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doi:10.2204/iodp.proc.317.102.2011

Stratigraphic correlation

As part of standard IODP shipboard procedure when coring multiple holes at individual drill sites, stratigraphic correlation was carried out on board to correlate adjacent holes using Correlator software. This software allows depth adjustment of individual or multiple cores to enable ties to be made between cores in separate holes based on the expression of onboard core logging data. In accordance with standard IODP protocol, onboard correlation did not involve depth adjustment of individual core sections, nor were individual core/section depth scales stretched or compressed to force the correlation of intracore tie points. Rather, this first-order approach to stratigraphic correlation established a framework from which more detailed correlation can be carried out postcruise.

Depth adjustment of individual cores is often necessary for a variety of reasons, such as differences in seafloor topography between holes or inconsistent mudline definition. Equally, stratigraphic variation may occur between holes because of slight differences in sedimentation rates and erosion histories. Further complication in the correlation procedure may stem from a lack of unambiguous correlation tie points between holes. Ambiguities may arise from differences in the physical properties of contemporaneous strata, the occurrence of noncontemporaneous strata at similar depths (e.g., incised channel fills), inaccuracies/imprecision in physical property data, and drilling disturbances that affect the fidelity of the data.

The first-order objective of correlation across holes at Expedition 317 sites was to establish a common CCSF depth scale for cores from multiple holes. Furthermore, the possession of two or more overlapping records from adjacent holes provided an opportunity to construct a spliced stratigraphic record against a CCSF scale. This spliced record has the potential to optimize both the completeness of the site's stratigraphic record and the fidelity of the associated stratigraphic data. For the four sites drilled during Expedition 317, the key physical property data used to aid correlation were shipboard NGR, magnetic susceptibility, color, and GRA bulk density. In general, NGR and magnetic susceptibility typically provided the best data for correlation.

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