Previous   |    Next

doi:10.2204/iodp.proc.347.109.2015

Stratigraphic correlation

At Site M0065 three holes were drilled: Holes M0065A (73.9 mbsf), M0065B (49.3 mbsf), and M0065C (47.9 mbsf). The uppermost 2 m of sediment was open holed at each location because of the possibility of chemical contamination in surface sediments (see “Operations”). Hole M0065C was a microbiology hole, and core material was partly consumed by subsampling to 34 meters composite depth (mcd). Composite depths for Site M0065 were based on correlation of magnetic susceptibility (Fig. F22). The correlation was checked against scanned core slab images as well as sedimentology, and continuity of the composite record was checked against downhole log data. All major features from Hole M0065A were correlated to downhole log data (Fig. F23). Based on this, within the uppermost 47.5 mcd, correlation was possible within a 0.1 m error margin. The depth offsets that define the composite section for Site M0065 are given in Table T12 (affine table). No compression or expansion corrections were applied to the data.

It was possible to construct one continuous splice for Site M0065 to 47.5 mcd (Table T13). Deeper core material was sampled for optically stimulated luminescence but was not logged through the MSCL because of the hammer sampling coring method acquiring very little core material.

Seismic units

Seismic sequence boundary-sediment core-MSCL log (magnetic susceptibility) correlations are shown in Figure F24. Two-way traveltime values for each lithostratigraphic unit boundary were calculated using sound velocity values measured during the OSP (z-axis velocities, see “Physical properties”) (Table T14). Correlations are based on the integration of seismic data and lithostratigraphy (see “Lithostratigraphy”). Uncertainties in the time-depth function could have resulted in minor inconsistencies between seismic features, sedimentological observations from cores, and MSCL logs.

Seismic Unit I

• Two-way traveltime: 0.128 ms
• Lithology: organic-rich dark greenish clay with weak laminations (lithostratigraphic Unit I)
• Depth: 2.00–9.18 mbsf (M0065A), 3.00–9.63 mbsf (M0065B), 2.00–9.22 mbsf (M0065C)

Unit I corresponds to the seismic unit that is transparent and weakly stratified. In sediment cores, this unit displays low magnetic susceptibility values.

Seismic Unit II

• Two-way traveltime: 0.134 ms
• Lithology: laminated gray to dark gray clay (lithostratigraphic Unit II)
• Depth: 9.18–13.20 mbsf (M0065A), 9.63–13.85 mbsf (M0065B),

Unit II shows increasing magnetic susceptibility values downcore. A distinctive spike at 10 mbsf may indicate the presence of greigite (Fe₃S₄) magnetofossils, which have been reported from different various sites in the Baltic Sea (Reinholdsson et al., 2013). In the seismic profile, this spike corresponds to a well-defined reflector. This seismic unit is crudely stratified.

Seismic Unit III

Seismic Subunit IIIa

• Two-way traveltime: 0.168 ms
• Lithology: grayish brown laminated clay, massive to contorted (lithostratigraphic Subunit IIIa)
• Depth: 13.20–36.05 mbsf (M0065A), 13.85–36.10 mbsf (M0065B)

Seismic Subunit IIIa shows slight rhythmic variations in magnetic susceptibility values. It also shows somewhat irregular internal structures, which could be related to deformation of originally laminated clay.

Seismic Subunit IIIb

• Two-way traveltime: 0.169 ms
• Lithology: dark gray laminated clay (lithostratigraphic Subunit IIIb)
• Depth: 36.05–36.65 mbsf (M0065A), 36.10–36.80 mbsf (M0065B)

Seismic Subunit IIIb is difficult to correlate with a well-defined seismic unit. However, it approximately coincides with the strong reflector at the transition from seismic Unit III to seismic Unit IV.

Seismic Unit IV

• Two-way traveltime: 0.182 ms
• Lithology: laminated grayish brown silt and clay with sand content increasing downcore. The lowermost few meters consist of massive sand (lithostratigraphic Subunit IIIc)
• Depth: 36.65–46.60 mbsf (M0065A), 36.80–49.20 mbsf (M0065B)

Unit IV is characterized by high magnetic susceptibility values that are quite variable, possibly reflecting variations in sand and silt content within the laminated fine-grained unit. In the seismic profile, the upper boundary of Unit IV roughly corresponds to a very strong reflector. In the lower part of Unit IV, several nonparallel reflectors can be seen and the unit seems to continue further down.

Top of page   |    Previous   |    Next