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doi:10.2204/iodp.proc.323.102.2011

Stratigraphic correlation

It was necessary to recover complete stratigraphic sections to meet the scientific objectives of Expedition 323. Continuous sedimentary sections cannot be recovered from a single IODP borehole because core-recovery gaps occur between successive APC and XCB cores despite 100% or more nominal recovery (Ruddiman et al., 1987; Hagelberg et al., 1995). The construction of a complete section, referred to as a splice, requires the combination of stratigraphic intervals from two or more holes cored at the same site. To maximize the probability of bridging gaps between successive cores in individual holes, the starting depths below seafloor from which cores are recovered are offset between holes. This practice ensures that most missing sedimentary sections from inter-core gaps within a given hole are recovered in one or more adjacent holes. Usually at least two complete holes and a third partial hole must be cored to recover a complete section in the APC portion at a site. Additional complete holes are cored to allow for the construction of alternate splices, where possible.

The composite section and splice construction methodology employed during Expedition 323 follows the basic strategy originally developed during ODP Leg 138 (e.g., Hagelberg et al., 1992) and later refined during other legs. This strategy is now common practice on all high-resolution paleoceanographic expeditions. Our approach was modeled after that of ODP Leg 202, in which initial STMSL analyses of magnetic susceptibility and (new to Expedition 323) GRA bulk density were used to develop preliminary composite depths to inform drilling decisions; a more refined scale was developed as more detailed information became available (Mix, Tiedemann, Blum, et al., 2003).

The goals for stratigraphic correlation, in order of priority, were to

1. Guide drilling to ensure recovery of a complete stratigraphic section;

2. Establish a composite depth scale;

3. Define a stratigraphically complete and representative sampling splice and, if possible, one or more alternate splices;

4. Link core depths to logging depths at sites where logging was done; and

5. Evaluate and, if possible, refine shipboard stratigraphic age models by synthesizing all available age information, including the potential for tuning lithologic variations to reference records.

As a result of this stratigraphic correlation process, several depth models were created. Table T5 summarizes these depth models and their ODP equivalents. Detailed discussion of the definitions of these depth scales and the process by which they are created follows.

Core depth below seafloor (CSF-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 the recovered sediment. In IODP this depth scale is referred to as CSF-A, which is equivalent to the ODP scale mbsf. As a convention, recovered core is reported in mbsf throughout this volume.

The zero depth 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 recovers the true sediment/water interface. Some holes are purposely (or inadvertently) started 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 successfully recover 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 (i.e., the "catwalk"). A different length of section, referred to as "last observed length," is recorded as the section length in the LIMS, but the created lengths are used for calculating CSF-A depths. Another length measurement called "curated length" is available in the LIMS but was not used during Expedition 323.

Stratigraphic correlation relies on comparisons between holes and numerical adjustment of the depths of various data collected on several nonintrusive sensing tracks (detailed below). Depths assigned to individual data points on the various tracks are based on offsets from the top of each section end cap, as sensed by a laser indexer unique to each track, as well as relative movements of the core sections or sensors in each track system. Early in the cruise, some discrepancies in the offsets within sections for the various tracks were observed. The software for each track was adjusted to attempt to correct these discrepancies. When discrepancies occurred, the depth registration of WRMSL magnetic susceptibility was used as the reference depth when possible. This depth was chosen as the reference because WRMSL magnetic susceptibility was generally the data set with the highest signal-to-noise ratio and the measurements were relatively insensitive to issues of calibration drift. Errors in depth assignment of track data within sections are small—in most cases <1 cm. Errors between sections caused by the use of created lengths to determine core depth may be as large as several centimeters, especially deep in each core, where the sum of several section-created lengths is used in the depth calculation. In spite of these potential errors, the use of created lengths provides 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. Tidal influence on shot depth was first predicted during Leg 138 (Hagelberg et al., 1995) and was proven by correlation of affine offset changes and tide height during Leg 202 (Mix, Tiedemann, Blum, et al., 2003). In an attempt to remove tidal influence from the drilling depths, deep-ocean tidal predictions for all Expedition 323 drill sites were obtained from Dr. Gary Egbert at Oregon State University (pers. comm., 2009). Dr. Egbert provided output on sea-surface height from two different tidal prediction models to optimize drilling strategy. These site-specific tidal corrections were used throughout Expedition 323 to adjust drilling depths relative to the ship.

Core composite depth below seafloor (CCSF-A)

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

The CCSF-A scale is initiated by choosing the uppermost sediment in the first core (commonly referred to as the "mudline") in a single hole that best represents the sediment/water interface. At each site, this selected core becomes the "anchor" in the composite depth scale for all holes and is usually the only core for 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. For each core, depth offsets, or affine values, are chosen to best align observed lithologic variations among the equivalent cores in adjacent holes. An affine value specific to each core is added to the CSF-A depth in sequence downhole.

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

Data sources for depth scale construction

An initial composite depth scale was created during Expedition 323 using data from loop magnetic susceptibility and GRA bulk density from the ship's STMSL, based on recommendations from Leg 202 (Mix, Tiedemann, Blum, et al., 2003). This initial scale was developed during drilling for the primary purpose of guiding drilling in subsequent holes in an attempt to recover the full stratigraphic section.

The final CCSF-A scale and the splice for each site are based primarily on correlation of data from the JOIDES Resolution's WRMSL, which measures GRA bulk density (a function of grain density and porosity) and loop magnetic susceptibility. These WRMSL GRA bulk density and magnetic susceptibility data generally replaced the STMSL data; however, cores from holes dedicated to microbiology experiments were only analyzed on the STMSL, and these data were recorded and archived so that these special-purpose cores could be incorporated into the composite depth framework. Comparison of STMSL and WRMSL core logging data also led us to detect calibration drift in the sensor, which was corrected during the expedition whenever possible. The WRMSL also measures P-wave velocity and noncontact resistivity, but these data were not useful for correlation purposes, and to save time these sensors were used only intermittently during Expedition 323.

Correlations based primarily on GRA bulk density and magnetic susceptibility data were augmented by data from the NGRL and digital color reflectance parameters L*, a*, and b* measured on the SHMSL, which also records high-resolution reflectance spectra. This track also normally measures high-resolution point-source magnetic susceptibility (MSLP), but the MSLP system was not functioning during Expedition 323. High-resolution color digital images were analyzed on the SHIL, which provides color digital images that were incorporated, for the first time, in the correlation procedures during Expedition 323. These measurements are described in greater detail in "Physical properties." Core logging data were collected at 2.5, 5, or 10 cm intervals depending on time and core flow.

Composite depth scale construction

Raw stratigraphic data were imported into the beta-test versions of the specialized shipboard software program "Correlator" (versions 1.65, 1.652, 1.655, 1.656, and 1.657) and linked to digital core images with the program "Corelyzer" (versions 1.2.9.1, 1.3.2, and 1.3.3). Input data for these programs were downloaded from the ship's LIMS database using several versions of "Correlator Download." Upload of the resulting depth models to the LIMS was attempted with "AffineSpliceUploader," and management of various depth scales was attempted with "Depth Manager." These programs were sequentially modified to resolve problems that arose during Expedition 323, and they will be further modified before they become fully operational programs for IODP expeditions.

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 plotting results, data were masked to avoid incorporating anomalous data influenced by edge effects at section boundaries or at core tops or in voids where no sediment was present; however, all original data were retained in the LIMS database.

Depth intervals within cores are not squeezed or stretched by Correlator; thus, it is not possible to align all 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 the extent of lithification. In addition, sediment of unknown age occasionally falls from higher levels in the borehole onto the tops of cores as they are recovered, and as a result the top depths of some cores are unreliable. Gas expansion caused voids within some core sections. When this happened, holes were drilled into the liners and the sediment was manually pushed into the core liners to minimize the occurrence of these voids, which compromise track data. Where holes were drilled, pressurized sediment often extruded in wormlike streams. Although drilling holes in the core liner improves the continuity of data used for correlation in gassy sediments, it may also cause artificial distortion of depths within cores. Gas expansion also frequently extruded sediment out of the top of cores before the cores could be archived. The extruded material was recovered from the drilling rig floor and reassembled as Section 1 in many cores. Although great care was taken to assemble these sections in stratigraphic order, these assemblies are severely disturbed and may be missing some intervals. Where possible, the splice excludes these disturbed sections.

Correlation among cores from adjacent holes was evaluated visually and statistically (by cross-correlation). The depth offsets for each core that are needed to convert CSF-A to CCSF-A scales are recorded in an affine table for each site. The CCSF-A depth for any point within a core equals the CSF-A depth 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 numerical 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 logging data (e.g., Harris et al., 1995) can be done after the development of the composite section.

Ideally, the base of the continuous CCSF-A scale is the bottom of the deepest core recovered from the deepest hole. In practice, however, the base often occurs where core recovery gaps align across all holes, and the construction of a splice is impossible. Cores below this interval cannot be directly tied into the overlying "continuous" CCSF-A scale. However, below the base of the continuous section in CCSF-A units, cores from two or more holes can sometimes be correlated with each other, allowing the generation of a "floating" CCSF-A scale for intervals deeper than the continuous CCSF-A scale. In these cases, cores in the interval with no overlap were "appended" using the affine value of the overlying cores, and a floating splice was added below the appended interval. Because Correlator could not handle situations with multiple appended cores, these values were calculated manually. In one case (Site U1339), the observation that gaps between cores (the differential affine offset plus the drill pipe advance less core recovery) within the correlated section were relatively constant (i.e., the growth rate in the affine values as a function of drilling depth was linear) was used to assign cores below the splice an extrapolated affine value that yields an extrapolated CCSF-A scale with gaps. At other sites, long appended intervals use a constant affine value assigned to drilling depths.

The length of the CCSF-A scale at a given site is typically ~10%–20% 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, expansion is commonly attributed to rebound that follows the release of overburden in the deeper sections, stretching during the coring process, gas expansion during the core recovery process, and other factors (Moran, 1997).

Corrected core composite depth below seafloor (CCSF-B)

Scaled CCSF (CCSF-B) is a depth scale intended to correct the CCSF-A, CCSF-C, or CCSF-D scales for empirically observed expansion. CCSF-B can be produced by dividing various CCSF depth values by the average affine growth value of the CCSF-A scale relative to the CSF-A scale over a sufficiently long interval for which random variations in drill pipe advance caused by ship heave, tides, and other factors are negligible. This produces a complete stratigraphic sequence that is the same length as the total depth cored. The CCSF-B scale is assumed to be a close approximation of the actual drilled interval in the sediment column, although this is difficult to verify in intervals not covered by wireline logging. During Expedition 323, CCSF-B scales were not created, but affine growth factors were provided so that users could create them as needed.

Splice (CCSF-D)

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 splice does not generally contain coring gaps, and an effort was made to minimize inclusion of disturbed sections by examining core photographs. The shipboard splice is ideally suited to guide core sampling for detailed paleoceanographic studies. A splice table and a figure that summarize the tie points between intervals from each hole used to construct the splice are presented in each site chapter. The portion of the CCSF-A depth scale applied to the splice is referred to as 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 employed three rules. First, when possible, the top and bottom 50 cm of core, where disturbance from drilling artifacts (even if not apparent in core logging data) was most likely, were not used. Reassembled first sections of cores that were severely disturbed were also not used unless they were the only representations of a sedimentary interval. Second, attempts were made to incorporate the realizations of the stratigraphic section that were judged most representative of the holes recovered. Third, a minimum number of tie points was used (i.e., the longest possible sections within individual cores were used) to simplify sampling.

When possible, additional shipboard splices were constructed so that more than one copy of a complete stratigraphic section was available for high-resolution sampling. When this was done, multiple splices were given depth scales designated as CCSF-D. Note, however, that because of stretching and squeezing within cores, some features may not correlate precisely between the different splices. 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 CCSF-D scales, but these scales are slightly different than CCSF-D in the primary splice. To avoid confusion, alternate splices can be designated numerically (e.g., CCSF-D2, CCSF-D3, etc.) 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 (CCSF-C). Such scales are applied to individual holes and thus may be discontinuous. In the ODP program such a scale was referred to as revised meters composite depth (rmcd). Splice-correlated scales were not created during Expedition 323 because of software limitations.

Core-log integration (CCSF-L)

For sites that were logged, high-quality logging data allow for core-log integration in which cored intervals are squeezed or stretched and the zero depth point of the wireline log is linearly shifted up or down to maximize correlation between core logs and wireline logs. Core-log integration produces yet another depth scale. In ODP such depths were referred to as equivalent logging depths (eld). In IODP such a depth scale might be referred to as CCSF-C because it is a variant of the CCSF scale modified by correlation. However, this presents the potential for confusion between a CCSF-C scale developed to link all CCSF depths to the primary splice and a CCSF-C scale developed to correlate cores and logs. A new definition should be created specifically for depths modified to correlate with logging depths. This construct is tentatively referred to as CCSF-L.

The CCSF-L scale typically begins at ~100 mbsf, where the drill pipe is positioned during logging operations. When available, and when logging data are of sufficient quality, CCSF-L is the best estimate of in situ depth and so is ideal for calculating MARs and other products that require knowledge of absolute depths. Errors in wireline log depths can occur when ship heave is significant relative to heave compensation of the wireline.

A specialized module in Correlator is used to determine CCSF-L. Raw data are culled and smoothed as necessary to ensure that bad data are not included in the integration and that log and core data are compatible. Because core logging data are typically higher resolution than downhole logging data, it is necessary to smooth core logs before comparing them with downhole logs. Correlator allows correlation of individual cores within different holes with the data series recovered from logging. Core-log integration was attempted for all Expedition 323 holes in which downhole logging occurred. Unfortunately, although Correlator is capable of correlating cores and logs, it could not output the data correctly. Given time limitations, efforts at core-log integration were abandoned during the expedition pending corrections to the software. Determination of the CCSF-L depth scale will be accomplished during postcruise study when the problems with Correlator are resolved.

Depths in splice tables versus LIMS depths

Discrepancies in CSF-A and CCSF-A (CCSF-D) depths of as much as ±50 cm were found between the LIMS and Correlator. These discrepancies resulted from incorrect calculation on the Correlator-referred onboard splice table provided by the stratigraphic correlators, even though the onboard affine tables for each hole at the particular sites provided correct CCSF-A depths. After the new version of Correlator (version 1.66) was developed during postcruise study, output depths in the splice tables for all sites from Correlator were consistent with CSF-A depths in the LIMS. The shipboard splice tables include small differences (±5 cm) between calculated depth and measured depth that were derived from a calculation of interpolation of the tie points by using shipboard nondestructive measurements.

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