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doi:10.2204/iodp.proc.319.103.2010

Cuttings-Core-Log-Seismic integration

Seismic velocity structure and well tie

We combined the zero-offset VSP (check shot) with wireline sonic velocity to derive a velocity-depth function at Site C0009 (Fig. F109). Check shot measurements only extend to 1137 m WMSF (red squares, Fig. F109). However, wireline sonic data extend to ~1600 m WMSF (black line; Fig. F109) (refer to "Logging and data quality" for a discussion of different depth references). To derive the velocity structure from the seafloor to 2000 m WMSF, we extended check shot measurements with five velocity zones estimated from the wireline sonic data (green dashed line, Fig. F109). The velocities that we used (green dashed line) are lower than those used for seismic processing prior to the cruise (black dashed line, Fig. F109). Figure F110 illustrates one-way traveltime versus depth below the seafloor. Red squares indicate check shot measurements. We use a time-depth model (green dashed line) that combines check shot data and wireline sonic data. For the drilled sequence (below 712.7 m) the traveltimes are greater below 1000 m WMSF in our time-depth correlation (green dashed line) than was estimated from precruise seismic processing (black dashed line) because of the lower velocities obtained from check shots than those used to process the seismic data. This is largely due to the low-velocity layer present at ~1200 m WMSF in the wireline sonic data (Fig. F109). This layer corresponds to a zone of increased free gas in lithologic Unit III (see below; also see "Geochemistry" and "Physical properties").

We correlate the wireline logging, cuttings, and core data to time-based seismic data (Figs. F111, F112). We shifted the time-based seismic data 50 ms upward so that the seafloor reflector correlated to a two-way traveltime (TWT) of 2720 ms, the TWT inferred from the check shot data for the seafloor. Four references for our correlation are presented: WRF, WMSF, SSF, and TWT. The correlation between time and depth is also presented in C0009_T1.XLS in CCLS in "Supplementary material."

Seismic Surface S1 is a prominent reflector within Unit II that corresponds to a depth of ~550 m WMSF (see "Background and objectives;" Fig. F111). There are no wireline sonic data in this interval. Seismic Surface S2 correlates to ~750 m WMSF, 40 m above the interpreted Unit II/III boundary and near the top of the depth range for which we collected wireline log data (Figs. F112, F113). Compressional velocity from the sonic log (V_P) and bulk density (ρ_b) are shown to the right of the seismic data; these are used to compute the impedance (the product of velocity and density). The amplitude and sign of the seismic reflection are proportional to the impedance contrast for a vertically incident wave. In this part of the borehole, the obvious surface for a significant reflection lies at the Unit II/III boundary, where there is a drop in velocity and density, which results in a marked decrease in impedance (Figs. F112, F113). It is not clear why the Surface S2 reflector lies above the Unit II/III boundary. One interpretation is that there is an abrupt impedance contrast close to the base of the casing that is causing the strong reflection (~730 m WMSF) but was not imaged by the logs because of the presence of the casing. Another possibility is that Surface S2 is actually slightly deeper than mapped in the region around Site C0009, where the geometry of seismic reflections within Units II and III is somewhat complicated. A third possibility is that the check shot tie is not correct at this depth.

There are several very bright reflections between 3700 and 3900 ms within Subunit IIIA, but the unit is generally less reflective than overlying units (Figs. F111, F112). In Subunit IIIB, density shows minor change; however, V_P decreases significantly (Figs. F112, F113). The compressional velocity to shear velocity ratio (V_P/V_S) is also low in this zone (see "Physical properties"). Elevated methane concentrations derived from mud gas measurements correlate closely to this low-velocity zone (Fig. F114), and abundant wood fragments are found just below the first appearance of the low-velocity zone (Fig. F114). All of these data imply that this is a zone where free gas is present, and preliminary calculations suggest gas saturations reach ~10% (see "Physical properties"). The upper part of Subunit IIIB has a relatively transparent character that corresponds to a zone with uniformly low velocity (Figs. F114, F115). The transparent character may result from lack of any contrast in impedance and/or attenuation due to the presence of gas.

Unconformity UC1 is a prominent positive (blue) reflection that lies at the base of the low-velocity zone within Subunit IIIB (Figs. F111, F112, F115). Close analysis of P-wave velocity, resistivity, V_P/V_S ratios, and Poisson's ratio suggests a significant decrease in gas across the unconformity (Figs. F75, F76). Unconformity UC1 truncates underlying reflectors and can be locally correlated laterally, although biostratigraphic analysis was unable to resolve the presence of a hiatus, if present. We therefore interpret Unconformity UC1 to be an erosional unconformity with limited missing section.

Finally, Unconformity UC2 lies at the Unit III/IV boundary, where there is an increase in impedance (Fig. F112). The Unconformity UC2 surface actually lies slightly above the Unit III/IV boundary (Fig. F112) after applying the corrected velocity model, but this is likely to be a function of an imperfect velocity model and we would expect the surface and unit boundary to be coincident. The boundary is marked by a decrease in density and an increase in velocity; the net result is an increase in impedance with depth. XRD analysis of cuttings samples records an increase in clay fraction across this boundary (Fig. F114), and SP values decrease at the same interface. The increase in clay fraction may drive the decrease with depth in density and SP.

Seismic facies

Each lithologic unit (see "Lithology") is imaged as a distinct seismic facies. Lithologic Unit I is characterized by laterally continuous, high-amplitude seismic reflections (Fig. F115). These reflections are largely parallel; however, in the upper part of the section, they converge and onlap to the south-southeast as Unit I thins (Fig. F115). Lithologic Unit I is defined as interbedded sand and mud as inferred from the gamma ray MWD log (see "Lithology"). We interpret that the laterally continuous reflections are caused by the abundant 10–40 m thick sand cycles recorded in the gamma ray log. The lateral continuity of the reflections suggests that these units are well stratified and laterally extensive. The present water depth is >2000 m, and we interpret that Unit I was also deposited in deep water as turbidites.

Reflections in lithologic Unit II have lower frequency and slightly lower amplitude than those in overlying Unit I (Fig. F115). The reflections are not laterally continuous, and many intersect each other when traced laterally. There is one prominent and laterally continuous reflector in the upper third of lithologic Unit II that we refer to as seismic Surface S1. Although not apparent in Figure F115, Surface S1 is an onlap surface; overlying reflections terminate on this reflector to the south-southeast (apparent direction). The overall low amplitude in Unit II is broadly consistent with the interpretation that it is dominated by silty clay with silt and sand interbeds (see "Lithology").

Lithologic Unit III is a relatively transparent seismic facies (Fig. F115) with a few high-amplitude reflectors that correlate with the presence of gas (see above) in this unit. Surface S2 overlies the Unit II/III boundary by ~50 m (Figs. F112, F113, F115). It truncates underlying reflectors, and overlying seismic reflectors downlap onto it with an apparent direction from south-southeast to north-northwest (Fig. F115).

Above Unconformity UC1, which lies near the base of Unit III, the seismic data are largely transparent, whereas below Unconformity UC1, high-amplitude reflectors record a distinct stratified new tilted basin (Fig. F115).

Unconformity UC2 lies at the base of Unit III; it separates reflective strata above from much less reflective but more chaotic and discontinuous reflectors below. Reflections within Unit III onlap Unconformity UC2 with an apparent north-northwest–south-southeast direction. Lithologic Unit IV consists mainly of thin-bedded, fine-grained turbidites (see "Lithology"). Unit IV is more lithified and has more clay than overlying Unit III. It also contains interbedded mudstone and silt/sandstone interbeds, and dip increases downhole within this unit. Gamma ray values in Unit IV increase downhole, suggesting increased clay content, which is supported by XRD measurements (see "Lithology").

Correlation between Sites C0009 and C0002

We correlated Unconformity UC2, seismic Surfaces S1 and S2, and representative downlapping Reflectors S-A and S-B between Sites C0009 and C0002 (Table T22; Fig. F116). Surface S1 is a prominent reflection that carries laterally across the basin. Evidence of onlap is found in the vicinity of Site C0009 (Figs. F115, F116); however, it is difficult to see clear evidence of truncation or onlap to the south-southeast in the vicinity of Site C0002. At Site C0009, Surface S1 lies within the mudstone of lithologic Unit II at 620 m WMSF. No cuttings were recovered at this depth, and the lithologic unit is interpreted from MWD gamma ray logs only. At Site C0002, Surface S1 is shallower, at only ~20 m WMSF, where it correlates with interbedded mud and ash with an age <0.5 Ma (Ashi et al., 2009) (Fig. F116).

Reflectors S-A and S-B downlap onto Surface S2 near Site C0009 (Fig. F116B), and cuttings samples acquired from this depth interval (~710 m MSF) have an age younger than ~0.9 Ma (see "Biostratigraphy") (Fig. F116B). At Site C0002, seismic Reflectors S-A and S-B lie at ~85 and 92 mbsf, respectively, and the Unit I/II boundary, at ~134 mbsf and below Reflectors S-A and S-B, is interpreted to have an age of ~0.9–1.0 Ma (Ashi et al., 2009) (Fig. F116B).

Seismic Surface S2 is a prominent seismic boundary that separates laterally continuous downlapping reflections above from a more acoustically transparent unit of variable thickness below (Fig. F116). Downlap proceeded with an apparent direction from south-southeast to the north-northwest. The apparent downlap angle is steepest to the south-southeast (near Site C0002) and gradually flattens to the north-northwest (toward Site C0009). We expect that sediments above Surface S2 at Site C0009 will be younger than sediments above Surface S2 at Site C0002 because of this downlap relationship.

The seismic character of the material between Surface S2 and Unconformity UC2 can be traced between Sites C0009 and C0002, but individual surfaces are not easily interpretable, making specific correlations more difficult. Units III and IV, which lie below Surface S2, also have more variable seismic stratigraphic character than overlying units. At Site C0002, Unit III is a homogeneous mudstone that was interpreted as a condensed section of basal/early forearc basin or slope apron origin (Tobin et al., 2009b; Ashi et al., 2009). The age of Unit III at Site C0002 ranges from 1.7/2.0 to 3.65 Ma (Fig. F116B, inset). At Site C0002, the Unit II/III boundary lies ~35 m above seismic Surface S2, and the Unit III/IV boundary lies slightly below Unconformity UC2 (Tobin et al., 2009b; Ashi et al., 2009) (Fig. F116B, inset). At Site C0009, Unit III ranges in age from ~0.9 to 3.8 Ma and is five times the thickness (~500 m versus ~100 m) of Unit III at Site C0002, although it spans only twice the duration. Subunit IIIB at Site C0002 is approximately coeval with Unit III at Site C0002. Unit III at Site C0002 is composed of mudstone, with minor volcanic ash. However, Unit III at Site C0009 has some silt and ash beds. Unit III at Site C0009 is also distinguished from Unit III at Site C0002 by the presence of terrigenous organic matter. Unconformity UC2 is significant: age gaps of 1.4 and 1.8 m.y. are present at Sites C0002 and C0009, respectively (Fig. F116B, inset). Below Unconformity UC2, sediments are slightly older at Site C0009 (5.59 Ma) than at Site C0002 (5.04 Ma). The hiatus between deposition of Units IV and III is of approximately equal age and duration at both Sites C0009 and C0002, implying a tectonic event of regional significance at this time.

Unit IV at Site C0002 is considerably more deformed than overlying units based on observations from cores and seismic reflection data and is interpreted as accretionary prism material (Ashi et al., 2009; Tobin et al., 2009b). It also exhibits limited evidence of carbonate deposition (e.g., CaCO₃ tests), suggesting deposition below the CCD. In comparison, Unit IV at Site C0009 is only mildly deformed (see "Structural geology"), and calcareous microfossil tests are present, indicating deposition above the CCD. We interpret Unit IV at Site C0009 as either frontally accreted trench sediment or as slope sediment deposited on the accretionary wedge or part of the earliest Kumano Basin.

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