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Core-log-seismic integration

Hole C0012H is located ~30 m southeast of Hole C0012A; ~40 m south of Holes C0012C–C0012E; and ~40 m east of Holes C0012F and C0012G (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012c). Cores recovered from Holes C0012A–C0012E provided detailed lithologic information for the Shikoku Basin sedimentary sequence from 0 to 530 mbsf. Intermittent recovery from Holes C0012A, C0012E, C0012F, and C0012G also provided some lithologic information about the underlying basement rock of the Philippine Sea plate starting at 530 mbsf.

The upper and lower Shikoku Basin strata of Ike et al. (2008) were correlated with seismic Units A–C and D–F identified on Institute for Research on Earth Evolution (IFREE) 3-D prestack depth migration (PSDM) (Park et al., 2008) In-line 95 by Expedition 322 Scientists (2010c). Seismic Unit G represents the basement, and although some samples were recovered from coring during Expeditions 322 and 333, correlation with logging data is not attempted here. Instead, the seismic and lithologic unit correlation (predominantly Hole C0012A) is revisited with integration of LWD data from Hole C0012H. Mismatches between the depths of the seismic unit boundaries and the logging or lithologic unit boundaries are predominantly a result of errors in the velocity field used to generate the 3-D PSDM data. Figure F12 presents an overview of the correlation between the LWD, seismic, and core data. The results are also summarized in Table T4.

Seismic Units A and B were first defined at Site C0011 and then extrapolated to Site C0012 (Expedition 322 Scientists, 2010c). The boundary between the two seismic units is unclear at Site C0012 but is interpreted to be at ~70 mbsf. The seismic character of the A–B interval (0–120 mbsf) exhibits low-amplitude reflectivity with some irregular and discontinuous reflectors. This interval, cored in Hole C0012A, shows a dominant lithology of hemipelagic mudstone with a few volcanic ash/tuff beds, although no cores were taken from 0 to 60 mbsf. The shallow section was cored in Holes C0012C and C0012D (Expedition 333 Scientists, 2012c), where a large number of thin volcanic ash/tuff beds were recovered within the background hemipelagic mudstone. The change in abundance of volcanic ash/tuff layers, many within the topmost 60 mbsf and few from 60 to 150 mbsf (Fig. F12), could represent the seismic Unit A/B boundary. The corresponding logging unit defined in Hole C0012H (logging Unit I, 0–144.3 mbsf) does include a slight increase in average gamma ray value at ~70 mbsf, but the correlation is uncertain.

Logging Unit II (144.3–188.1 mbsf) has a greater variability in gamma ray values (between 35 and 95 gAPI). The baseline for Hole C0012H was set at 95 gAPI (see “Logging while drilling”), so the variations represent lows with a characteristic thickness of ~5 m. This interval corresponds to lithologic Unit II in Hole C0012A, which contains several volcanic sandstone beds (each ~5 m thick), and seismic Unit C, which contains a series of high-amplitude, continuous reflections. The top of this section is also marked by a small step increase in sonic velocity in Hole C0012H from 1650 to 1750 m/s (Fig. F4). High density values and P-wave velocities (1.9 g/cm3 and 2000 m/s, respectively) were also reported from Hole C0012A sandstone samples (Expedition 333 Scientists, 2012c). The density and velocity measurements from both cores and logging data show a good correlation with the observation of strong reflections between 120 and 200 mbsf in the IFREE seismic data (Fig. F12).

Lithologic Units III and IV (215–415 mbsf) comprise hemipelagic mudstone and hemipelagic mudstone with interbedded siltstone turbidites, respectively. Seismic Unit D at the corresponding depth interval of 200–405 mbsf is transparent (i.e., no strong reflections), indicating no major impedance contrasts, as would be expected for a hemipelagic section. There is evidence in the LWD data for the increasing number of siltstone turbidites between 330 and 415 mbsf (lithologic Unit IV) from spikes in gamma ray to low values (~30 gAPI) within logging Subunit IVA (339.6–372.1 mbsf). In contrast, no distinction was found in Hole C0012A cores to match the high gamma ray (>100 gAPI) and resistivity (>1 Ωm) of logging Subunit IVB.

Logging Units V and VI (403.3–530.3 mbsf) show a similar pattern of increasing gamma ray values with increasing depth from 75 to 100 and 65 to 90 gAPI, respectively (Fig. F12). Lithologic Unit V (415.58–528.51 mbsf) contains a series of sandstone turbidites overlying a series of volcanic sandstones, all within a background of hemipelagic mudstone. An increase in the ratio of mudstone to volcanic sandstone or sandstone turbidite with depth could explain the observed increases in gamma ray. Although the lithologic column does not match this hypothesis, incomplete core recovery may have resulted in some interpolation, especially at the base of lithologic Unit V (Expedition 322 Scientists, 2010c). Seismic Unit E contains a series of strong reflections between 405 and 530 mbsf, corroborating the observation of interbedded sandstones/turbidites and hemipelagic mudstone.

Lithologic Unit VI (~530–537 mbsf) was identified as a thermal alteration of the hemipelagic mudstone, resulting from contact with the basement rock of lithologic Unit VII (seismic Unit G, logging Unit VII). Although not picked as a discrete logging unit, there is evidence for a 3–4 m thick layer with P-wave velocities of ~2050 m/s directly overlying the basement in Hole C0012H. Finally, the basement seismic character is a consistently low frequency below ~530 mbsf.