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doi:10.2204/iodp.proc.338.107.2014

Core-log-seismic integration

Site C0022 is situated at the tip of the megasplay fault, ~300 m southeast of Site C0004, and ~500 m northwest of Site C0008 (Fig. F20). LWD data from Hole C0022A and cores from Hole C0022B document the shallow extension of the megasplay fault through the slope sediment. The Kumano 3-D prestack depth migration (PSDM) seismic volume (Moore et al., 2009) ties to Site C0022 at the intersection of In-line 2675 and Cross-line 5268. Figure F45 presents an overview of the correlation between the LWD, seismic, and core data.

Only a single unit was defined from analysis of both core samples and LWD data (“Lithology” and “Logging while drilling”). Two subunits were identified based on core lithology, whereas three subunits were identified from the LWD data (Fig. F45); however, none of these boundaries are coincident.

The dominant lithology of cores from Hole C0022B is silty clay, with thin interbedded layers of sand and volcanic ash. The lithologic Subunit IA/IB boundary (383.5 mbsf) is located at the top of several layers of gravel that are not observed elsewhere in the drilled interval at this site. This boundary corresponds to a discrete decrease in gamma ray values from ~75 to 60 gAPI as well as to a low-amplitude reflection in the seismic data at ~400 mbsf (Fig. F45).

A positive reflection is observed at 35 msbf, and it is possible that this correlates to a sandy layer observed in the core from Hole C0022B (Fig. F45), although there is no clear signal in the LWD data. The logging Subunit IA/IB boundary instead correlates with the top of a package of sand layers in the core lithology, although the seismic data are reflectively transparent at this depth.

Several shear zones and microfaults were observed in cores between ~75 and 150 mbsf (see “Structural geology”). The lack of core recovery between 95.5 and 99.5 mbsf despite overall excellent recovery from 20 to 123 mbsf may be indicative of a highly deformed zone. The depth of this interpreted deformation zone correlates with subtle offsets in the seismic reflections at this depth (Fig. F45), suggesting that it may represent the updip extension of the megasplay fault. A zone of high deformation and fracturing was also identified in the LWD resistivity images between 85 and 115 mbsf, centered about a low-resistivity fracture at ~100 mbsf (Fig. F4).

A package of strong reflections between 150 and 220 mbsf corresponds to a zone of higher variability in resistivity and some broad intervals with locally low gamma ray values (176–182 and 192–205 mbsf) (Fig. F45). No cores were collected between 156.8 and 190.5 mbsf, so in this interval seismic-lithology correlation is not possible.

Finally, a strong, negative reflection at 300 mbsf may be correlated to a thick sand layer at Site C0008 and thin sand layers at Site C0004 (Kimura et al., 2011; Strasser et al., 2011). It is possible that this reflection also correlates with sand or volcanic ash layers identified in the core from Hole C0022B (Fig. F45). Gamma ray values show no major change, although a pair of high gamma ray spikes at 295 and 302 mbsf may correlate with two of the volcanic ash layers identified in the core.