A two-dimensional seismic line, NT62-2, crosses Site 1177 and allows correlation of seismic stratigraphy with Site 1177 (Fig. F29). As with the other Leg 190 seismic ties, 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.
Hole 1177A was washed to a depth of 300 mbsf before coring began. The seismic section shows that this upper section of the hole contains the outer trench facies as well as the top of the upper Shikoku Basin hemipelagic section.
Lithostratigraphic Unit I is recognized on the seismic line as a sequence bounded at its base by two strong, continuous reflections (Fig. F29). The unit is characterized by low-amplitude, discontinuous horizontal reflections.
Lithostratigraphic Unit II is bounded by a strong reflection at its top and base and is <50 m thick. The reflection character is indicative of a negative impedance contrast, indicating that there is an inversion of either velocity of density at the top of the unit. This horizon is traced laterally toward the accretionary prism, where it becomes the décollement.
The series of high-amplitude, laterally continuous reflections between 450 mbsf and 750 mbsf correlates with lithostratigraphic Unit III. The strong reflections correlate with individual sand packages.
Lithostratigraphic Unit IV is correlated with the three high-amplitude, laterally continuous reflections between 750 and 850 mbsf. They are conformable at their base with the underlying basement reflection.
Basaltic basement is identified as a strong double reflection at 850 mbsf. This reflection is laterally continuous under the entire region.