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doi:10.2204/iodp.proc.313.202.2018

Results

The identified overlaps in Holes M0027A, M0028A, and M0029A, including lithology and the degree of overlap for each core section, are illustrated in Figure F1. It is clear from this representation that overlapping intervals were identified across several lithologies, but almost all clay intervals display some overlap (e.g., around 200 mbsf) in Hole M0027A and 230 mbsf in M0028A. Relatively small overlaps occur at the base of each core run and are interpreted to be due to clay mineral expansion. This interpretation is supported by the drilling notes and drilling parameters, indicating that drilling was progressing well (Fig. F1, column 7). The expansion amount is generally <10% (e.g., 7.52% in Hole M0027A and 11.23% in Hole M0028A; upper clays). Some larger overlaps also occur, generally in nonclay lithologies, and are interpreted to be due to slipped core, an interpretation generally confirmed by the drilling notes (e.g., at 517 mbsf in Hole M0027A, 298 mbsf in Hole M0028A, and 439 mbsf in Hole M0029A) (Fig. F1, column 7). These overlapping cores are characterized by gaps in recovery adjacent to the slipped core. In other intervals of overlap, drilling notes mention difficulties in drilling operations, such as needing to ream or reenter the hole (e.g., at 265 mbsf in Hole M0028A). Here the underlying core could be considered to be likely affected by drilling disturbance. A large interval of overlap also occurs in Hole M0028A between 322 and 334 mbsf and was caused by reentry of the drill pipe that created a slightly deviated borehole, as discussed in the “Site M0028” chapter (Mountain et al., 2010c), and has been shown as a separate narrow column to the left of the standard overlap columns (Fig. F1, Column 5*).

Each core run during drilling is marked by an increase in head rpm, water flow, and torque force. Where drilling went smoothly, there tends to be a fairly regular gap between core runs, and without anything external altering, any observed difference in drilling parameter values can be attributed to a change in the material being drilled (e.g., a lithologic change). This scenario can be observed on Figure F2 where sand changes to silt downhole at 414 mbsf in Hole M0027A, which is recognized by a decrease in drilling parameters as the lithology changes from sand to silt, with fairly regular runs both above and below this depth. Overlaps in the silt, continuing deeper (Fig. F1), are therefore interpreted to be due to clay minerals within the silt, resulting in expansion. This is consistent with XRD measurements that indicate the presence of clay minerals in this interval (see the “Site M0027” chapter [Expedition 313 Scientists, 2010b]).

Where larger overlaps are identified, they are often due to slipped cores or difficulty during drilling operations. At 548 mbsf in Hole M0027A (Fig. F3), a gap in recovery occurs above Core 195R; therefore, moving this core upward is the appropriate depth shift. This core run can be identified in the drilling record by a brief spike in torque force and a note that a short (10 cm) run collected material from the previous run (Fig. F3). Similarly, the overlap at 517 mbsf in Hole M0027A (Fig. F3) also correlates with the observation in the drilling notes that Core 183R is a slipped core. At 524 mbsf, an interval of identified overlap corresponds to a gap in time (3 h) in the drilling parameter record where the recording switch was turned off, which is common at any point where drilling and coring pauses in order to solve a technical issue. The initial run after the pause is likely to have also collected surplus material that had fallen into the hole; therefore, it is appropriate to treat the core from the upper overlapping core as the better quality core.

Cores in their conventional depth position and two different methods for dealing with the overlapping sections are illustrated in Figures F4 and F5, showing the upper intervals of clay in Holes M0027A and M0028A, respectively. Digital linescan images are shown in their conventional position (both as a single image and separated) and as an expanded image column. In the expanded image column, core below an overlap has been moved down by a linear shift corresponding to the amount of the overlap. Hence, the shift progressively increases as more overlaps are accounted for. The conventional image column omits the upper part of each underlying image if an overlap is present; therefore, any measurements taken in the overlap intervals should be treated with caution. The overall expansion amounts are, respectively, 7.52% and 11.23%. These amounts are lower than for some clay-rich lithologies, and yet the expanded image column illustrates that the progressive increase in shift required to account for the expansion soon results in considerable displacement of cores relative to their conventional position. Figures F4 and F5 illustrate that without additional information, it is difficult to establish how to correct the overlap interval and where to use conventional expanded depths or scaled depths.

Expedition 313 downhole logs can provide the data to decide how to adjust the core depths appropriately by providing an independent depth control. Spectral gamma ray logs acquired throughout the Expedition 313 boreholes mostly display a good correspondence to NGR measurements taken on cores (Fig. F1, column 8). In intervals of lower recovery, where core is likely to be less accurately positioned, especially if overlaps also complicate the recovered sequence (e.g., 70–100 mbsf in Hole M0027A or 265–295 mbsf in Hole M0028A; Fig. F1), the correspondence is harder to identify between downhole logs and core, with the logs having better depth control. However, in some intervals, NGR measurements do not display a sufficient degree of variation for unambiguous depth control (e.g., 225–255 mbsf in Hole M0028A). Here, other downhole logs can be more helpful, such as magnetic susceptibility.

To assess the optimum correction for core depths, Figure F6 shows in detail an example of a 6 m interval in Hole M0027A, with magnetic susceptibility measured on the recovered core (dark blue) as well as downhole logging magnetic susceptibility (light blue) overlain on (1) conventional, (2) scaled, and (3) expanded core sequences. The core magnetic susceptibility is shown on a logarithmic scale when overlain on the conventional and scaled images. A linear scale is selected for the core magnetic susceptibility overlain on the expanded image column to illustrate how clearly this picks up the color banding in the clay in Core 69X and at the top of Core 70X. The core magnetic susceptibility measurements indicate three main changes across this 6 m interval, designated by letters A through C (Fig. F6, Columns 5–7). From the base of the figure, Cores 73X through 70X display low, near-constant magnetic susceptibility until two successive increases uphole in Core 70X, first a change (at A) to a slightly higher, more variable magnetic susceptibility followed by a further increase uphole (at B) within the clay. Between Cores 70X and 69X, magnetic susceptibility slightly increases (at C), and larger variations correspond to the visible color banding. VCDs do not identify major changes in the sediment at A or B, with silty layers disappearing uphole at C (see the “Site M0027” chapter [Expedition 313 Scientists, 2010b]). The downhole magnetic susceptibility log displays two significant increases uphole (Fig. F6, Column 8) that are likely to correspond to the changes at A and C in the core logs; the change at B likely coincides with more minor changes in the downhole log. To discuss the complexity of correlation within the overlap interval illustrated in Figure F6, conventional, scaled, and expanded depths are compared:

• Conventional depth (left): the depth of change A in the conventional column corresponds well with the lowermost significant change in the downhole log (Fig. F6, track 8). The change in core magnetic susceptibility at B does not clearly correspond to either a visible change in the image or in the downhole log. Note that the change at C could be placed at either the top or base of the overlap interval, depending on whether data from the upper or lower core are used or an average is used (for numeric data). In the overlap, the overlying cores overprint the underlying cores (e.g., Core 69X overprints Core 70X). For numeric data (e.g., core magnetic susceptibility), a further option is to use an averaged value, and therefore caution is required to ensure contrasting data types are treated equivalently.
• Scaled depth (center): the core magnetic susceptibility change at A correlates with a minor change in the downhole magnetic susceptibility log (Fig. F6, track 8). For changes B and C, the scaled image (center) shows a good correlation to the downhole log, with change B coinciding with a minor change in the downhole log and change C reflecting the significant increase uphole in both core and downhole magnetic susceptibility.
• Expanded depth (right): all three significant changes (A through C) in the core magnetic susceptibility logs occur below the depth of the equivalent changes in the downhole magnetic susceptibility log. Hence, this depth scale is inappropriate for any scientific studies requiring knowledge of the true depth because the successive summation of overlaps quickly leads to a large depth offset.

The scaled depth avoids any ambiguity in which data are being used within an overlap and allows a continuous record of the succession. Although either conventional or scaled depths could represent a better correlation to the downhole logs for the change at A, for the changes at B and C the scaled depth displays a closer correspondence. It should be noted that although downhole logs represent independent depth control, they are not without depth issues (e.g., wire expansion or sticking), and the true depth may be a combination of core and logging depths.

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