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Stratigraphic correlation and sedimentation rates

Meeting the scientific objectives of Expedition 346 required recovery of complete stratigraphic sections. Such sections cannot be constructed from a single IODP hole because core-recovery gaps on the order of 20–50 cm occur between successive cores despite 100% or greater nominal recovery (e.g., Ruddiman, Kidd, Thomas, et al., 1987; Hagelberg et al., 1995; Acton et al., 2001). The construction of a complete stratigraphic section, referred to as a splice, requires combining stratigraphic intervals from two or more offset holes cored at the same site.

Core depth below seafloor (CSF-A) scale

The depth to the top of each core is based on the DSF scale. DSF is a drill string measurement, determined by the length of the drill string below the rig floor to the top of the cored interval minus the length of the drill string from the rig floor to the mudline (which is assumed to be the seafloor). The depth to a given position within a core is determined by adding to the core top depth the distance that position occurs from the top of the core. The distance the position occurs from the top of the core includes expansion due to relief of overburden as well as gas expansion. This depth scale is referred to as CSF-A (Fig. F16) and is equivalent to the historical DSDP, ODP, and IODP mbsf scale (IODP Depth Scales,​program-policies/​procedures/​guidelines/). Error in the CSF-A scale includes pipe and BHA stretch and compression, tides, and uncompensated heave, as well as incomplete recovery and core expansion as a result of elastic rebound and gas. Tidal influence on this depth measurement was first predicted during ODP Leg 138 (Hagelberg et al., 1995) and proven during ODP Leg 202 (Mix, Tiedemann, Blum, et al., 2003). Because tidal variations are predictable, pipe offsets can be adjusted to account for these cyclical changes in sea level, which helps avoid core gap alignment among holes. During this expedition, tidal ranges were not a factor and were not accounted for. In any case, the goal of avoiding core gap alignment can be difficult to achieve with just two holes and sometimes even with three.

Core composite depth below seafloor (CCSF-A) scale

The goal of constructing a composite depth scale for a given site is to place specific coeval, laterally continuous features identified in all drilled holes into a common frame of reference by depth shifting individual cores (each starting on the CSF-A scale). The resulting CCSF-A scale is equivalent to the historical ODP and IODP meters composite depth (mcd) scale. In constructing the CCSF-A scale, the depths of the individual cores are shifted by a constant amount (there is no stretching or squeezing within individual cores). This provides good first-order correlation between cores from different holes, provides estimates of the length of coring gaps, and provides a basis upon which higher order composite depth scales are constructed; compositing is a prerequisite to developing the CCSF-D scale (spliced record). The CCSF-A scale is built by correlating features downhole from the sedimentological mudline (typically the top of a core with a well-preserved sediment/water interface). This establishes the top of the stratigraphic section and anchors the entire composite depth scale for all cores from all holes at a site. In practice, compositing is accomplished sequentially by establishing specific tie points among the various holes, working from the mudline (anchor) core to the bottom of the drilled section, as illustrated by the red tie point arrows in Figure F16. Core intervals between the tie points are used to construct the splice as discussed below. As such, tie points should be chosen such that disturbed intervals are avoided. The mudline (or anchor) core is typically the only core in which the depths are the same for both the CSF-A and CCSF-A scales. The CCSF-A scale very rarely (if ever) results in alignment of all features because of the differing effects of coring-induced stretching and squeezing among cores as well as hole-to-hole sedimentological differences (Fig. F16).

The vertical depth offset of every core in every hole is tabulated in an affine table. Conceptually, it should be possible to correlate (“tie”) each successive core in one hole to a core from an adjacent hole, all the way to the bottom of a drilled section, provided between-core gaps never come into alignment among all holes at a site and recovery is sufficiently high. Aligned coring gaps across all holes cored at a site may still occur, in which case cores below the gap are no longer tied to the mudline core but can often still be tied to one another. Such intervals are commonly referred to as “floating” spliced sections.

During the process of constructing the composite section, the CCSF-A depth becomes systematically larger than that of the CSF-A depth at equivalent horizons. This expansion, which is typically ~5%–15%, is mostly caused by decompression of the cores as they are brought to the surface, gas expansion, stretching that occurs as part of the coring process, and/or curation when, for example, 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).

Core composite depth below seafloor (CCSF-D) scale; the splice

Once the CCSF-A scale has been developed and the between-core gaps identified, a complete stratigraphic section (splice) is constructed by combining selected intervals between the previously established tie points. The depth scale is designated the CCSF-D scale (Fig. F16).

Although CCSF-D is merely a subset of the CCSF-A, this -D designation applies only to intervals included in the splice. Intervals not included in the primary splice, including any alternate splice(s), retain the CCSF-A scale. Should coring gaps happen to align across all holes drilled at a site, the spliced sections below are “appended” to those above and referred to as floating splice sections (i.e., not tied to the mudline). In this case, the amount of missing material between floating sections can be measured using downhole logs. Where no logs are available, the CSF-A scale provides a reasonably accurate estimate of the length of the missing section, especially when coring in calm seas.

Core composite depth below seafloor (CCSF-C) scale

Once the splice is constructed, it is sometimes possible (and often useful) to map into the splice those intervals not included in the splice itself. This can be accomplished using the core-log integration functionality in the Correlator software by treating the splice as a downhole log. The methodology is based on identification of correlative tie points at very high resolution (cycle by cycle), with linear adjustments of data between ties. This is designated as the CCSF-C scale and can be thought of as an “equivalent splice depth.” In the best case scenario, the CCSF-C and CCSF-D scales are equivalent. In reality, they are only equivalent to the extent that the correlations are accurate. These depth-depth mapping functions, if available, are provided in table form for specific sites. At core boundaries, where strong stretching and compression are prevalent, these detailed depth maps are less accurate. Users are encouraged to plot data on the CCSF-C scale along with the same set of data on the splice (CCSF-D scale) in order to assess whether or not the CCSF-C scale is sufficiently accurate for the intended use.

Measurements and methods for correlation

The composite sections and splices are based on the stratigraphic correlation of data sets acquired from the WRMSL, STMSL, digitized red, green, and blue (RGB) color data extracted from core images acquired from the SHIL, and reflectance data from the SHMSL. During this expedition we used the WRMSL and STMSL in parallel as described in detail in “Physical properties.” This allowed stratigraphic correlation to take place in real time such that bit depth could be adjusted as necessary to avoid alignment of core gaps among holes.

The data sets used for compositing and splicing include magnetic susceptibility, GRA, and various color reflectance data sets (e.g., R, G, and B and L*, a*, and b*). Magnetic susceptibility and GRA were measured at 2.5 or 5 cm intervals, whereas R, G, and B values were calculated at 0.5 cm intervals in the form of 0.5 cm averages measured continuously along a 1.5 cm wide strip centered on digital color SHIL images. Details on instrument calibrations, settings, and measurement intervals for Expedition 346 are given in “Lithostratigraphy” and “Physical properties.”

Compositing and splicing was accomplished using Correlator software (v 1.695), from which we generated standard affine tables (listings of the vertical offset [m] added to each core in order to generate the CCSF-A scales) and splice interval tables (listings of the specific core intervals used to construct the splice). These tables were uploaded into the LIMS database, from which all users are able to attach the appropriate depth scale to any data set. In most cases, the detailed splice was constructed on the basis of blue (B) data acquired after the cores were split and imaged.

At sites with significant gas expansion, where voids developed within the liner, holes were drilled to relieve the pressure, and voids were reduced using plungers. In addition, the splitting process sometimes caused sediment to be moved upsection (sections are split bottom to top). Curators then further consolidated voids within each section on both the working and archive halves. For these reasons, the WRMSL and STMSL magnetic susceptibility and GRA data may not agree with the blue data when plotted on the CCSF-D scale.

Age models and sedimentation rates

Preliminary age models and sedimentation rates were generated for each site considering all available datums generated by the Biostratigraphy, Paleomagnetism, and Lithostratigraphy (tephra) groups. Based on these shipboard results, the most likely depth-age relationship was constructed based on the following criteria:

  • Paleomagnetic datums, where present, were used preferentially.

  • Where appropriate, the depth-age line was set between the lower and upper limits given by LO and FO, respectively, of biostratigraphic datums.

  • Within a lithologic unit, the number of inflection points was minimized.

Linear or higher order fits were applied to the data satisfying above criteria to generate preliminary age models. The age model was then used to assess sedimentation rates between lithologic boundaries established by the Lithostratigraphy group.