to-hole correlation and to construct a composite section for Site M0061 (Fig. F21). Before analysis/correlation, all magnetic susceptibility data were cleaned to remove instrumental outliers. The depth offsets that define the composite section for Site M0061 are given in Table T11 (affine table).

Correlation between the susceptibility anomalies/data in Holes M0061A, M0061B, and M0061C is good, and it was possible to produce a continuous splice record for this site (Table T12). The splice was constructed mainly from Holes M0061A and M0061C. Hole M0061B was used in the splice to cover larger gaps at 8.5–10 and 14.2–15.3 mcd and in the lowermost part of the splice.

Accuracy of correlation was visually checked from scanned core slab images using the Corelyzer software. At Site M0061, correlation was straightforward to 25 mcd. The lowermost ~3 m of sand lacked distinctive MSCL data or sedimentological features to validate the correlation. A correction for either compression or expansion was not applied to the data, so offsets within each core were 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 F22. Correlations are 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 offshore and during the OSP (see “Physical properties”; Table T13). Lithostratigraphic unit boundaries were examined at the calculated two-way traveltime values to define the extent of agreement between seismic boundaries and actual lithologic transitions and physical properties disconformable surfaces. Uncertainties in the time-depth function may have resulted in minor inconsistencies between seismic features, sedimentological observations from cores, and MSCL logs.

Seismic Unit I

• Two-way traveltime: 0.123 ms
• Lithology: greenish gray to black clay (lithostratigraphic Unit I)
• Depths: Hole M0061A = 0–2.30 mbsf, Hole M0061B = 0–2.40 mbsf, Hole M0061C = 0–2.44 mbsf

The Unit I base matches the first clear boundary within a transparent and stratified seismic unit. It is also evident in physical properties, such as low magnetic susceptibility values, measured in sediment cores.

Seismic Unit II

• Two-way traveltime: 0.131 ms
• Lithology: dark greenish gray weakly varved clay with weak sulfide banding (lithostratigraphic Subunit IIa); iron sulfide-rich gray clay (lithostratigraphic Subunit IIb); and gray clay (lithostratigraphic Subunit IIc)
• Depths: Hole M0061A = 2.30–8.10 mbsf, Hole M0061B = 2.40–7.89 mbsf, Hole M0061C = 2.44–8.74 mbsf

Unit II shows increasing magnetic susceptibility values downcore and a distinctive spike related to an iron sulfide clay subunit at 6.46–6.90 mcd. In the seismic profile, the lower limit corresponds to a very strong reflector, which could be a basin-wide erosional surface. This strong reflector could possibly be connected to a ~10 cm thick sandy bed observed between Units II and III (see “Lithostratigraphy”).

Seismic Unit III

• Two-way traveltime: 0.140 ms
• Lithology: dark greenish gray clayey silt with a varvic rhythmic lamination (lithostratigraphic Unit III)
• Depths: Hole M0061A = 8.10–14.50 mbsf, Hole M0061B = 7.89–14.50 mbsf, Hole M0061C = 8.74–15.20 mbsf

Unit III shows increasing rhythmic variations in downcore magnetic susceptibility values. The lower boundary of seismic Unit III is characterized in the seismic profiles by closely spaced dark reflectors, possibly indicating gradual thickening of rhythmic laminae, and an increase in silt and sand content.

Seismic Unit IV

• Two-way traveltime: 0.159 ms
• Lithology: dark gray silty sand and fine sand with a few dispersed pebble-sized clasts (lithostratigraphic Unit IV)
• Depths: Hole M0061A = 14.50–25.15 mbsf, Hole M0061B = 14.50–28.70 mbsf, Hole M0061C = 15.20–23.10 mbsf

Unit IV is characterized in the seismic images by an upper laminated part and a lower homogeneous part. However, the cores did not reveal this lamination/bedding, as the core liners were in many cases partly filled with water, so original sedimentary structures could not be verified.

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