Reflections on three-dimensional (3-D) seismic line 281 can be correlated with lithostratigraphic boundaries at Site 1174 (Fig. F48). Specific correlations should be considered preliminary at this time because the velocity structure at this site is not well known. Any inaccuracies in velocity will produce errors in depth conversion. Additional velocity work will be carried out postcruise to improve depth conversions and stratigraphic correlations.
Unit I (slope-apron facies) is a very thin interval (<10 mbsf) just below the seafloor reflection. The axial trench-wedge facies (Unit II) extends to the strong reflection just below 300 mbsf (core depth of 314.55 mbsf) and is characterized by high-amplitude, laterally continuous reflections. Subunits IIB and IIC (outer trench-wedge facies and trench to basin transition facies) cannot be easily distinguished in the seismic data as there is not a distinct change in reflection character between them. The base of Subunit IIC is the top of a strong reflection representing the top of Unit III (upper Shikoku Basin facies) at ~485 mbsf. Unit III consists of a series of three strong reflections that are offset by a protothrust at ~540 mbsf at Site 1174. Note that the depth of the seismically defined protothrust does not match the depth of the thrust identified in the cores between 467.10 and 469.95 mbsf, possibly due to either a mismatch between navigation from the 1999 3-D seismic cruise and Leg 190 or incorrect velocities. The lower part of this unit has less distinct reflections. The transition between Units III and IV (lower Shikoku Basin facies) is marked by the last laterally continuous reflection at ~660 mbsf. Unit IV is characterized by indistinct, noncontinuous reflections in the upper part and stronger, more continuous reflections in its lower part. The structurally defined décollement is represented by a strong negative-polarity reflection at ~820-860 mbsf. Unit V (volcaniclastic facies) is represented by an indistinct reflection at ~1120 mbsf that overlies the strong basement reflection at ~1170 mbsf.