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doi:10.2204/iodp.proc.338.105.2014

Core-log-seismic integration

Hole C0018B is located ~25 m north-northwest of Hole C0018A (Expedition 333 Scientists, 2012a). Site C0018 is the primary site proposed in the Nankai Trough Submarine Landslide History ancillary project letter to investigate MTDs within the shallow slope cover of the Nankai accretionary prism (Strasser et al., 2011, 2012). Cores recovered from Hole C0018A during Expedition 333 (Expedition 333 Scientists, 2012a) provided detailed lithologic information for the slope cover sedimentary sequence and intercalated MTDs from 0 to 314 mbsf. LWD to 350 mbsf was added during Expedition 338, providing new data for characterizing in situ internal structure and properties of MTDs and for core-log-seismic integration.

The Kumano 3-D prestack depth migration (PSDM) seismic volume (Moore et al., 2009) ties to Site C0018 at the intersection of In-line 2315 and Cross-line 4950. A prominent seismic reflection was identified as the boundary between two lithologic/logging subunits of differing seismic character, defined by Kimura et al. (2011) and Strasser et al. (2011) as seismic Subunits IA and IB. The correlative two subunits were also identified at Site C0018 following analysis of the core and LWD data from Holes C0018A and C0018B, respectively. Figure F12 presents an overview of the correlation between the LWD, seismic, and core data.

Seismic Subunit IA is characterized by several packages (generally <100 m thick) of chaotic, low-amplitude reflectivity. These packages were originally discovered from analysis of the seismic data in the region of scarps identified in the high-resolution bathymetry data and are interpreted as MTDs (Strasser et al., 2011).

The dominant lithology of lithologic Subunit IA is greenish-gray silty clay. Variability in the abundance of volcanic ash was used to further divide this subunit into individual facies (Expedition 333 Scientists, 2012b). Analysis of X-ray computed tomography images and structural measurements highlighted a number of contorted, folded sections, providing evidence for a total of six intervals with characteristics of MTDs, the largest being an ~60 m interval from 128 to 188 mbsf (Fig. F12).

Resistivity images of logging Subunit IA (Fig. F12) corroborate observations from the core and seismic data. Generally, bedding dip angles were found to be low to moderate (<20°) except for two intervals that displayed high-angle (>60°) bedding surfaces (many nonplanar) with variable dip direction, indicative of large-scale deformation. The upper interval (61.5–83.0 mbsf) only had a few clear bedding planes at its base, with its upper surface defined by the disappearance of coincident high-angle fractures and the reappearance of low-angle (<10°) bedding planes. The lower interval (127–176 mbsf) is more clearly defined, with high-angle bedding surfaces throughout. These two intervals correspond to the locations of the two largest MTDs (5 and 6) that were identified from cores.

Although both intervals correlate with the core and seismic data and are therefore identified as MTDs, they display different characteristics in resistivity. The upper MTD (5) has a low and relatively constant resistivity of ~1.0 Ωm, whereas resistivity in the lower MTD (6) shows an overall increase with depth but fluctuates between 0.5 and 1.5 Ωm (Fig. F12). The higher variability in resistivity of the lower MTD might have resulted from a larger amount of internal deformation during transport, occurring as a function of its larger size and greater transport distance.

A positive seismic reflection correlates to the base of MTD 6 at 188.5 mbsf in Hole C0018A, defined by a thick volcaniclastic sand layer, and also ties well with the change to sand-dominated lithology documented in cores (i.e., Subunit IA/IB boundary). The base of logging Subunit IA was picked at the slightly shallower depth of 179.8 mbsf based on the departure of the resistivity curve to lower values and an increase in its short wavelength variability (Fig. F12).

However, there is contrasting evidence from the gamma ray curve for distinct sand packages below ~205 mbsf (Fig. F12), which may correlate to the first downhole occurrence of Subunit IB sand layers in Hole C0018B. Thus, correlation of the Subunit IA/IB boundary between Holes C0018A and C0018B is not fully conclusive. The mismatch could be explained by the distinct geometry of the MTD. Interpretations of the 3-D seismic volume show that this MTD had a south-southwest flow direction with lateral ramps that developed near Site C0018 (Strasser et al., 2011). The short lateral offset between holes may lead to Hole C0018A being situated on the southeast edge of the deposit and Hole C0018B being located within the main flow area, where the basal part of the MTD would represent eroded or off-scraped material incorporated into the MTD. For further discussion on regional correlation of the base of MTD 6, see “Core-log-seismic integration” in the “Site C0021” chapter (Strasser et al., 2014c).

Lithologic Subunit IB comprises interbedded fine-grained to medium sand, silty sand, silty clay, and clay (Expedition 333 Scientists, 2012b). Distinct sand layers are not distinguishable in logging data, and overall, the gamma ray values for logging Subunit IB follow a higher baseline (85 gAPI) compared to logging Subunit IA (75 gAPI) with only a few clear spikes to lower gamma ray values (<60 gAPI; see “Logging while drilling”). The mismatch between core lithology and LWD data characteristics may occur because the thin sand layers (4–20 cm) are significantly smaller than the vertical resolution of the gamma ray tool (~30 cm, see the “Methods” chapter [Strasser et al., 2014a]), so the dominant signal is that of the muddy sediment, which locally is as thick as 4 m (Expedition 333 Scientists, 2012b).

Further subdivision of logging units was based on the resistivity data, which show two breaks in the overall high variability at ~231 and ~293 mbsf. Although there is no clear tie between these boundaries and the core lithology, it is possible that they correlate with reflections in the seismic data at these depths (Fig. F12). A third reflection at ~265 mbsf may also correlate with reduced variability in resistivity.

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