Reflections on 3-D seismic line 281 can be correlated with lithostratigraphic boundaries at Site 1175 (Fig. F35). 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.
Lithostratigraphic Unit I is recognized on the seismic line as a sequence bounded at the top by the seafloor and at the bottom by an angular unconformity (Fig. F35). Dips within the sequence increase from horizontal at the seafloor to 8°-10° at its base. The upper ~40 m of the sequence around Site 1175 has a somewhat irregular and discontinuous character that correlates with the slump deposits in the drill hole. The strong negative reflection at ~75 mbsf is likely to be caused by a major coarse ash bed encountered at ~90 mbsf in Hole 1175A, indicating that the depth conversion velocities may be ~10% too high in the shallow section.
Lithostratigraphic Unit II is interpreted to be the seismic stratigraphic unit that is onlapped by Unit I at its top and onlaps the underlying unit at its base. The shape of the unit is a northwest-thickening wedge that has relatively continuous and high-amplitude internal reflections.
The lowest identified seismic stratigraphic unit correlates with lithostratigraphic Unit III. It is defined by an angular unconformity at its top; the base of the unit is unclear, but it may be <100 m thick.