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doi:10.2204/iodp.proc.347.106.2015 Stratigraphic correlationAt Site M0062 four holes were drilled: M0062A (35.9 mbsf), M0062B (24.1 mbsf), M0062C (3.3 mbsf), and M0062D (21.0 mbsf). The meters composite depth (mcd) scale for Site M0062 is based on correlation of magnetic susceptibility between holes (Fig. F18). In a similar methodology to that used at previous sites, when on location the offset between adjacent holes was monitored by measuring Fast-track magnetic susceptibility (see “Physical properties”) from every other core. These data proved to be an efficient tool to monitor and adjust the drilling process in order to maximize core overlap and composite core recovery. Sediment cores were also logged with a standard multisensor core logger (MSCL) to enable more precise hole-to-hole correlation and to construct a composite section for Site M0062 (Fig. F18). Before analysis and correlation, all magnetic susceptibility data were cleaned for the top of each section, removing any outliers from the measurements. The depth offsets that define the composite section for Site M0062 are given in Table T7 (affine table). Correlation between the susceptibility anomalies/data in Holes M0062A, M0062B, M0062C, and M0062D is good and enabled us to produce a continuous splice record for this site (Table T8). The splice was constructed mainly from Holes M0062A and M0062D. Hole M0062C was used in the splice to cover the uppermost part of the sediment column as Holes M0062A and M0062B were open holed in this interval. The accuracy of the correlation was visually checked using scanned core slab images in Corelyzer software. At Site M0062, correlation was straightforward to 23.56 mcd (Section 347-M0062A-8H-3, 12.8 cm). The lowermost part of Hole M0062A (interval 347-M0062A-9H-1, 7 cm, to 13H-1, 8 cm) was appended in the splice record. No compression or expansion corrections were applied to the data, so offsets within each core are equal for all points. Thus, it is possible that some features are not similarly aligned between holes. Seismic unitsSeismic sequence boundary-sediment core-MSCL log (magnetic susceptibility) correlations are shown in Figure F19. Correlation is based on the integration of seismic/acoustic 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 T9). 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 could have resulted in minor inconsistencies between seismic features, sedimentological observations from the cores, and the MSCL logs. Seismic Unit I
Unit I ends at the strong reflector below this stratified seismic unit. Two acoustic subunits can be distinguished (namely acoustic Subunits AUIa and AUIb). In the seismic profile, the upper subunit (AUIa) (0 to ~5 mbsf) is characterized by high-amplitude parallel reflectors and the lower subunit (AUIb) (~5–10.72 mbsf) by low-amplitude parallel reflectors. In Subunit AUIa, magnetic susceptibility values measured in sediment cores (see “Physical properties”) stay at similar levels. In Subunit AUIb, magnetic susceptibility values are more variable, peaking in the lowermost part of this subunit. Seismic Unit II
The top boundary of Unit II is a strong reflector, which could possibly represent a basin-wide erosional surface, probably related to the erosional surface documented at Site M0061. As in Unit I, two acoustic subunits (AUIIa and AUIIb) can be distinguished. The upper subunit (AUIIa) (10.72 to ~13 mbsf) is acoustically relatively structureless, whereas the lower subunit (AUIIb) (~13–16.17 mbsf) is characterized by low-amplitude parallel reflectors. The boundary between these subunits is slightly irregular and probably erosional. Magnetic susceptibility values of this unit increase downcore. The lower boundary of Unit II is characterized by closely spaced dark reflectors, possibly indicating increases in silt and sand content. Seismic Unit III
Seismic Unit III is in acoustic images characterized by parallel reflectors in an upper part and a lower homogeneous part, and it could potentially be a continuation of the lower acoustic subunit (AUIIb) of Unit II. The acoustic pulse does not penetrate deep into this unit. |
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