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doi:10.2204/iodp.proc.341.104.2014

Stratigraphic correlation

The composite depth scale at Site U1418 was constructed from 0.0 to 941.44 m core composite depth below seafloor (CCSF-A). The splice consists of one complete and continuous interval from the mudline to 271.40 m core composite depth below seafloor (CCSF-D). The CCSF-A and CCSF-D depth scales are defined in “Stratigraphic correlation” in the “Methods” chapter (Jaeger et al., 2014a).

The splice ranges from the top of Core 341-U1418C-1H (the mudline) to the base of Core 341-U1418D-26H (Tables T10, T11). To the extent possible, the splice was constructed from Holes U1418C and U1418D because Hole U1418A was sampled at sea and Hole U1418B consisted of two special-purpose cores for pore water sampling. It was necessary to include four short intervals of Hole U1418A in the splice. Hole U1418E was spot cored to cover specific gaps in the composite depths and was used for three intervals in the splice. Hole U1418F consists of rotary drill cores at depths deeper than those included in the splice.

Weather was calm and ship heave was negligible during coring at Site U1418; thus, weather did not contribute core-to-core variability to the affine growth curve. Nevertheless, many of the APC cores contain evidence for stretched or compressed intervals relative to equivalent depths in other holes; we interpret these discrepancies as piston effects from coring. In support of this hypothesis, we observed that the drill string bounced on the order of 1 m during some APC shots (recorded as variations in heave compensator position and wireline tension), implying that the piston was not always stationary during penetration of the core barrel. Such decoupled variations in motions of the piston and the core barrel could result in decimeter- to meter-scale compression (“plowing” effects) or expansion (“suction” effects) within cores. We also observed instances of significant flow-in, generally near the base of cores (for example, Core 341-U1418-32H is mostly flow-in). Flow-in is often associated with failure of the core barrel to fully stroke, for example, because of incomplete penetration of a relatively stiff formation. The resulting suction during core pull out creates flow-in. For the splice, when multiple copies of specific sediment sequences were available, we avoided apparent stretched or compressed intervals and excluded intervals of flow-in.

Correlations between holes were accomplished using the Integrated Ocean Drilling Program (IODP) Correlator software (version 1.695), and all splice tie points were checked with digital line-scan images with Corelyzer (version 2.0.2), linked to Correlator. During coring, real-time development of composite depths and guidance for coring operations relied on Special Task Multisensor Logger (STMSL) gamma ray attenuation (GRA) bulk density and magnetic susceptibility data. The final composite depth scale (CCSF-A) and the splice (CCSF-D scale) are based primarily on the stratigraphic correlation of magnetic susceptibility and GRA density from the Whole-Round Multisensor Logger (WRMSL) (Figs. F21, F22) and verified with whole-round NGR and reflectance spectroscopy color data from the Section Half Multisensor Logger (SHMSL). Anomalously low GRA density from the WRMSL was also used as an indicator of core disturbance.

The CCSF-A and CCSF-D scales were constructed by assuming that the uppermost sediment (the mudline) in Core 341-U1418C-1H represented the sediment/water interface. This assumption was supported by the presence of an ~6 cm thick layer of brownish sediment at the core top. An approximate mudline was also recovered in Cores 341-U1418A-1H and 341-U1418B-1H, which confirms the fidelity of the top of the recovered interval. Core 341-U1418C-1H serves as the “anchor” in the composite depth scale and is the only core with depths that are the same on the CCSF-A and CCSF-D scales. From this anchor we worked downhole, matching variations in core logging data between holes on a core-by-core basis using Correlator.

In the splice, a few tie points are tentative. In particular, the tie points between 341-U1418A-11H-6, 99.91 cm, and 341-U1418C-15H-1, 35.14 cm (109.95 m CCSF-A), and between 341-U1418E-6H-6, 124.35 cm, and 341-U1418D-14H-1, 66.47 cm (131.81 m CCSF-A), are supported by little core overlap, include potentially disturbed sediment in Section 1 of the deeper cores, and should be used cautiously. The tie point between 341-U1418C-31H-5, 23.52 cm, and 341-U1418D-26H-1, 73.81 cm (262.71 m CCSF-A), also includes a potentially disturbed Section 1 and is based on very subtle structures in magnetic susceptibility. This tie point seems supported by NGR and GRA density but is considered very uncertain because of disturbance.

Within the splice, the composite CCSF-A depth scale is (by definition) equivalent to the CCSF-D depth scale. Note that the CCSF-D scale rigorously applies only to the spliced interval. Intervals outside the splice, although available with CCSF-A composite depth assignments, should not be expected to correlate precisely with fine-scale details within the splice or with other holes because of normal variation in the relative spacing of features in the recovered intervals from different holes. Such apparent depth differences may reflect coring artifacts or fine-scale spatial variations in sediment accumulation and preservation at and below the seafloor.

The cumulative offset between the CSF-A depth scale and the CCSF-A or CCSF-D depth scales is nonlinear (Fig. F23). The affine growth factor (a measure of the fractional stretching of the composite section relative to the drilled interval; see “Stratigraphic correlation” in the “Methods” chapter (Jaeger et al., 2014a) is 1.121 on average (i.e., the sediment is expanded by ~12% relative to the interval drilled) at Site U1418 in the APC-cored interval from 0 to 152.39 m CCSF-A or CCSF-D. Anomalies around this relatively uniform affine growth relationship are plausibly explained by imprecision of tide corrections on coring depths, potential distortion of cores by core-pipe rebound and piston effects, and fine-scale spatial variations in sediment accumulation and preservation at and below the seafloor.

Over the interval from 152.39 to 272.00 m CCSF-A, the affine growth factor is 1.20 (i.e., the sediment is expanded by ~20% relative to the interval drilled). This high rate of expansion at Site U1418 does not appear to be an artifact introduced by the construction of the CCSF-A depth scale; for the most part the splices between holes were good in this interval. Rather, the high rate of expansion in this interval likely reflects either coring artifacts in the borehole or gas expansion of sediments during recovery (see “Geochemistry” for discussion of methane at this site). Deeper than 272.00 m CCSF-A, intervals of possible correlation were found between a few cores in the interval from ~322 to ~336 m CCSF-A (Cores 341-U1418D-35X through 341-U1418F-4R and 341-U1418D-36X through 341-U1418F-5R), but these intervals were not considered sufficiently continuous nor were the correlations certain enough to warrant core-by-core adjustment of their affine values. Thus, deeper than 272.00 m CCSF-A, the affine value is held constant at 42.09 m. This value applies to APC, XCB, and RCB cores in Hole U1418F. This does not imply a lack of sediment expansion deeper than 272.00 m CCSF-A, just that no constraint on its extent is available.

Calculation of mass accumulation rates based on the CCSF-A or CCSF-D scales must correct for the affine growth factor. To facilitate this process, we developed an additional depth model, CCSF-B, which compressed the CCSF-A and CCSF-D scales into a scale that has the same total depth of sediment column as the actual interval drilled (see “Stratigraphic correlation” in the “Methods” chapter (Jaeger et al., 2014a). This depth scale is approximately consistent with depths defined by borehole logging but was not correlated directly to the logs. The following three equations define transformation of the CCSF-A or CCSF-D depth scale into CCSF-B depths (Fig. F23):

From 0 to 152.39 m CCSF-A/CCSF-D:

From 152.39 to 272.00 m CCSF-A/CCSF-D:

For 272.00 m CCSF-A/CCSF-D:

The depth boundaries between these equations are chosen to coincide with splice points (341-U1418C-21H-6, 92.0 cm, tied to 341-U1418D-16H-1, 90.1 cm, at 152.39 m CCSF-A) and between Cores 341-U1418D-26H (base at 271.40 m CCSF-A) and 27H (top at 272.09 m CCSF-A).

Initial age model

All available paleomagnetic and biostratigraphic age datums (Tables T5, T12) were integrated to construct minimum and maximum shipboard age models (Fig. F24; Table T13); together, these preliminary age models span most of the uncertainty in the shipboard datums. The age models were constructed in the CCSF-B depth scale but are also reported in the CCSF-A depth scale (Table T13). All paleomagnetic datums observed in Hole U1418F (Matuyama/Brunhes polarity reversal and the top and base of the Jaramillo Chron) are included in the shipboard minimum and maximum age models.

The shipboard minimum and maximum age models are calculated in increments of 0.2 m.y. between 0 and 1.2 Ma. When averaged within these intervals, inferred sediment accumulation rates in the interval 0–0.2 Ma are 1041–1505 m/m.y. (Table T13; Fig. F25). Over the interval 0.2–1.2 Ma, sediment accumulation rates are on average 450–825 m/m.y., but uncertainty ranges between 345 and 1139 m/m.y. In Cores 341-U1418F-70R, 71R, and 72R, the sediment layering is disturbed, which is inferred to represent a MTD (see “Lithostratigraphy”). Extrapolation of the age models or sedimentation rates deeper than Core 341-U1418F-69R is thus unjustified.

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