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doi:10.2204/iodp.proc.318.102.2011

Stratigraphic correlation and composite section

When coring multiple holes at Sites U1357, U1359, and U1361, stratigraphic correlation was carried out onboard to correlate adjacent holes using Correlator software (version 1.68). This software allows for the depth adjustment of individual or multiple cores to enable correlations to be made between cores in separate holes based on the patterns of shipboard core logging data.

Depth adjustment of individual cores is often necessary for a variety of reasons, including gaps between two adjacent cores, incomplete recovery, and differences in seafloor topography between holes or inconsistent mudline definition. Genuine stratigraphic inconsistencies also may occur between holes because of slight differences in sedimentation rates and erosion histories. Further complication of the correlation procedure may stem from a lack of unambiguous tie points between holes. Ambiguities may arise caused by genuine inconsistencies in the physical properties of contemporaneous strata, inaccuracies/imprecision of the physical property data, and drilling disturbances that affect the quality of the data.

Possession of two or more overlapping records from adjacent holes provides an opportunity for the construction of a spliced stratigraphic record against a common composite core depth below seafloor (CCSF; see below and Table T1). This spliced record has the potential to optimize both the completeness of the site’s stratigraphic record and the reliability of the associated stratigraphic data. For Sites U1357, U1359, and U1361 drilled during Expedition 318, the key physical property data that were utilized to aid correlation were NGR, magnetic susceptibility, and GRA bulk density. The recovery of a complete sediment section of APC-cored intervals was crucial to the paleoceanographic objectives of Sites U1357, U1359, and U1361. Drilling of parallel holes at these sites was planned to ensure that intervals missing from one APC hole as a result of recovery gaps between cores could be recovered in an adjacent hole. A composite depth section has been developed for these multiply cored sites to confirm continuity of recovery.

Adjustments to the shipboard core depth below seafloor, method A (CSF-A; see below and Table T1), depth scale are required for several reasons (e.g., Ruddiman et al., 1987; Hagelberg et al., 1992). Elastic rebound and gas expansion of the sediment following core recovery causes the cored sediment sequence to be expanded relative to the cored interval. As a result, the composite depth scale increases downhole relative to the CSF-A scale, typically on the order of 10%. In addition, the ship’s motion, which is affected by tides and heave, can influence the in situ depth at which a core is cut. Portions of the sediment sequence are usually missing, even between cores that have >100% recovery.

A composite depth scale places coeval, laterally continuous stratigraphic features into a common frame of reference by shifting the CSF-A depth scales of individual cores to maximize the correlation between holes. The individual cores are shifted vertically without permitting expansion or contraction of the relative depth scale within any individual core. A horizontal feature recovered from several adjacent holes will have approximately the same core composite depth below seafloor, appended (CCSF-A), which is equivalent to the ODP meters composite depth scale. However, it will most likely have a different CSF-A scale, which is equivalent to the ODP meters below seafloor depth scale. Horizontal features will have exactly the same CCSF-A in two holes at the correlation tie points. Most other intervals cannot be precisely correlated (offsets of a few millimeters to several centimeters) because of differential stretching and squeezing within cores. APC recovery gaps between cores in one hole are typically from 0.5 to 2 m and rarely exceed 5 m. After establishing a CCSF-A scale, a complete stratigraphic record is spliced from the data of the multiple holes.

The methods used during Expedition 318 to construct composite depth and spliced sections were similar to those used during ODP Leg 138 (Hagelberg et al., 1992) and numerous subsequent paleoceanographic ODP legs and IODP expeditions. For each site, the hole to hole correlation was based on 2.5 or 5 cm spaced physical property data (see “Physical properties”).

Correlations were carried out visually by selecting a tie point from primarily the WRMSL data in one hole and comparing it directly with the same point measured from the length of the drill string advance, on a core by core basis. The core that has the most pristine record of the upper portion of the upper few meters of the sedimentary record, particularly the mudline, was chosen as the first (anchor) point of the composite section. The CCSF-A of the first core is thus the same as its CSF-A depth. A tie point that gave the preferred correlation was selected between data from this core and a core in an adjacent hole. All data from the second hole below the correlation point were vertically shifted to align the tie points horizontally between the holes. Once the depth adjustment was made, the shifted section became the reference section and a tie was made to a core from another hole. The process continued downhole, vertically shifting the data from one core at a time relative to data from the other hole. The tie points were added to the “affine” table, which records all of the depth adjustments (“offsets”) that define the composite depth scale in CCSF-A. The composite depth scale is presented in tabular form in the “Stratigraphic correlation and composite depth” section of the chapters for the multiply cored sites (see the “Site U1357,” “Site U1359,” and “Site U1361” chapters). For each core, the depth adjustment required to convert from the CSF-A scale to the CCSF-A scale is given as the cumulative depth offset added to the IODP curatorial subbottom depth (in CSF-A).

During the composite section construction procedure, a spliced record was assembled that provides a single representative sedimentary record suited for postcruise sampling and studies. Splice ties were established as close to the composite depth tie points as the Correlator program allowed. Intervals were chosen for the splice so that section continuity was maintained and disturbed intervals were avoided. Further adjustments to the composite depth section by detailed correlation that includes expanding and compressing the depth scale within individual core intervals are required to align all features exactly.

This practice ensures that missing sedimentary sections from most intercore gaps within a given hole are recovered in one or more adjacent holes. At least two complete holes and a third partial hole are usually necessary to recover a complete section in the APC portion of a site. Additional complete holes are cored to allow options for construction of alternate splices, where possible.

Our goals for stratigraphic correlation, in priority order are

• To guide drilling to ensure recovery of a complete stratigraphic section,

• To establish a composite depth scale,

• To define a stratigraphically complete and representative sampling splice (and if possible one or more alternate splices), and

• To evaluate and (if possible) refine shipboard stratigraphic age models by synthesizing all available age information, including the potential for tuning of lithologic variations to reference records.

As a result of this stratigraphic correlation process, several different depth models are created. The following section provides detailed discussion of the definitions of these depth scales and the process by which they were created (see also “Core recovery and depths” and Table T1).

Depth scales

Core depth below seafloor, method A

Initial depths for the top and bottom of each core are assigned during drilling, based on the length of the drill string below the ship’s drilling rig floor and the length of recovered sediment. In IODP this is referred to as CSF-A, which is equivalent to the ODP meters below seafloor depth scale. The zero point on the CSF-A scale is defined by the mudline in the first core of each hole. It is often difficult to tell whether this empirical mudline recovered the true sediment/water interface. Some holes are started from below the sediment water interface. In this case the zero depth point in CSF-A units may be substantially offset from the zero depth point in adjacent holes that recovered the sediment/water interface.

Within each core, CSF-A depths are calculated based on the drilling depth to the top of the core, and by adding the depth offset of each data point within a section to the sum of the overlying sections. “Created lengths,” which are the lengths of the core liner sections measured with a ruler on the core-receiving platform (“catwalk”), are used for calculating CSF-A depths. Errors between sections because of the use of created lengths to determine depth in a given core may be as much as several centimeters, especially lower in each core where the sum of several section created lengths is used for the depth calculation. In spite of these potential errors, the use of created lengths provides for consistency in the database (i.e., the depths are precise, even if not completely accurate).

The CSF-A scale is also inaccurate because of ship heave (which is not compensated for in APC coring), tidal variations in sea level, and other sources of error.

Core composite depth scale, appended

Before a splice can be constructed, the cores from the various holes must be stratigraphically correlated with each other. Such correlation transfers initial estimates of depths of drilling (CSF-A) into a composite depth scale referred to as CCSF-A. This IODP depth scale is approximately equivalent to the ODP meters composite depth scale.

At each site, the first selected core becomes the “anchor point” in the composite depth scale for all the holes and is usually the only core in which depths are the same on both the CSF-A and CCSF-A scales. From this anchor, core-logging data are correlated among holes downsection. An affine value specific to each core is added to the CSF-A in sequence down the holes.

It is not possible to align all features perfectly between holes, so where possible the affine values are chosen to maximize correlations at the levels that splice tie points are defined between holes (see below).

Composite depth scale construction

The raw stratigraphic data were imported into the shipboard Correlator software program. Input data for this program were downloaded from the ship’s database (LIMS) using the Web-based LIMS to Correlator tool, but we also directly uploaded comma-delimited Microsoft Excel files. Upload of the resulting depth models to LIMS was attempted with the AffineSpliceUploader program.

Correlator enables the graphical construction of a composite depth scale for each hole at a given site by depth-shifting individual cores to maximize the correlation of core logging data among multiple holes. For hole to hole correlations and for plotting results, data were filtered to avoid incorporating anomalous data influenced by edge effects at section boundaries or core tops or in voids where no sediment was present; however, all original data were retained in LIMS.

Depth intervals within cores are not squeezed or stretched by Correlator; therefore, it is not possible to align all the correlative features within each core. Stretching or squeezing between cores from different holes may reflect small-scale differences in sedimentation and/or distortion caused by the coring and archiving processes. For example, the tops of APC cores are often stretched and the bottoms compressed, although this depends on lithology and extent of lithification.

Correlations among cores from adjacent holes are evaluated visually and statistically (by cross-correlation). The depth offsets for each core that are necessary to convert CSF-A to CCSF-A are recorded in an affine table for each site. The CCSF-A for any point within a core equals the CSF-A plus the affine offset. Correlation at finer resolution is not possible with Correlator because depth adjustments are applied linearly to individual cores; at this stage of depth scale development, no adjustments in the length of each core, such as numerically squeezing and stretching, are made within cores. Such finer scale adjustment of individual cores relative to the splice (e.g., Hagelberg et al., 1995; Pälike et al., 2005) or relative to logging data (e.g., Harris et al., 1995) can be done following the development of the composite section.

The length of the CCSF-A scale at a given site is typically ~10% greater than the length of the cored section in any one hole, as indicated by the CSF-A scale. Although the exact reasons for this apparent expansion of the sediment column are unknown, it is commonly attributed to rebound following release of overburden in the deeper sections, stretching during the coring process, gas expansion during the core recovery process, and other factors (Moran, 1997).

Splice

The splice is a composite core section representing the best available representation of a complete stratigraphic column at a site. It is composed of core sections from adjacent holes such that coring gaps in one hole are filled with core from an adjacent hole. The objective of the splice is to fill coring gaps and an effort has been made to minimize inclusion of disturbed sections by using the core disturbance classification made by the sedimentologists. The shipboard splice is ideally suited to guide core sampling for detailed paleoceanographic studies. A splice table and figure are presented in the site chapters that summarize the tie points between intervals from each hole used to construct the splice. The portion of the CCSF-A scale that is applied to the splice is referred to as core composite depth below seafloor, method D (CCSF-D). Within the splice sections, CCSF-D is identical to CCSF-A.

The choice of splice tie points is somewhat subjective. The method used in the construction of a splice followed three rules. First, where possible we avoided using the top and bottom 50 cm of cores, where disturbance caused by drilling artifacts (even if not apparent in core logging data) was most likely. We also avoided reassembled first sections of cores that were severely disturbed unless it was the only representation of a sedimentary interval. Second, we attempted to incorporate those realizations of the stratigraphic section that in our judgment were most representative of the holes recovered. Third, we tried to minimize tie points (i.e., to use the longest possible sections within individual cores) in order to simplify sampling.

Where possible, additional shipboard splices may be constructed, so that more than one copy of a complete stratigraphic section is available for high-resolution sampling. Where this is done, multiple splices would all have depth scales designated as CCSF-D. Therefore, the final composite depth scale, CCSF-D, is only formally defined in the primary shipboard splice. Composite depth scales for any alternate splices are also assigned CCSF-D scales, but this scale is slightly different than CCSF-D in the primary splice. To avoid confusion, alternative splices can be designated numerically, CCSF-D2, D3, and so on, depending on the number of additional splices. A solution to the problem of comparing different splices is to create an additional depth scale that numerically stretches and squeezes features into a splice-correlated scale (core composite depth below seafloor, method C). Such scales are applied to individual holes and thus may be discontinuous. In ODP, such a scale was referred to as “revised meters composite depth.” During Expedition 318, we did not create splice-correlated scales because of software limitations.

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