Site 1193 penetrated 538.9 m of sediment and 5.45 m of acoustic basement through seismic Megasequences D, B, and A, (Fig. F53). Megasequence C is not present at this site as it onlaps the adjacent slope sediments of the NMP farther to the east (Fig. F2 in the "Leg 194 Summary" chapter). The site is located at the intersection of multichannel seismic lines MAR13 (shotpoint 4755; Fig. F53) and MAR34 (shotpoint 2241). Site 1193 was chosen to sample the sediments of the NMP. This site is located ~31 km east of Site 1194 (Fig. F2 in the "Leg 194 Summary" chapter).
Because of deteriorating hole conditions that developed during retrieval of the drill string, the pipe became stuck in the drill hole and, as a result, needed to be severed with explosives; thus, Site 1193 was not logged and no check shot information is available to accurately tie the seismic record to the sedimentary section. The time-to-depth conversion used here is calculated by integrating shipboard velocity measurements collected with the P-wave sensor (PWS) (Figure F54) (see "Core Physical Properties"). Velocity values for the platform section were overestimated by the shipboard data so that lower values were used. To constrain the resulting time-to-depth function, two tie points, one at the basement-sediment contact (909 ms two-way traveltime [TWT] and 531 m) and one at the top of the NMP (510 ms TWT and 35 m) were used for calculation of this synthetic seismogram. Both horizons are recognized in the seismic data by unique and strong reflections, so they can be used as accurate ties.
The PWS velocity, together with GRA bulk density, was also used for the calculation of a synthetic seismogram. A normal polarity, zero-phase wavelet of 80-ms length was statistically extracted from the seismic data of line MAR13. The result is superimposed onto line MAR13 and displayed at left on Figure F54. Because of the limited data set and the lack of check shot information, both the time-depth conversion and the synthetic seismogram carry considerable uncertainty.
The seismic facies of Megasequence D (Fig. F53) is characterized by laterally continuous reflections that dip gently eastward. The top portion of this onlapping package has a wedge-shaped geometry that appears to be truncated by erosion. This conclusion is supported by paleontological evidence indicating that sediments at 2.1 mbsf are 0.33 Ma in age and that a major hiatus occurs at 5.5 mbsf, lasting at least from 1.7 to 2.8 Ma (see "Age Model").
At Site 1193, Megasequence D is thin (48 ms TWT) and is seen to infill the erosional surface on top of the middle Miocene NMP. Reflection amplitudes are generally low, with the most prominent intramegasequence high-amplitude reflection directly overlying the top of the NMP at 504 ms TWT. Megasequence D thickens to the east as sediments onlap the paleotopographic surface of the NMP. The lower Megasequence D sequence boundary directly overlies the upper boundary of Megasequence B, which at Site 1193 coincides with the top of the NMP, resulting in a large hiatus that spans the entire Megasequence B.
Megasequence D is a thin, ~40-m-thick layer of hemipelagic sediment that overlies the NMP. Seismic Megasequence D incorporates lithologic Unit I, which is composed of a foraminiferal nannofossil ooze deposited in water depths >200 m (see "Lithostratigraphy and Sedimentology"). Paleontological evidence indicates that the sediments of Megasequence D are Pliocene-Pleistocene in age (see "Biostratigraphy and Paleoenvironments"). As with Site 1192, Holocene sediments were absent at this site, confirming that modern sedimentation is strongly reduced or nonexistent and that the seafloor represents an unconformity.
The exact magnitude of the hiatus is unknown but has been estimated by tracing the first reflection on top of the NMP platform to Site 1192, where sediments of Megasequence D were thicker and exhibited a more complete biostratigraphic succession. The initial sediments overlying the NMP that correspond to this reflection provide an age of ~5 Ma. Because the reflection traced did not lie directly on the platform surface but slightly above, this reflection is likely to provide an age for initial sedimentation on top of the NMP that is younger than the true age of the flooding. Biostratigraphically the base of the sediments overlying the NMP are dated as 5.6 Ma (see "Biostratigraphy and Paleoenvironments" in the "Site 1192" chapter). The top of the underlying NMP is biostratigraphically not well constrained, but benthic foraminifer assemblages give an age of >12 Ma. Thus, the hiatus is at least 5.6 to 10.5 Ma in age or as much as 5.6 to >12 Ma.
The top of Megasequence B is characterized by a high-amplitude unconformity at 508 ms TWT that marks the top of the NMP. The upper portion of this platform (508-610 ms) is characterized by a seismically transparent to chaotic carbonate platform section with hummocky reflections (Fig. F53). The B/D boundary forms the top of the NMP. The top of Megasequence B was traced into the basin to Site 1192, where it could be assigned an age of 10.5 Ma (see "Age Model" in the "Site 1192" chapter). The time-depth correlation used at this site places the seismic sequence boundary on top of the platform at 37 mbsf (Fig. F54). Below the platform interval is a sequence of moderately continuous reflections dipping to the east, indicating an eastward progradation of the platform slope (610-830 ms TWT) (Fig. F53). The seismic character of reflections within this portion of Megasequence B is disturbed by the overlying platform facies. Despite this, it is clear that these sediments grade eastward into a prograding sequence with more continuous reflections that can be consistently traced through the study area. The basinward transition of the upper "reefal" NMP facies is not as continuous as it is for the lower prograding slope sediments and, in many instances, shows evidence of erosional channels adjacent to the platform margin, indicating subaerial exposure and possibly current reworking after flooding (Figs. F53 and F7 in the "Leg 194 Summary" chapter). The time-depth correlation used at this site places the lower boundary of Megasequence B at 440 mbsf (Fig. F54).
Megasequence B incorporates lithologic Units II through IV, all of which are Miocene in age. The upper carbonate platform facies observed on the seismic data correlates well with lithologic Unit II, which is composed of skeletal rudstones and floatstones with lesser amounts of packstone and clay-rich mudstone (see "Lithostratigraphy and Sedimentology"). This section has been interpreted as a shallowing-upward interval of subtropical carbonate platform deposition dominated by benthic foraminifers and bryozoans. Physical properties data of this interval are characterized by highly varying P-wave velocity (2.7-5.2 km/s) and porosity (10%-45%) (see "Core Physical Properties"). Several irregular surfaces were observed in the cores, indicating subaerial and submarine erosion and reworking. The lack of regular bedding, together with the large range of physical properties values, explains the hummocky, chaotic seismic facies.
The sequence of dipping reflections below the platform interval correlates with lithologic Units III and IV, representing a deepening-upward sequence of platform-derived slope sediments (see "Biostratigraphy and Paleoenvironments") characterized by bioclastic packstones to mudstones that are dominated by benthic foraminifers and bryozoans.
At the location of Site 1193, Megasequence A is difficult to characterize as a result of the seismic interference from the overlying NMP. Megasequence A is characterized by a nearly horizontal series of reflections of varying amplitudes that can be traced laterally underneath sediments of Megasequence B. Megasequence A overlies and infills basement irregularities resulting in an overall variability in thickness. The time-depth correlation used at this site places the lower boundary of Megasequence A at 531 mbsf (Fig. F54).
The sediments of Megasequence A were deposited in a shallow-water inner-shelf setting and consist of a series of grainstones with quartz sand, bryozoans, and benthic foraminifers (see "Biostratigraphy and Paleoenvironments").
Acoustic basement at Site 1193 is difficult to characterize as a result of the interference of the overlying carbonate platform sediments and the seafloor multiple (Fig. F53). Despite this interference, the basement top surface is recognized as a high-amplitude reflection at the interface with overlying sediments and numerous diffractions caused by the irregular bedrock surface.
Approximately 5 m of acoustic basement was recovered at Site 1193 (Fig. F53). These rocks consisted of volcaniclastic red breccia with fine-grained layers (see "Lithostratigraphy and Sedimentology").