Previous   |    Next

doi:10.2204/iodp.proc.341.103.2014

Stratigraphic correlation

The composite depth scale at Site U1417 was constructed from 0.0 to 750.82 m CCSF-A (as defined in “Stratigraphic correlation” in the “Methods” chapter [Jaeger et al., 2014]). The splice consists of one complete and continuous interval from the mudline to 224.2 m core composite depth below seafloor (CCSF-D). At greater depths, intervals of correlation were found between holes but were not considered sufficiently continuous to warrant development of a deeper “floating” shipboard splice.

The splice ranges from the top of Core 341-U1417D-1H (the mudline) through Core 341-U1417C-22H (Tables T10, T11). To the extent possible, the splice was constructed from Holes U1417C and U1417D because Hole U1417A was sampled at sea and the shallowest 10 cores from Hole U1417B were recovered during heavy weather, when ship heave compromised the quality of many of the cores. In Hole U1417C, many of the APC cores taken deeper than ~150 m CCSF-A had significant flow-in, likely due to incomplete penetration. To the extent possible, these intervals were identified and excluded from the splice.

Correlations between holes were accomplished using IODP Correlator software (version 1.695). All splice tie points were checked by examining 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 initially on Special Task Multisensor Logger (STMSL) GRA 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) with verification from NGR and reflectance spectroscopy color data from the Section Half Multisensor Logger (SHMSL).

The CCSF-A and CCSF-D scales were constructed by assuming that the uppermost sediment (the mudline) in Core 341-U1417D-1H represents the sediment/water interface. An approximate mudline was also recovered in Cores 341-U1417A-1H and 341-U1417B-1H and 2H, confirming the fidelity of the top of the recovered interval. Core 341-U1417D-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, correlating variations in core logging data between holes on a core-by-core basis using Correlator.

A few splice tie points are uncertain. In particular, the splice points between 341-U1417C-7H-7, 16.86 cm, and 341-U1417D-7H-1, 34.84 cm (62.18 m CCSF-A), between 341-U1417C-8H-7, 57.10 cm, and 341-U1417A-10H-1, 32.74 cm (76.12 m CCSF-A), and between 341-U1417A-21H-3, 135.07 cm, and 341-U1417B-18H-1, 12.14 cm (186.76 m CCSF-A), all have little overlap to constrain the splice points and should be used cautiously. The tie point between 341-U1417B-22H-4, 47.19 cm, and 341-U1417C-22H-1, 16.37 cm (211.29 m CCSF-A), is equivocal.

Within the splice, the CCSF-A depth scale is defined as 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 depth assignments, should not be expected to correlate precisely with fine-scale details within the splice or with other holes because of variation in the relative spacing of features in different holes. Such apparent depth differences of specific features may reflect coring artifacts or fine-scale spatial variations in sediment accumulation and preservation at and below the seafloor.

The cumulative offset between CSF-A 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., 2014]) averages 1.1348 at Site U1417 in the APC-cored interval from 0 to 208.8 m CCSF-A or CCSF-D. Anomalies around this relatively uniform affine growth relationship may be explained by significant ship heave (especially during coring of Hole U1417B), imprecision of tide corrections on coring depths, and potential distortion of cores by piston effects.

The affine growth factor is 1.2608 in the interval from 208.8 to 266.75 m CCSF-A. This high rate of affine growth is an artifact introduced by construction of the CCSF-A depth scale; because few features correlated well among the holes in this interval, we sometimes appended cores either within or between holes rather than making firm correlations of specific features within cores. This method artificially expands the apparent length of recovered material. At depths greater than the splice, we observed significant correlation among holes between 220.4 and 232.4 m CCSF-A (Cores 341-U1417B-25H and 341-U1417D-28H and 30H) and from 259.0 to 263 m CCSF-A (Cores 341-U1417B-31H, 341-U1418C-26H and 27H, and 341-U1417D-35X and 36X). We considered these correlations not sufficiently reliable to warrant construction of a floating splice or precise tie points between holes.

Deeper than 266.75 m CCSF-A, cores were recovered with either the XCB or RCB tools. In this interval, affine values were nominally constant at 43.26 m, which is the highest affine value within the APC-cored interval (Core 341-U1417C-29H). Exceptions are Cores 341-U1417B-40X and 44X and 341-U1417D-45X, 46X, 58X, and 59X. These six cores all include minor adjustments to their affine values to align significant sedimentary features or shipboard observations of paleomagnetic reversals relative to those found in Hole U1417E (which was given a constant affine value of 43.26 m). Correlations between holes based on XCB or RCB cores should be considered uncertain.

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, the CCSF-B depth scale compressed the CCSF-A and CCSF-D scales into a scale that has the same total depth of sediment column as the interval actually drilled (see “Stratigraphic correlation” in the “Methods” chapter [Jaeger et al., 2014]). The following three equations define transformation of the CCSF-A or CCSF-D depth scale into CCSF-B depths (Fig. F23):

From 0 to 208.80 m CCSF-A/CCSF-D:

From 208.80 to 266.75 m CCSF-A/CCSF-D:

For 266.75 m CCSF-A/CCSF-D:

Initial age model

All the available paleomagnetic and biostratigraphic age datums (Tables T5, T12) (see “Paleontology and Biostratigraphy” and “Paleomagnetism”) were considered while constructing 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 and are also provided in the CCSF-A depth scale.

When geomagnetic polarity boundaries and/or reversals were observed in several holes, the average depth and its uncertainty were calculated in the CCSF-B scale. All identified paleomagnetic reversals were included in the shipboard minimum and maximum age models.

Uncertainty in the biostratigraphic datums mostly reflects the shipboard sampling interval and the presence of barren zones. Two biostratigraphic datums were considered outliers relative to well-constrained paleomagnetic datums. The LO of the diatom Actinocyclus oculatus (Sample 341-U1417D-9H-3W, 80 cm) and the LO of the diatom Neodenticula koizumii (Sample 341-U1417D-15H-3, 130 cm) were not included in the maximum and minimum age models. All other biostratigraphic datums were accommodated within the uncertainties of their published ages and relatively coarse shipboard sampling.

Relatively few biostratigraphic datums were detected in the interval from 460 m CCSF-A (416.74 CCSF-B) to the base of the site. For purposes of constraining the oldest ages at Site U1417, we assumed that basaltic basement was at 838 ± 8 m CCSF-A (794.74 m CCSF-B based on site survey geophysical data). The oceanic basement is Pacific plate basaltic crust that can be associated with Chron C17–C18 based on the EMAG2 magnetic anomalies (Maus et al., 2009), which span from 37 to 41.2 Ma (Ogg, 2012).

Based on these initial shipboard minimum and maximum age models, the interval older than ~10 Ma (averaged over 10–39 Ma) has sediment accumulation rates of <10 m/m.y. Sediment accumulation rates (averaged within equally spaced increments of 0.5 m.y.) increase through time, peaking at values >110 m/m.y. in the intervals 0–1 and 2–2.5 Ma (Fig. F25). Such a rise in sedimentation rates is consistent with the gradual northward tectonic drift of the site, as well as acceleration of sediment inputs from regional tectonics and glacial forcing (based on the presence of lonestones and diamicts of probable ice-rafted origin within the last ~2.5 m.y.) and further acceleration of glacial erosion in the last ~1.0 m.y., following the MPT (Clark et al., 2006); Fig. F16).

Top of page   |    Previous   |    Next