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doi:10.2204/iodp.proc.338.106.2014

Core-log-seismic integration

Site C0021 is located ~2 km northwest of Site C0018, where coring during Expedition 333 characterized the slope basin sedimentary succession as hemipelagic deposits comprising several MTDs (Expedition 333 Scientists, 2012b; Strasser et al., 2012). LWD and coring at Site C0021 during Expedition 338 aimed to further investigate the MTDs in a more proximal site with respect to the interpreted mass transport direction (Strasser et al., 2011) (Figs. F3, F4, F40). LWD data were collected in Hole C0021A from the seafloor to a TD of 294.0 mbsf, and cores were recovered from 0 to 5.9 and 80.0 to 194.5 mbsf in Hole C0021B. Because of operational time constraints, cores from Hole C0021B were not analyzed on board the ship during Expedition 338; they were analyzed during a shore-based core opening, description, and sampling party at the JAMSTEC KCC.

The Kumano 3-D prestack depth migration (PSDM) seismic volume (Moore et al., 2009) ties to Hole C0021A at the intersection of In-line 2269 and Cross-line 5094. The seismic data in this region are characterized by several packages of chaotic, low-amplitude reflectivity separated by moderate to high-amplitude negative polarity reflections, which are sometimes truncated in the updip direction (Strasser et al., 2011) (Fig. F40). Figure F41 presents an overview of the correlation between LWD data and lithostratigraphy from Holes C0021A and C0021B, respectively, and seismic data.

According to seismic stratigraphic mapping and previous drilling and coring in the slope basin seaward of the megasplay fault zone (Screaton et al., 2009; Expedition 333 Scientists, 2012b; Kimura et al., 2011; Strasser et al., 2011), a single unit was defined from analysis of both core samples and LWD data (Unit I [slope basin]; see “Lithology” and “Logging while drilling”). Two lithologic subunits were identified based on core data, whereas three subunits were identified from the LWD data (Fig. F41).

Lithologic Subunit IA is composed of mottled greenish gray silty clay with local thin interbeds of fine sand and ash layers. Subunit IB is composed of a succession of thin sand beds interbedded with silty clay and local ash layers. The Subunit IA/IB boundary is defined in cores at 176.16 mbsf as the first downhole occurrence of repeating sand beds with spacing of 20–30 cm. This boundary correlates to the logging Subunit IA/IB boundary, characterized by a change in the resistivity log character (lower to higher amplitude variability above and below, respectively). This boundary also reflects an abrupt change in bedding dip between an overlying well-defined zone of high-angle bedding, observed in resistivity images (see “Logging while drilling”) and measured in cores (see “Structural geology”), and an interval of predominantly low to moderately dipping bedding planes below. In the seismic data, this boundary correlates to a low amplitude, positive reflection within the uppermost part of seismic Subunit 1b (Strasser et al., 2011). Below this reflection, and within seismic Subunit 1b, a more prominent high-amplitude negative polarity reflection is observed at ~205 mbsf, which may correspond to a distinct ~35 gAPI decrease in the gamma ray curve, as well as a local minimum in resistivity (Fig. F41).

Within lithologic Subunit IA, two MTD intervals have been identified in Hole C0021B cores based on the occurrence of mud clasts, chaotic bedding, and steeply dipping beds (see “Lithology”); an increase in shear strength and a decrease in porosity (see “Physical properties”); and the occurrence of shear zones/faults (see “Structural geology”) (MTD A: 94.16–116.57 mbsf; MTD B: 133.76–176.16 mbsf [see “Lithology”]). Distinct log character in the lower part of logging Subunit IA (144.0–176.8 mbsf) appears to correlate to MTD B (Fig. F41), which also matches a zone of low-amplitude reflectivity observed in the seismic data at this interval and is inferred to represent a large, regional MTD (Strasser et al., 2011). The top of MTD B is defined in the cores by a thin cogenetic turbidite overlying chaotic bedding intervals containing mud clasts. This boundary may correlate to the logging data at ~144 mbsf, characterized by a slight downsection increase in both resistivity and natural gamma ray values. This boundary appears to tie to a prominent reflection in the seismic data (Figs. F40, F41).

In logging data, moderate to high bedding dip angles are also present between 97 and 130 mbsf, but these are more variable, within a larger range (20°–75°). The top part of this interval appears to correlate with MTD A identified in cores from Hole C0021B and also matches the chaotic, low-reflective character of the seismic data in this interval (Fig. F41).

The equivalent depth of the logging Subunit IB/IC boundary, defined by a distinct drop in resistivity values and a change in the resistivity log character (from higher to lower amplitude variability above and below, respectively), has not been cored. This boundary, however, appears to correlate with a high-amplitude, negative polarity reflection observed at ~280 mbsf and may reflect the transition from seismic Subunit 1b (sand-rich slope basin) to seismic Subunit 1c (lower slope basin) (Strasser et al., 2011).

Comparison with Site C0018

Site C0021 is located ~2 km northwest of Site C0018 and is situated in an upslope position, closer to the scarp surface for the MTDs (Strasser et al., 2011, 2012). Based on the spatial coverage of the MTDs predicted from the seismic data, it should be possible to correlate events between the two sites. MTD A identified in Hole C0021B and the upper zone with chaotic bedding observed in Hole C0021A does not have the same characteristics as MTD A identified from structural analysis of the resistivity image data in Hole C0018B (see “Logging while drilling” and “Core-log-seismic integration,” both in the “Site C0018” chapter [Strasser et al., 2014c]), which was postulated to correspond to several of the smaller MTDs observed in Hole C0018A cores (see “Core-log-seismic integration” in the “Site C0018” chapter [Strasser et al., 2014c]; Expedition 333 Scientists, 2012b). The mismatch between the upper sections of these sites is corroborated by the seismic data (Fig. F40), which show that the package of low reflectivity at the corresponding depth at Site C0021 is truncated to the southeast and does not extend to Site C0018.

In contrast, core observation and analyses of MTD B in Hole C0021B and the character of the MTD B correlative interval in the logging data (144–176.8 mbsf in Hole C0021A) is very similar to MTD 6 of Hole C0018A and correlative to the logging interval between 103 and 170 mbsf in Hole C0018B (Fig. F40). This MTD has been traced throughout the Kumano 3-D seismic volume (Strasser et al., 2011, 2012). The base of the MTD does not remain within the same stratigraphic horizon and suggests that the processes associated with MTD emplacement at Site C0021 eroded deeper into the underlying strata than at Site C0018. The reflection defining the top of the MTD, in contrast, is well correlated throughout the seismic data set. Using seismic stratigraphy and bio-, magneto-, and tephra-stratigraphic age constraints from Site C0018, this MTD is inferred to have emplaced between 1.05 and 0.85 Ma (Strasser et al., 2012).

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