Previous   |   Next

doi:10.2204/iodp.proc.320321.102.2010

Stratigraphic correlation and composite section

The scientific objectives of Expedition 320/321 required the recovery of complete stratigraphic sections, yet it has been demonstrated that a continuous section is rarely recovered from a single borehole because of core recovery gaps between successive APC and XCB cores despite 100% or more nominal recovery and other offsets (tides, core expansion, and deformation) (e.g., Ruddiman et al., 1987; Hagelberg et al., 1995; Lisiecki and Herbert, 2007). Construction of a complete stratigraphic section, referred to as a composite splice, requires combining stratigraphic intervals from two or more holes cored at the same site. To maximize the probability of bridging core recovery gaps in successive holes, the depths below the seafloor from which cores are recovered are offset between the holes. This practice ensures that most between-core intervals missing within a given hole are recovered in at least one of the adjacent holes. During Expedition 320/321, at least two holes were cored at all sites and used to construct composite sections.

Our composite sections and splice construction methodology follows one which has been successfully employed during a number of previous ODP legs and IODP expeditions (e.g., Hagelberg et al., 1992; Curry, Shackleton, Richter, et al., 1995; Jansen, Raymo, Blum, et al., 1996; Lyle, Koizumi, Richter, et al., 1997; Wefer, Berger, Richter, et al., 1998; Wang, Prell, Blum, et al., 2000; Lyle, Wilson, Janecek, et al., 2002; Mix, Tiedemann, Blum, et al., 2003; Zachos, Kroon, Blum, et al., 2004; Channell, Kanamatsu, Sato, et al., 2006). Assembly and verification of a complete composite stratigraphic section requires construction of a composite depth scale. Other depth scales (discussed below) can be implemented using downhole logging data and/or combining splicing and drill pipe depths.

Once a composite depth scale has been completed, a stratigraphically continuous and complete composite splice can be created from representative intervals from the multiple holes. Ideally, both core recovery gaps and intervals with coring deformation are absent in the spliced section. The continuity and completeness of the spliced section are dependent on avoiding coeval core recovery gaps in multiple holes, which can be done through real-time adjustments to the depths at which a core is collected, as discussed below.

Core composite depth scale

The goal of constructing a composite depth scale is to place coeval, laterally continuous stratigraphic features into a common frame of reference by depth shifting the CSF depth scales (see Table T1) of individual cores to maximize correlation between holes. In the composite depth scale used by IODP, referred to as the core composite depth below seafloor (CCSF-A) the depths of the individual cores can only be shifted by a constant amount without permitting expansion or contraction of the relative depth scale within any core (Fig. F14). Ultimately, this provides good first-order correlation between cores from different holes while also avoiding more subjective, and potentially erroneous, interpretations that might arise without applying this restriction first. The CCSF-A scale, once established, provides a basis upon which higher order depth composite scales can be built. The CCSF-A depth scale also allows a direct translation between lengths as measured in cores that are ordered in the splice and the new depth scale.

In essence, the CCSF-A scale overcomes many of the inadequacies of the CSF depth scale, which is based on drill pipe measurements and is unique to each hole. The CSF depth scale is based on the length that the drill string is advanced on a core by core basis and is often inaccurate because of ship heave (which is not compensated for in APC coring), tidal variations in sea level, and other sources of error. In contrast, the CCSF-A scale is built by assuming that the uppermost sediment (mudline) in the first core from a given hole is the sediment/water interface. This core becomes the "anchor" in the composite depth scale and is typically the only one in which depths are the same on both the CSF and CCSF-A scales. From this anchor, physical property core logging data (e.g., bulk density and magnetic susceptibility) are correlated among holes downsection. For each core, a depth offset (a constant) that best aligns the physical property variations to the equivalent cores in adjacent holes is added to the CSF depth in sequence down the holes. Depth offsets are often chosen to optimize correlation of specific features that define splice levels in cores from adjacent holes.

For Expedition 320/321, the CCSF-A scale and the splice are based on the stratigraphic correlation of data from the IODP STMSL core track, WRMSL, SHMSL, and long-core magnetometer (STMSL data were only used for real-time continuity assessment and are not presented in this report). For each of these track instruments, data were collected every 2.5 or 5 cm. We used magnetic susceptibility (Fig. F14), GRA bulk density, NGR, reflectance (L*, a*, and b*), and the intensity of magnetization following 20 mT AF demagnetization. All of these measurements are described in "Physical properties" and "Paleomagnetism." Other data were used as appropriate (see "Biostratigraphy" and "Lithostratigraphy").

The raw stratigraphic data were imported into Correlator (versions 1.43 for Expedition 320 and 1.61 for Expedition 321) and culled as necessary to avoid incorporating anomalous data influenced by edge effects at section boundaries and by coring disturbance. Correlator was used to assess the stratigraphic continuity of the recovered sedimentary sequences at each drill site and to construct the CCSF-A scale and composite splice.

Because depth intervals within cores are not squeezed or stretched by Correlator, all correlative features cannot be aligned. 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. The tops of APC cores are generally stretched and the bottoms are compressed, although this effect is lithology dependent. In addition, sediment (especially unconsolidated mud, ash, sand, and gravel) occasionally falls from higher levels in the borehole onto the tops of cores as they are recovered, and as a result the top 0–100 cm of many cores are not part of the stratigraphically contiguous section.

Correlations among cores from adjacent holes are evaluated visually and statistically by cross-correlation within a 2 m depth interval, which can be adjusted in length when appropriate. Depth-shifted data are denoted by CCSF-A. A table is presented in each site chapter that summarizes the depth offsets for each core. These tables are necessary for converting CSF to CCSF-A scales. The CCSF-A depth for any point within a core equals the CSF depth plus the cumulative offset. Correlation at finer resolution is not possible with Correlator because depth adjustments are applied linearly to individual cores; no adjustments, such as squeezing and stretching, are made within cores. Such fine-scale adjustment is possible postcruise (e.g., Hagelberg et al., 1995; Pälike et al., 2005).

Splicing

Once all cores have been depth shifted and stratigraphically aligned, a composite section is built by splicing segments together from multiple holes to form a complete record at a site (Fig. F14). The record is composed of core sections from adjacent holes so that coring gaps in one hole are filled with core intervals from an adjacent hole. The splice should contain no coring gaps, and an effort has been made to minimize inclusion of disturbed sections. The shipboard splice is ideally suited to guide core sampling for detailed paleoceanographic studies. A table and a figure presented in each site chapter summarize the intervals from each hole used to construct the splice. Additional splices may be constructed postcruise as needed.

The choice of tie points (and hence of a splice) is partly a subjective exercise. Our method in the construction of a splice followed three rules. First, where possible we avoided using the top and bottom ~0.5 m of cores, where disturbance resulting from drilling artifacts (even if not apparent in core logging data) is most likely. Second, we attempted to incorporate those portions of the recovered core that were most representative of the overall stratigraphic section of the site. Third, we tried to minimize the number of tie points to simplify postcruise sampling.

The length of the spliced section (on the CCSF-A scale) at a given site is typically ~5%–15% greater than the length of the cored section in any one hole as indicated by the CSF scale (Fig. F15). This increase is commonly attributed to sediment expansion resulting from elastic rebound, stretching during the coring process, gas expansion during the core recovery process, and curation practices, in which soupy core material commonly occurring at the top of many cores is curated as part of the core (e.g., Moran, 1997; Acton et al., 2001; Lisiecki and Herbert, 2007). In reality, much of the soupy material results from sediment falling into the hole or from sediment being stirred at the bottom of the hole as the BHA is advanced.

Ideally, the base of the 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 or the data quality does not allow reliable correlations between holes. Cores below this interval cannot be directly tied into the overlying and continuous CCSF-A scale. However, below the base of the continuous CCSF-A scale, cores from two or more holes can sometimes be correlated with each other to create a floating splice. In this case a CCSF-A depth was assigned to the section below the splice by adding the greatest cumulative offset to the first core below the splice and beginning the floating splice from that point in the section.

Corrected core composite depth scale

To correct the CCSF-A scale for empirically observed core expansion, a growth factor is calculated by fitting a line to CSF versus CCSF-A depth. By dividing the CCSF-A scale by the growth factor a corrected core composite depth scale (CCSF-B) can be evaluated that is a close approximation of the actual drilling depth scale. It should be noted, however, that the actual growth factor is not linear and varies with lithology.

Special Task Multisensor Core Logger

To permit rapid stratigraphic correlation for real-time drilling adjustments, the STMSL was used. The STMSL is a simple track system with a magnetic susceptibility loop and a GRA sensor (see "Physical properties"). The GRA sensor failed after use at Site U1331 during Expedition 320, was repaired during the Honolulu port call, and used again during Expedition 321. To avoid interference with the susceptibility loop installed on the WRMSL, the STMSL operates at a different frequency (513 versus 621 Hz). It was developed to permit rapid measurement of susceptibility and GRA on whole-core sections as soon as possible following recovery. The concept was first attempted during ODP Leg 202 using a similar instrument from Oregon State University (Mix, Tiedemann, Blum, et al., 2003).

We ran the track for all sections measured using a sampling interval of 5 cm on all cores from the second and third hole at each site. Three measurements were made at each interval using SI units and the short (~1 s) measurement setting on the Bartington susceptibility meter. Using these settings, we could log a typical 9.5 m long core in ~30 min.

STMSL susceptibility is used in Correlator for preliminary stratigraphic correlation with the goal of determining if between-core gaps in one hole correspond with those in a hole that is being cored. If so, drilling adjustments can be made to ensure that coeval coring gaps are avoided in subsequent cores. These preliminary susceptibility and GRA data sets are then superseded by the same measurements collected on the WRMSL, which is done after the cores have equilibrated to room temperature and generally at higher resolution than on the STMSL. Likewise, the preliminary stratigraphic correlation is superseded by more careful evaluation following the collection of a suite of physical property data and core descriptions.

Depths in splice tables versus LIMS depths

The depth of a core interval recorded for a tie point in a splice table is not always the same as the depth for the same core interval returned by most database queries. This is because the tie point depth is based on the liner length, measured when the cores are cut into sections on the catwalk. The cores are analyzed on the STMSL almost immediately after this liner length measurement. At some later time, typically 10–36 h after being analyzed by the WRMSL, core sections are split and analyzed further (see "Core handling and analysis"). At this time, section lengths are measured again and archived as "curated lengths." General database queries return depths based on curated liner lengths. Because the sections may have expanded during the period between the two measurements or shifted during splitting and handling, the curated length is almost always longer than the initial liner length. Thus, depths associated with the WRMSL data used to construct the splice table are not identical to the final depths assigned to a given interval by the database. This leads to small differences, usually between 0 and 5 cm, between the CSF and CCSF-A depths recorded in a splice table and the depths reported in other places for the same core interval. We have chosen not to change these depths to be compatible with the LIMS database because this would not improve their accuracy. For consistency, we recommend that all postcruise depth models use or build on CCSF-A values provided in the LIMS database.

Top of page   |   Previous   |   Next