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doi:10.2204/iodp.proc.303306.110.2006

Stratigraphic correlation

Paleomagnetic and paleoclimatic objectives of Expedition 306 required the recovery of complete stratigraphic sections, yet stratigraphers have demonstrated that a continuous section is rarely recovered from a single ODP borehole because of core-recovery gaps between successive APC and extended core barrel cores despite 100% or more nominal recovery (Ruddiman et al., 1987; Hagelberg et al., 1995). Construction of a complete 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 306, two or more 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 legs of the ODP (e.g., Hagelberg et al., 1992; Curry, Shackleton, Richter, et al., 1995; Jansen, Raymo, Blum, et al., 1996; Lyle, Koizumi, Richter, et al., 1997; Gersonde, Hodell, Blum, et al., 1999, Wang, Prell, Blum, et al., 2000; Mix, Tiedemann, Blum, et al., 2003; Zachos, Kroon, Blum, et al., 2004; among others) and, most recently, during Expedition 303 (see “Composite section” in the “Site U1302–U1308 methods” chapter). 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 is dependent on avoiding coeval core-recovery gaps in the multiple holes, which can be done through real time adjustments to the depths at which a core is collected as discussed below.

Meters 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 mbsf depth scales of individual cores to maximize correlation between holes. In the composite depth scale used by ODP and IODP, referred to as the mcd scale, 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. 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 mcd scale, once established, provides a basis upon which higher-order depth composite scales can be built.

In essence, the mcd scale overcomes many of the inadequacies of the mbsf depth scale, which is based on drill pipe measurements and is unique to each hole. The mbsf 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 mcd scale is built by assuming that the uppermost sediment (commonly referred to as the 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 mbsf and mcd scales. From this anchor, core logging data are correlated among holes downsection. For each core, a depth offset (a constant) that best aligns the observed lithologic variations to the equivalent cores in adjacent holes is added to the mbsf 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 306, the mcd scale and the splice are based on the stratigraphic correlation of data from the IODP whole-core MSCL, MST, AMST, and long-core magnetometer. For each of these track instruments, data were collected every 2, 2.5, 4, or 5 cm. We used magnetic susceptibility (the data are referred to as MSCL if collected using the MSCL or MS-MST if collected using the MST), gamma ray attenuation (GRA) bulk density, natural gamma radiation (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.”

The raw stratigraphic data were imported into Splicer (version 2.2) and culled as necessary to avoid incorporating anomalous data influenced by edge effects at section boundaries. Splicer was used to assess the stratigraphic continuity of the recovered sedimentary sequences at each drill site and to construct the mcd scale and splice.

Because depth intervals within cores are not squeezed or stretched by Splicer, 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 compressed, although this 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 size when appropriate. Depth-shifted data are denoted by mcd. A table is presented in each site chapter that summarizes the depth offsets for each core. These tables are necessary for converting mbsf to mcd scales. The mcd for any point within a core equals the mbsf plus the cumulative offset. Correlation at finer resolution is not possible with Splicer 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).

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. It 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 does not contain 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 a somewhat 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) was 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 sampling.

The length of the spliced section (on the mcd 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 mbsf scale. 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). 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 mcd 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 mcd. However, below the base of the continuous mcd, cores from two or more holes can sometimes be correlated with each other to create a floating splice. In this case an mcd 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 meters composite depth

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

Magnetic Susceptibility Core Logger

In order to permit rapid stratigraphic correlation for real-time drilling adjustments, the MSCL was used for only the second time during an IODP expedition. The MSCL is a simple track system with two magnetic susceptibility loops separated by 45 cm (see “Physical properties”). It was developed to permit rapid measurement of susceptibility 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. During Expedition 303, the current track system was used, but the system did not function properly in dual sensor mode and so only single-sensor data were collected (see “Physical properties” in the “Site U1302–U1308 methods” chapter).

Unlike Expedition 303, during Expedition 306 we did not experience any problems with the dual sensors. We were thus able to run the track in dual-sensor mode for all sections measured using a sampling interval of 10 cm for each loop, which results in a 5 cm overall data spacing. Five measurements were made at each interval using SI units and the short (~1 s) measurement setting on the Bartington susceptibility meters. At these settings, we could log a typical 9.5 m long core in ~30 min.

Rare data spikes occurred for reasons yet to be determined, although shielding the cables to the susceptibility meters and keeping cables away from the loops appeared to reduce the frequency of these spikes. These are manifested by single negative values or spikes that differ significantly from real susceptibility values. They were therefore easy to detect and to remove before the data were uploaded into the database. Once in the database, relative offsets between the two meters are mostly removed, providing a consistent data set with higher resolution than could be obtained with a single susceptibility meter in the same amount of time. During Expedition 306, we noted that small systematic differences existed between the two meters even after the offset correction, but these are virtually imperceptible when the data are loaded into Splicer using the five-point smoothing option.

The MSCL susceptibility data are used in Splicer 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. Basically, the stratigraphic correlators are able to guide the drilling in real time to avoid having coeval coring gaps in multiple holes cored at a site, which would result in an incomplete stratigraphic section. This preliminary susceptibility data set is then superceded by the magnetic susceptibility measurements collected on the MST, which is done after the cores have equilibrated to room temperature and generally at higher resolution than on the MSCL. Likewise, the preliminary stratigraphic correlation is superceded by more careful evaluation following the collection of a suite of physical property data and core descriptions.

Depths in splice tables versus Janus 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 MSCL almost immediately after this liner length measurement. At some later time, typically 10 to 36 h after being analyzed by the MST, core sections are split and analyzed further (see “Core handling and analysis”). At this time, the section lengths are measured again and are archived as “curated lengths.” General database queries return depths based on the 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, the depths associated with the MST 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 mbsf and mcd 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 Janus because this would not improve their accuracy. For consistency, we recommend that all postcruise depth models use or build on mcd values provided in the Janus database.

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