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doi:10.2204/iodp.proc.347.102.2015

Stratigraphic correlation

The objectives of Expedition 347 include examination of the sedimentary record at high resolution. Thus, recovery of a stratigraphically complete/composite geological record is important. Stratigraphic correlation consisted of the following:

1. Ensuring the maximum core recovery for each site by monitoring core overlap and identifying recovery gaps in each hole that must be targeted in an adjacent hole,
2. Seismic-core (sedimentary facies) correlation, and
3. Generating composite depth scales and splice records for each site to aid postcruise sampling at the OSP and postcruise research.

Ensuring optimal core recovery

Core recovery from a single hole is insufficient to generate a complete geological record because even with nominal 100% core recovery there are recovery gaps between adjacent cores. To obtain a complete sedimentary record, multiple adjacent holes were cored, offset in depth by 0.5–1.5 m between cores from different holes. The continuity of recovery was assessed by generating composite sections that aligned prominent features in physical property data from adjacent holes. With the data gained from the Fast-track MSCL (see “Physical properties”), it was possible to adjust the coring strategy before moving to a new hole. This ensured that intervals missing in previous cores could be recovered from an adjacent hole to optimize recovery, using an offset coring strategy with the offset amount advised by the stratigraphic correlators.

Seismic-core correlation

Correlation between seismic profiles and cores utilized a number of different aspects:

• Measured acoustic/sound velocity vs. sediment type, Acquired depth and stratigraphic observations tested by comparison with major core surfaces visible as peaks in the geophysical data,
• Downhole logs (mainly total gamma ray and velocity) (see “Downhole logging”), and
• MSCL logs (density and magnetic susceptibility) (see “Physical properties”).

Data integration required interpretation of sedimentary units and then correlation to observed physical property boundaries.

MSCL data provide indicators of lithologic variation. Bulk density in water-saturated sediments is related to porosity and is partially controlled by grain size. Acoustic velocity is controlled by porosity and density, and magnetic susceptibility may reflect changes in lithology—for example, the proportion of organic/biogenic components relative to lithogenic components. Downhole logging data (see “Downhole logging”) proved useful in filling core gaps due to limited recovery in unconsolidated sandy intervals or where spot-coring strategy was necessary.

Composite

Core depths are recorded in meters below seafloor from the upper part of the first core in each hole. Consecutive depth measurements were determined by the length of each core run. To align similar features, which record physical (geological) properties between different holes (or even different sites), MSCL physical property measurements were correlated to create a composite depth (mcd) scale. This was achieved by loading the MSCL data files from the Expedition DIS into the stratigraphic correlation software (Correlator v1.693; developed at Lamont-Doherty Earth Observatory) and shifting cores relative to each other using prominent signals as tie points. During this step, the total length on the meters composite depth scale was typically longer than the original meters below seafloor scale. With Correlator software, data sets from adjacent holes could be correlated simultaneously. A tie point, which gives the preferred correlation, is selected between data from different cores (e.g., from the first core in the first hole drilled and a core in a second hole). All the data from the second hole (that will be correlated with the chosen core) below the correlation point are vertically shifted to align the tie points between the holes. After choosing an appropriate tie point and adjusting the depths, the shifted section becomes the next “reference” section and a tie is made to a core from the first hole. Working downhole in an iterative fashion, each core is vertically shifted. Where there is no overlap, consecutive cores are appended. The tie points are recorded in an output (“affine”) table in units of meters composite depth. Confidence of correlation/composite was estimated for each tie point by the Stratigraphic Correlator using a value/number from 1 to 4:

• 1 = poor.
• 2 = fair.
• 3 = relatively good.
• 4 = good.

Fast-track MSCL measurements run immediately on cold cores at low resolution provided essential information on correlating the microbiology holes with the paleoenvironmental holes, as most of the cores/sections were later subsampled (whole rounds) for microbiology studies prior to being run through the normal MSCL slow track, reducing the number and length of core sections for detailed correlation and splicing.

Splicing

The next step in the stratigraphic correlation workflow was splicing. Splice records were generated by selecting sections from adjacent holes to avoid core gaps or disturbed sediment, resulting in a continuous (single) composite record. The splice is typically ~10%–15% longer relative to the original raw data, mainly because of expansion of sediment cores and the linear transformation of the composited features. The splice table defines tie points between core sections in meters composite depth. Confidence of splicing was also estimated using values from 1 to 4:

• 1 = poor.
• 2 = fair.
• 3 = relatively good.
• 4 = good.

The splicing formed the basis for onshore sediment sampling for postcruise research.

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