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doi:10.2204/iodp.proc.347.103.2015 Stratigraphic correlationMagnetic susceptibility, NGR, and gamma ray attenuation (GR) density data collected from Holes M0059A–M0059E were used to correlate between each hole and to construct a composite section for Site M0059 (Figs. F29, F30). The meters composite depth (mcd) scale for Site M0059 was based primarily on correlation of magnetic susceptibility between holes. NGR and GR density data provided confirmation of the correlation in some intervals and were used as constraints in other intervals where the magnetic susceptibility was low or lacked correlative features. All magnetic susceptibility data were cleaned for the top of each section, removing any outliers. The depth offsets (the affine table) that define the composite section for Site M0059 are given in Table T15. The magnetic susceptibility anomalies could be well correlated to ~87 mcd. The uppermost ~27 mcd, however, has some uncertainties, which were reevaluated using cyclic variations in the NGR data (Fig. F30). At ~27 mcd, there is a prominent spike in susceptibility data. However, this spike is not associated with a definitive lithologic change. The next prominent feature in the susceptibility data is at 47 mcd, reflecting a sequence boundary (see “Lithostratigraphy”). A general increase in susceptibility values at ~53 mcd is coincident with a lithologic change from silty clays to laminated clays. After constructing the composite depth section for Site M0059, a single spliced record from 0 mcd (top of Section 347-M0059D-1H-1) to 87 mcd (bottom of Core 347-M0059D-27H) was made from the aligned core intervals from all five holes (Table T16). If possible, cores that had been disturbed by the coring process (e.g., significantly compressed or expanded) were not used for the splice. Deeper than 87 mcd, Site M0059 relies on cores from only Hole M0059C. Therefore, Cores 347-M0059C-28S through 84S were appended to the record. Deeper than 87 mcd, coring gaps occur between each core, as well as larger sections with no recovery. No correction factor was applied to any of the data to allow for localized compression or expansion, so any offset within each core was equal at all points. Therefore, some features do not appear to align between holes. Because of this, the meters composite depth values will approximately, but not precisely, correspond to the same stratigraphic horizons in adjacent holes. To test the robustness of the spliced record, the NGR core data from Holes M0059A and M0059E were compared with the logging total spectral gamma ray (HSGR) data of Hole M0059E (Fig. F31). Because of its relatively low resolution (~20 cm or less), all NGR measurements in both holes were included and plotted on the meters below seafloor depth scale using a five-point moving average. Hole M0059A core data were used as the primary reference, as sediments from Hole M0059E were almost completely sampled for microbiology, resulting in reduced core lengths for later NGR measurement. HSGR data were plotted on the logging meters below seafloor scale, which was directly derived from the tool string. Characteristic trends within both records between 0 mbsf (top of the logging operations) and 57 mbsf correlate well to one another, suggesting that the spliced core data contains an almost continuous stratigraphic succession of the uppermost 57 mbsf. Seismic unitsSeismic sequence boundaries were identified by onlap, downlap, and erosional truncation of reflectors. Seismic sequence boundary-sediment core correlations are shown in Figure F32. 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 offshore and during the OSP (see “Physical properties”; Table T17). Cores and/or logs were examined at these calculated depths to define the extent of agreement between seismic boundaries and actual lithologic or log disconformable surfaces. Uncertainties in the time-depth function have resulted in minor inconsistencies between sedimentological observations from cores, seismic features, and the multisensor core logger, as well as downhole logs. Seismic Unit I
This relatively structureless seismic unit in the uppermost part of the seismic profile correlates to lithostratigraphic Subunit Ia. Seismic Unit II
The seismic profile shows relatively strong parallel structures in the lowermost part of this unit, suggesting a possible unconformity/erosional contact between seismic Units II and III. That horizon could be a silty sand unit described in “Lithostratigraphy”. Seismic Unit III
This seismic unit, which correlates to lithostratigraphic Unit III, shows a slightly irregular internal structures. Seismic Unit IV
This seismic unit, which correlates to lithostratigraphic Unit IV, shows a slight parallel structure in the seismic profile. Seismic Unit V
This seismic unit shows strong irregular internal structures. Seismic Unit VI
This seismic unit shows strong irregular internal structures but also some strong parallel reflectors. The very strong parallel reflector occurring in the middle of this unit might suggest a possible unconformity/erosional contact. Therefore, it might be possible to divide seismic Unit VI into subunits. Because of the very poor core recovery within this unit, it was difficult to make reliable seismic sequence boundary-sediment core correlations. The bedrock surface was found at 169.03 mbsf (two-way traveltime = 0.257 ms), at approximately the same depth at which the seismic data show a very strong parallel reflector. Overall, the sedimentary sequence was shorter than expected, most likely because of the thick organic-rich clay unit (~0–47 mcd) containing high concentrations of gas, which likely resulted in a slower attenuation of seismic waves. |
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