Site 1149 is located on the Pacific plate in the Nadezhda Basin southeast of Japan. It resides on a slight bathymetric high ~100 km east of the Izu-Bonin Trench where the Pacific plate is flexed upward before it enters the subduction zone (see "Background and Objectives" and Fig. F4). Nakanishi et al. (1992) charted magnetic lineations in the Site 1149 area that are part of a long, continuous reversal sequence within the Kashima fracture zone compartment, which includes Site 801 ~2200 km to the southeast (Fig. F5). This sequence ranges in age from ~125 to 180 Ma and becomes monotonically older from Early Cretaceous magnetic lineation M5 in the Nadezhda Basin near the Japan Trench, southeast through the Jurassic magnetic quiet zone in the Pigafetta Basin. This lineation pattern is part of a larger set of magnetic lineations east of Japan and north of Shatsky Rise called the Japanese magnetic lineations. These lineations were accreted to the northern boundary of the Pacific plate at a location near the paleoequator (Larson and Chase, 1972).
Magnetic anomalies are strongly lineated in a northeast-southwest direction (050°) in the vicinity of Site 1149. Figure F6 shows the majority of magnetic anomaly data collected by ships in the area superimposed on the predicted bathymetry. A fracture zone with a 5-10 km, right-lateral offset transects the area from north-northwest to south-southeast. This may not be an original feature of the seafloor spreading fabric as it is markedly nonorthogonal to the magnetic lineations, although it is much more orthogonal to the magnetic lineations farther south (Nakanishi et al., 1992). Thus, in the Site 1149 area the fracture zone may result from recent, northward propagation of an original transform fault because of bending stresses at the outer bulge of the subduction zone.
It appears that Nakanishi et al. (1992) identified magnetic Anomaly M12 in the vicinity of Site 1149 on their magnetic lineation chart, although that specific correlation is not described. Close inspection of the magnetic lineation pattern within the vicinity of Site 1149 suggests instead that Site 1149 lies on the older portion of magnetic Lineation M11 (Fig. F6). Using the magnetic reversal model of Channell et al. (1995), we generated the magnetic anomaly model profile with a constant half-spreading rate shown in Figure F6. The skewness (cross-sectional shape) of the profile is in accord with the original calculations of Larson and Chase (1972) and subsequent ones by Larson and Sager (1992).
We consider this model to be a good fit to the charted magnetic anomalies, although the width of M11 on the chart is slightly less than in the constant spreading-rate model. The constant spreading rate of 51 km/m.y. is consistent with other portions of the Japanese lineation pattern of this age. The shapes of the magnetic anomalies are also in reasonable agreement with the model. In contrast, if we assume that the positive magnetic anomaly over Site 1149 is M12, such a model would require a much slower spreading rate between Site 1149 and an assumed M11 to the north and, conversely, a much faster spreading rate between Site 1149 and an assumed M13 to the south. The required spreading-rate change would be more than a factor of 2. Also, the short polarity intervals within such a model would bias the shapes of the model anomalies in ways that would be poorer matches to the charted data. Thus, we prefer the model solution shown in Figure F6; however, we cannot completely exclude the other solution where Site 1149 and magnetic Anomaly M12 are coincident.
Inspection of the location of Site 1149 relative to the magnetic reversal model suggests that Site 1149 lies on the older half of the reversed polarity interval M11. This stratigraphic position has a late Valanginian biostratigraphic age and a radiometric age of ~132 Ma on the Channell et al. (1995) time scale.
Selection for Site 1149 was initially based on two multichannel seismic (MCS) lines and sonobuoy data obtained during Cruise C2005 of the Robert D. Conrad in 1976. A short, single-channel seismic (SCS) and 3.5-kHz survey was conducted on approach to Site 1149 that confirmed the general seismic character and unit thicknesses observed in the MCS records. Holes 1149A, 1149B, and 1149C (near proposed site BON-10) are located at the intersection of JOIDES Resolution (JR) SCS Lines 1 and 3 just 3.5 km northwest of the proposed site location (Figs. F3, F7). Hole collapse in the sedimentary section and drill-string sticking problems were jeopardizing our objectives of further basement penetration and prompted us to place Hole 1149D at a location where the sediment section is significantly thinned. Following a short 3.5-kHz survey, Hole 1149D was placed at a promising location on C2005 MCS Line 39, ~5 km southeast of Holes 1149A, 1149B, and 1149C (see "Seismic Reflection Profiling" in "Site Geophysics" in the "Explanatory Notes" chapter).
Ewing et al. (1968) originally defined the acoustic stratigraphy of large portions of the western Pacific as consisting of two or more of the following four seismic units: (1) an upper transparent layer (weakly reflective), (2) an upper opaque layer (highly reflective or well stratified), (3) a lower transparent layer, and (4) acoustic basement. Acoustic basement has been referred to as "Horizon B," the "deep opaque layer," and the "reverberant layer" and is characterized by an interval of flat-lying, smooth, high-amplitude, closely spaced reflections (Ewing et al., 1968; Houtz et al., 1970; Heezen, MacGregor, et al., 1973; Houtz and Ludwig, 1979). The reverberant nature of Horizon B is the result of a trailing bubble-pulse oscillation creating multiple reflections from each large impedance contrast and making it difficult to distinguish a relatively simple single interface (crust/sediment) from a sequence of stratified material. Reprocessing of the 1976-vintage MCS data included prestack predictive deconvolution, which significantly reduced the bubble-pulse amplitude but did not completely remove this often misleading artifact (Fig. F8). The use of water guns during the JR-185 SCS survey provided an implosive, bubble pulse-free source that produced a much higher resolution record of the sedimentary sequence and a clear image of the top of oceanic crust.
Early DSDP investigations (e.g., Legs 6, 7, 17, and 20) revealed the lithostratigraphic significance of the upper transparent and opaque layers in the western Pacific. The upper transparent layer corresponds to a variety of lithofacies: pelagic clay with ash in the west Pacific, pelagic clay in the central Pacific, turbidite sequences in the north and east, and biogenic oozes along the equator. The upper opaque layer has been correlated to the uppermost abundant chert in much of the north Pacific; however, DSDP Leg 32 Sites 303 and 304 (Fig. F5) on crust of Hauterivian age recovered chalk/limestone and chert within the upper opaque layer that lies directly on top of oceanic crust at these locations. Before Leg 32 in 1973 there was little direct knowledge of the lower transparent seismic unit and Horizon B in the oldest portions of the Pacific, and it was not until Legs 129 and 185 that these two deepest seismic facies were sampled in the East Mariana, Pigafetta, and Nadezhda Basins (Fig. F5). Results from DSDP Legs 6, 17, 61, 89, and 129 indicate that Horizon B in large areas of the Nauru, Pigafetta, and East Mariana Basins correlates to mid-Cretaceous volcanic material (sills, flows, and volcanogenic turbidites), which, according to magnetic anomaly identifications, significantly postdate the formation of oceanic crust (Winterer, 1976; Abrams et al., 1993; Shipley et al., 1993). DSDP Sites 303 and 304, 1600 km to the northeast, and ODP Site 801, 2200 km to the southeast, are the most proximal locations where Horizon B has proved to correlate to the top of oceanic crust of the Pacific plate (Larson, Moberly, et al., 1975; Lancelot, Larson, et al., 1992).
Reflections are most often interference patterns caused by the combined impedance effects of many thin beds, but exceptions include the sediment/oceanic crust interface or, in some cases, thick chert layers within a relatively uniform matrix. The correlations between lithologic units and the reflection image presented in Figures F3 and F8 were assigned without the guidance and constraints provided by a log-generated synthetic seismogram and should be considered a best estimate until an adequate synthetic seismogram can be generated postcruise.
The lowermost sequence, Horizon B, is characterized by a single or two to three closely spaced, high-amplitude, continuous reflector(s) when imaged with either air guns (C2005 MCS) (Fig. F8) or SCS water guns (Fig. F3). These continuous reflections range in appearance from relatively smooth and flat lying to diffractive and undulating, depending on seismic source and azimuth of profile direction. In Hole 1149B, Horizon B begins at 8.111 s two-way traveltime (s twt) or 0.42 s below seafloor (sbsf) (0.28 sbsf at the closest point of approach to Hole 1149D) and is interpreted to result from the impedance contrast between nannofossil chalk/marl and fractured basalt at 410 mbsf (307 mbsf at Hole 1149D). The velocity structure of this area is constrained by a single sonobuoy shot on the outer wall of the Izu-Bonin Trench ~50 km from Site 1149 (see Fig. F2 in the "Explanatory Notes" chapter). This sonobuoy records refracted arrivals with velocities of 4.1 km/s increasing to 6.1 km/s beginning at ~528 mbsf, which was interpreted as a normal upper crustal section below 528 m of sediment (assuming a velocity of 2 km/s for the sediment section).
The lower transparent layer, which corresponds to Jurassic to lowest Cretaceous radiolarite and claystone at Site 801, is absent at Site 1149 where the upper opaque layer directly overlies oceanic crust (Horizon B). The single upper opaque layer as originally defined by analog air-gun records appears as two distinct seismic facies in SCS water-gun data. The upper portion of the upper opaque layer in Holes 1149A, 1149B, and 1149C extends from 0.20 to 0.28 sbsf (7.891-7.971 s twt) and consists of high-amplitude, semicontinuous reflections that mimic the underlying basement relief and appear as a stratified pelagic drape deposit of generally uniform thickness. The lower portion of the upper opaque zone appears as discontinuous, chaotic to hummocky reflections extending down to Horizon B and also exhibits pelagic sheet drape character because it is generally of uniform thickness and concordant with the underlying basement topography. The MCS air-gun record (Fig. F8) only shows a single, continuous, high-amplitude reflection ~0.167 sbsf in Hole 1149D that marks the top of the upper opaque zone. We interpret the reflection that appears regionally at ~0.16-0.2 sbsf as the shallowest abundant chert/porcelanite interbedded with clay at ~180 mbsf (Holes 1149A, 1149B, and 1149C) and 155 mbsf (Hole 1149D). This reflection is most likely the consequence of the abrupt contrasts in density and velocity that mark the boundary between Subunit IIB and Unit III (see "Density," "Compressional Wave Velocity Measurements," and "Subunit IIB"). The change in seismic facies at ~0.28 sbsf (7.971 s twt) (Holes 1149A, 1149B, and 1149C) is interpreted as marking the increase in lithification and carbonate content (chalk/marl) of the matrix material interbedded with chert corresponding to lithologic Unit IV (see "Unit IV"). This boundary is marked by an abrupt increase in velocity at ~270 mbsf (see "Seismic Velocity"). The disrupted, hummocky character of this unit may be a consequence of postdepositional deformation related to undercompaction of the open, mechanically interlocked, siliceous/carbonate-microfossil framework of the primary pelagic sediment.
The upper transparent layer is relatively thick and extends from the seafloor to ~0.16-0.2 sbsf (7.891 s twt) (Holes 1149A, 1149B, and 1149C). This unit has a pelagic sheet drape form and a relatively reflection-free seismic character in the upper portion with semicontinuous reflections of variable, but generally low, amplitude apparent in the lower portion of this interval. The "transparent" character is indicative of a relatively homogenous interval containing no significant impedance contrasts and is correlated to the unlithified siliceous ash-bearing clay of lithologic Unit IA. The weak, discontinuous reflections beginning at ~0.15 sbsf (Hole 1149A) may correlate to the Subunit IA/IIA boundary at ~118 mbsf, which is marked by abrupt changes in porosity, water content, and velocity (see "Compressional Wave Velocity Measurements"). The relative thickness of this uppermost seismic unit is due to the significant input of volcanic ash, as well as biogenic silica, to an otherwise thin section produced from slow accumulation of pelagic clay.