Previous   |    Next

doi:10.2204/iodp.proc.339.102.2013

Stratigraphic correlation

Many of the scientific objectives of Expedition 339 depend on recovery of complete stratigraphic sections. Such sections cannot be recovered from a single IODP borehole because core-recovery gaps occur between successive cores despite 100% or more nominal recovery (e.g., Ruddiman, Kidd, Thomas, et al., 1987; Hagelberg et al., 1995; Acton et al., 2001). The construction of a complete composite 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 intercore 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 holes are cored to allow for the construction of alternate splices, where possible.

The composite section and splice construction methodology employed during Expedition 339 follows the basic strategy originally developed during ODP Leg 138 (e.g., Hagelberg et al., 1992, 1995) and later refined during many other ODP legs and IODP expeditions. This strategy is now common practice on all high-resolution paleoceanographic expeditions. Assembly and verification of a complete composite stratigraphic section requires construction of a composite depth scale, referred to as the meters composite depth scale. The composite depth scale provides a common depth scale for the holes at a site unlike the original meters below seafloor scale, which is based on drill string measurements that are unique to each hole.

Meters below seafloor depth scale

The depth to the top of each core is based on a drill string measurement, which is determined by the length of drill string below the rig floor to the top of the cored interval minus the length of drill string from the rig floor to the mudline (assumed to be the seafloor). The depth to a point along the core is determined by adding the distance the point occurs from the top of the core to the core top depth. This depth scale is referred to as meters CSF-A, which is equivalent to the ODP meters below seafloor depth scale.

The zero depth point of the meters below seafloor 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 inadvertently (or purposely) started below the sediment/water interface. In this case, the zero depth point in meters below seafloor units may be substantially offset from the zero depth point in adjacent holes that successfully recover the sediment/water interface.

The meters below seafloor scale may also be inaccurate because of ship heave, tidal variations in sea level, and other sources of error. Tidal influence on depth was first predicted during ODP Leg 138 (Hagelberg et al., 1995) and was proven by correlation of affine offset changes and tide height during ODP Leg 202 (Mix, Tiedemann, Blum, et al., 2003). Expedition 339 was fortunate to core during relatively benign seas, with ship heave remaining relatively small for all seven sites. Tidal variations were thus the largest source of variation in the meters below seafloor depth scale from one hole to the next. Because these are predictable, pipe offsets could be adjusted with the goal of avoiding core gap alignment. For a variety of practical reasons, such as failure to get a complete 9.6 m stroke with the APC system for one or more cores from each hole, the goal can be difficult to achieve with just two holes and sometimes even with three.

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

In essence, the meters composite depth scale overcomes many of the inadequacies of the meters below seafloor depth scale, which is unique to each hole and may be inaccurate as discussed above. Rather than using a drill string measurement, the meters composite depth scale is built by correlating features downhole from the mudline. The mudline is not merely taken as the top of the first core in a hole but is the top of the first core with the most representative or best-preserved mudline. This mudline establishes the top of the stratigraphic section and the core with this mudline becomes the “anchor” in the composite depth scale. It is typically the only core in which the depths are the same for both the meters below seafloor and meters composite scales. Each core downhole is tied to the composite section by adding or subtracting a depth offset (a constant) that best aligns the observed lithologic variations to the correlative variations for cores from adjacent holes. If between-core gaps are not aligned for the different holes and recovery is high, it should be possible to correlate each successive core downhole to a core from an adjacent hole. Gaps common to all holes cored at a site can occur, in which case cores downhole can be appended to the composite section. In such cases, the depth offset for the appended core can be adjusted to convey an estimate of the true size of the coring gap. For example, one could use the average growth factor (i.e., expansion) of the meters composite depth scale relative to the meters below seafloor depth scale or could use continuous logging data from a hole at the site or observations from other nearby sites.

In the process of constructing the composite section, the meters composite depth is virtually always expanded relative to the meters below seafloor scale. The expansion, which is typically ~5% to 15%, is mostly caused by real decompression of the cores as they are brought to the surface, by stretching that occurs as part of the coring process, and/or from curation practice, in which material that has fallen downhole or gas expansion voids are curated as part of the core (e.g., Hagelberg et al., 1995; Acton et al., 2001).

Goals

The goals for stratigraphic correlation for Expedition 339, in order of priority, were to

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

2. Establish a composite depth scale; and

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

This first-order correlation is commonly refined postcruise with the generation of adjusted or revised composite depth scales, which may also be used to link core and logging depths at sites where logging was done.

Measurements and methods specific to Expedition 339

The composite sections and splices are based on the stratigraphic correlation of data from the IODP whole-core logger systems that measure physical and magnetic properties. Initial correlation was based primarily on using magnetic susceptibility from the WRMSL and STMSL. Both loggers measure susceptibility and density, and the WRMSL also measures P-wave velocity. The primary whole-core physical property data are generated using the WRMSL, which is generally set to make higher resolution, higher precision, and more time consuming measurements. These measurements are only done after the temperature of the core has equilibrated to laboratory temperature, or roughly 3 h after the core is collected. The STMSL was set to make lower resolution, lower precision, and more rapid measurements of susceptibility and density, without the requirement that the core equilibrates. More details on the instruments and their settings and measurement intervals for Expedition 339 are given in “Physical properties”.

We typically only collected data with the WRMSL for cores from Hole A, given that the duration of coring did not require a rapid record from this hole. Core sections from subsequent holes were measured immediately after they were labeled using the STMSL. This provided a susceptibility record rapidly for correlation with the previously cored hole or holes at a site, with the resulting correlation used to make decisions about drill string offset during coring operations. The STMSL data were subsequently superseded by the WRMSL once those measurements were completed.

Final correlation involved the use of several data sets, with the magnetic susceptibility and NGR total counts per second providing anomalies that could be correlated most consistently for the Expedition 339 sites. Occasionally the whole-core density data and the lithostratigraphic variations were used to aid in the correlation. Other data, including color reflectance variations (L*a*b*), point magnetic susceptibility, and paleomagnetic inclination and intensity, were evaluated but did not have features as correlative between holes as did the whole-core susceptibility and NGR data.

Between-hole correlation of these data was accomplished mainly using Microsoft Excel or Correlator (version 1.695), although several other software packages were investigated, including Match (Lisiecki and Lisiecki, 2002), Autocomp (Lisiecki and Herbert, 2007), AnalySeries (Palliard et al., 1996), and KaleidaGraph. We found that use of dynamic tables and visual graphic correlation in Excel and visual and statistical correlation in Correlator worked well given the time constraints and goals.

For both methods, we generated standard affine tables, which list the offset that is added to each core to place it in the meters composite depth scale, and splice tables, which give the intervals that comprise the splice. The tables were uploaded into the LIMS database, which will provide composite depths in CCSF-A and CCSF-B, and spliced data sets for users directly from the LIMS database, although as of Expedition 339 those capabilities were lacking. The affine and splice tables provide all the necessary information needed to place any of the Expedition 339 data into the meters composite depth scale and for users to generate spliced data sets for each site.

Top of page   |    Previous   |    Next