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doi:10.2204/iodp.proc.341.104.2014

Core-log-seismic integration

For the purposes of shipboard data correlation, we compared data displayed in the following two depth scales: WMSF (see “Downhole logging”) and CCSF-B (a compressed composite depth scale; see “Stratigraphic correlation”) for logging and core data, respectively. Logging data are depth-matched between different tool strings using the gamma ray logs recorded on each logging run and then shifted to the WMSF depth scale based on the step increase in gamma radiation across the seafloor (see “Downhole logging” in the “Methods” chapter [Jaeger et al., 2014a]). For logging data at Site U1418, the maximum observed depth shift was <2 m. Core physical properties were measured in Holes U1418A–U1418F (see “Physical properties”), whereas logging data were recorded only in Hole U1418F (see “Downhole logging”).

For preliminary correlation between Site U1418 lithostratigraphic and logging units with features observed in seismic data, we converted lithostratigraphic and logging unit boundaries from depth in meters (CCSF-B/WMSF) to TWT using the average velocities of each unit. Average P-wave velocity was derived from core physical properties measurements using data from the PWC at depths shallower than ~257 m CCSF-B and the downhole sonic logs at depths shallower than ~548 m CCSF-B/WMSF (see “Physical properties” and “Downhole logging,” both in the “Methods” chapter [Jaeger et al., 2014a]). Deeper than ~548 m CCSF-B/WMSF, we used values calculated from the linear trendline of the downhole sonic log, though detailed correlations in this part of the core will require postcruise research.

Lithostratigraphy–downhole logging data correlation

Sediment core descriptions, whole-core physical properties measurements, and downhole logging data obtained from Site U1418 were combined in order to examine the coherence between the different data sets and evaluate the completeness of the recovered sediment record. We first compare the distribution of lithostratigraphic units with volume-corrected magnetic susceptibility measurements derived from cores on the WRMSL (see “Physical properties”) and standardized and temperature-corrected logging measurements made with the deep-reading magnetic susceptibility sensor on the triple combo tool string (see “Downhole logging”). In addition, standardized gamma ray measurements obtained from the triple combo and FMS-sonic tool strings were compared with the recovered sediment.

Figure F55 shows the correspondence between magnetic susceptibility derived from the sediment core splice appended with that derived from Holes U1418D and U1418F and the logging data obtained from Hole U1418F between 100 and 550 m CCSF-B/WMSF. In general, we note that intervals of high and low magnetic susceptibility values in the logging data correspond with similar variations in the sediment core. Good examples of this correspondence are located between 100 and 250 CCSF-B (inset diagram, Fig. F55), where low magnetic susceptibility is observed, and between 275 and 325 CCSF-B and 500 and 550 m CCSF-B, where we identify relatively high values in both data sets. At a finer scale (<10 m), we observe high magnetic susceptibility in the logging data associated with diamict and sand layers, whereas low values are observed in intervals with diatom ooze, mud with laminations, and bioturbated mud. It appears that between 430 and 475 m CCSF-B the correspondence between the core and log data is not as strong, particularly at 430 m CCSF-B, where there is a clear increase in the core magnetic susceptibility that is not seen in the log measurements. One possibility is that variations in borehole size, as revealed by caliper measurements, may influence the logged magnetic susceptibility signal. Additional postcruise work will attempt to account for these variations.

Figure F56 shows the correspondence between the lithostratigraphic units, the distribution of diatom ooze and mud with clasts, and downhole changes in K, Th, and U spectral gamma radiation and total gamma radiation. Overall, there is good agreement between gamma radiation measured on the triple combo and FMS-sonic tool strings. A primary observation is that standardized gamma ray parameters (total gamma radiation, K, Th, and U) appear to be significantly influenced by borehole dimensions (see “Physical properties–downhole logging data correlation”). Deeper than 250 m CCSF-B, borehole dimensions and downhole natural gamma ray measurements vary less, which may correspond to the lithostratigraphic Unit I–II transition at 260 m CCSF-B. Generally, gamma radiation, K, and Th are lower between 125 and 175 m CCSF-B, which corresponds to an interval where diatom-rich mud and ooze is prevalent in Unit I. At the base of the logged interval, there is a prominent increase in total gamma ray, Th, and U with a concomitant decline in K. There is no obvious change in lithology at this depth, but pore water geochemical changes within this interval indicate strong organic matter degradation (see “Geochemistry”), which may impact the logged spectral gamma radiation at the base of the borehole.

Physical properties–downhole logging data correlation

In general, there seems to be a good correspondence between data on the WMSF and CCSF-B depth scales, with a vertical offset on the order of a few meters (Fig. F57). However, between the base of the pipe and 218 m WMSF, the borehole has numerous washed-out intervals where the logging caliper measures its maximum extent (~18 inches). Both gamma ray and density logs present anomalously low values within these washed-out zones, compared to core measurements (Fig. F57). These two logging measurements are made through the detection of gamma rays from the formation; fewer gamma rays reach the tool detectors in a wider borehole, resulting in underestimation of gamma radiation and bulk density.

With the exception of the washed-out sections, the natural gamma ray log shows reasonable agreement with core NGR, with a similar range in values and similar features occurring within a few meters in depth (Fig. F57). The NGR data have been corrected for volume using GRA density (see “Physical properties”). The washouts shown in the caliper log generally correspond to lower gamma ray values in NGR data, which cannot be affected by borehole size the same way the logging data are affected. These washouts are likely partly controlled by lithology within the shallower part of the borehole, where the dominant lithology is interbedded silt and mud (see “Lithostratigraphy”). Silt or sand beds are more likely to washout during drilling, leading to thin intervals of enlarged borehole. This interpretation is supported by the lower gamma ray values measured on cores in some of the washouts (for example, between 170 and 250 m WMSF), which would be expected to correspond to a transition from mud to a more silt or sand rich formation. Deeper than this interval of frequent washouts, the gamma ray downhole logs show good agreement with the NGR data.

The density log (Fig. F57) is strongly affected by the irregular borehole size throughout much of the borehole, giving density values close to water density in washed-out intervals. However, the highest downhole density values show reasonable correspondence with maximum bulk density values in the range of the GRA density data and show good agreement with the trends in discrete MAD measurements. Even subtle trends in density deeper than ~270 m WMSF are mirrored in all three data sets within a few meters (for example, density variations between 500 and 550 m WMSF), suggesting that these are true reflections of changes in physical properties, likely corresponding to lithologic variability.

The P-wave velocity log indicates higher formation velocities than discrete P-wave core measurements over the logged interval (Fig. F57). The separation in velocity estimations increases with depth. The offset between these two measurements may be related to several factors. The discrete measurements may be biased toward lower velocity matrix material, whereas the downhole log integrates lower velocity matrix and higher velocity clasts, which were found to be a significant feature in all lithostratigraphic units described at this site (see “Lithostratigraphy”). The presence of gas in the formation may also play a critical role (see “Geochemistry” and “Physical properties”). Although gas in the formation would affect both core and downhole log velocity measurements, gas expansion in cores may have caused cracking and the development of void spaces in cores, leading to poor contact with the instrument transducers and resulting in anomalously low velocities. Despite the separation between core and log velocity estimates within the logged interval, when the general trend in P-wave velocity from downhole logs is extrapolated to the base of the drilled borehole, there is good agreement between extrapolated log velocity and discrete velocity data in the interval corresponding to lithostratigraphic Unit IV. This agreement suggests that the P-wave velocity data may be valuable for detailed postcruise correlation of core and log data with seismic images.

Magnetic susceptibility data show good agreement between the log and core measurements. Log magnetic susceptibility from the deep-reading sensor of the MSS has been corrected for the effects of tool temperature (see “Downhole logging”). Core magnetic susceptibility has been corrected for volume using GRA density, which reduced the variance in the original measurements (see “Physical properties”). General trends are reproduced in both data sets. There is a notable difference in the two methods of measurement of susceptibility between ~218 and 260 m WMSF, corresponding to logging Subunit 1B and the Subunit 1B/1C boundary. However, this difference could be related to local susceptibility differences in sedimentary features between holes, as the core susceptibility in much of this interval was measured in Hole U1418D and the logging susceptibility was measured in Hole U1418F.

Seismic sequences and correlation with lithostratigraphy and downhole logs

Two seismic lines cross Site U1418: high-resolution generator-injector gun Profile GOA3202, acquired in 2004 aboard the R/V Maurice Ewing during the site survey cruise for Expedition 341 (Fig. F58; Gulick et al., 2007), and Profile STEEP07, acquired in 2008 aboard the R/V Marcus Langseth (Fig. F59; Gulick et al., 2013). Site U1418 was primarily drilled through the regional Upper Surveyor Fan sequence (Seismic Sequence III) defined by Reece et al. (2011) and the chaotic seismic facies unit that lies unconformably below Sequence III. Here, we define this unit as seismic “Unit II*.” In preparation for integration with the core and downhole logging data, we further divided Seismic Sequence III into distinct seismic units that are defined by either changes in acoustic facies and/or regional correlative horizons associated with the Bering Channel and Aleutian Trench (see “Background and objectives”). Some of these seismic units are composed of multiple distinct internal packages, which are distinguished by a minor change in seismic character.

Generally, Seismic Sequence III on both profiles contains smooth, horizontal, parallel reflectors that dip slightly to the north, toward the Aleutian Trench and Bering Channel systems (see “Background and objectives”). The shallower strata of seismic Subunit IIIC gradually pinches out to the southwest, away from the abandoned Bering Channel (Fig. F59). Subunit IIIC also features a prominent reflection at ~5040 ms TWT that separates the more seismically transparent facies (in Profile STEEP07) at the top of the unit from the continuous horizons below the reflector. Line GOA3202 resolves an additional high-amplitude package of approximately four reflectors at ~5100 ms TWT (Fig. F58). Lithologically, these high-amplitude features could be associated with thick beds of mud separated by silt-rich intervals observed within lithostratigraphic Unit I (see “Lithostratigraphy”).

The boundary between seismic Subunits IIIB and IIIC at ~5260 ms TWT is located at the top of the youngest aggradational package that comprises the northwest flank of the Bering Channel (Fig. F59). At Site U1418, this boundary marks a subtle downsection change from higher to lower amplitude horizons on the regional seismic profile. This change in amplitude is more clearly resolved on high-resolution Profile GOA3202 (Fig. F60). Based on our traveltime-depth conversions, this boundary coincides with the boundary between lithostratigraphic Units I and II at ~257 m CCSF-B (Fig. F60). Lithologically, this boundary is defined by a change from thick beds of mud separated by silt-rich intervals (Unit I) to muddy diamict (Fig. F9I) interbedded with dark gray mud and mud with dispersed clasts (Subunit IIA).

Within seismic Subunit IIIB, two high-amplitude packages are resolved on Profile GOA3202 (Figs. F58, F60) at ~5310 and ~5410 ms TWT. Each of these packages corresponds to a subtle change in brightness within the unit on regional seismic Profile STEEP07 (Fig. F59). The boundary between lithostratigraphic Subunits IIA and IIB (~335 m CCSF-B) correlates to a reflector that lies between these two high-amplitude packages. Lithologically, this transition is characterized by a change from muddy diamict interbedded with dark gray mud (Subunit IIA) to dark greenish gray laminated mud (Subunit IIB) (Fig. F9H).

The seismic Subunit IIIB/IIIA boundary correlates with the boundary between logging Subunits 1D and 1E at ~500–510 m WMSF (Fig. F60); density and velocity log values decrease at this boundary. At the top of seismic Subunit IIIA, starting at ~5540 ms TWT, we observe both an increase in amplitude (Fig. F60) and a shift into slightly more chaotic facies (Fig. F59). Below ~5750 ms TWT, seismic reflections are less stratified and more heterogeneous. The reflections from strata here are no longer parallel, and internal truncations and lobate geometry indicate either a more energetic depositional environment or deformation.

Lithologic boundaries separating lithostratigraphic Subunits IIB–IID and Unit III are all located within seismic Subunit IIIA. Accurate correlation at this time is not possible because of uncertainties in velocity measurements downhole. The lithostratigraphic Subunit IIB/IIC boundary is defined by a change from dark greenish gray laminated mud (Subunit IIB) to dark greenish gray massive diamict interbedded with mud (Subunit IIC). The Subunit IIC/IID boundary is characterized by a change from dark gray massive diamict interbedded with mud (Subunit IIC) to dark greenish gray laminated mud interbedded with thin-bedded diamict (Subunit IID). The lithologic transition from Subunit IID to the Unit III boundary is defined as a change to laminated and bioturbated dark gray mud with interbedded sand and silt.

Seismic Unit II* is defined by chaotic seismic facies starting at ~5880 ms TWT. Some internal structure can be observed within the upper ~100 ms TWT of Unit II* (Fig. F60). The transition out of this structure into the noncoherent facies deeper may be equivalent to the lithostratigraphic Unit III/IV boundary. Lithologically, this boundary is marked by a transition from laminated and bioturbated mud with interbedded sand and silt to mud and muddy diamict characterized by soft-sediment deformation and intrastratal contortions, and seismically this unit is interpreted to mark the upper portion of a MTD.

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