Site 1198 penetrated a 523-m-thick sedimentary succession and recovered 5 m of volcanic basement. The site is located on regional multichannel seismic line MAR07 at SP 2262 and on local grid line MAR54 at SP 900 (Fig. F29; also see Fig. F8 in the "Leg 194 Summary" chapter). Site 1198 is located ~5 km northwest of the paleoplatform escarpment of the SMP (Fig. F29). The escarpment dips ~10° to the northwest. The lower section of this Miocene escarpment was buried by oncapping and downcapping sediments of Megasequence D. Strong currents close to the platform margin, however, reduced sedimentation rates adjacent to the escarpment, producing platformward-dipping reflections within Megasequence D. Consequently, a moat was formed at this current-exposed side of the margin so that the topmost section of the paleoescarpment is still outcropping at the seafloor (Fig. F29).
The main objective at this site was to drill the late Miocene platform proximal sedimentary record of the SMP within Megasequence C. The thickening of Megasequence C sediments toward the platform was interpreted to be a result of platform shedding to the northwest during active platform production, bypassing the nondepositional slope and creating an apron at the toe of slope. The sedimentary geometry indicates that these platform slope deposits, which were shed from the SMP, overlie platform-derived distal sediments that might have been shed from the NMP from the northwest.
Because of the deteriorating hole condition after drilling, Site 1198 could only be logged in the uppermost 160 m of the borehole and no check shot data are available. As a result, the time-depth conversion for this site was calculated for the upper 201 mbsf using the shipboard velocity data collected from cores (see "Core Physical Properties"). For the lower part, where velocity data are scarce because of low recovery, two prominent physical and lithological surfaces were used as tie points to relate the seismic traveltimes to depth in the cores. The first horizon is a hardground at 201 mbsf at the base of the hemipelagic drift deposits of lithologic Unit I (see "Lithostratigraphy and Sedimentology"). This first downcore hard surface is correlated with the high-amplitude reflection of the Megasequence C/D boundary at 655 ms two-way traveltime (TWT) (Fig. F29). The second fix point corresponds to the basalt at the bottom of the hole at 513 mbsf, which produces the strong basement reflection at 922 ms TWT. The resulting time-depth correlation is shown in Figure F10 in the "Leg 194 Summary" chapter. Since a continuous velocity record was not available, no synthetic seismogram was generated.
Megasequence D is characterized, in general, by laterally continuous low- to medium-amplitude reflections within a 200-m-thick mounded drift. Along line MAR07, 2 km to the northwest, erosional truncations cut into the top of Megasequence D, indicating ongoing erosion at the modern seafloor (Fig. F29). Such erosional features are only seen on the modern seafloor, and no major unconformities occur within Megasequence D northwest of Site 1198. Closer to the SMP margin, the reflections gently dip with progressively steeper angles to the southeast into the moat created in front of the platform margin. Several disturbances in these dipping reflections, such as contorted beds, document sediment slumping and accumulation of these slump sediments at the bottom of the of the paleomoat (Fig. F29). The basal reflections of Megasequence D downlap onto Megasequence C, indicating a hiatus at the Megasequence C/D boundary at 232 ms TWT, or 203 mbsf.
Megasequence D coincides with lithologic Unit I (0-201 mbsf), which consists of hemipelagic wackestone to grainstone with varying input of terrigenous sediments. In the bioturbated ooze, possible slumped intervals occur in the upper 70 m of this unit (see "Lithostratigraphy and Sedimentology"). Seismic data from line MAR07 show large-scale slump geometries on topographically lower levels toward the moat to the southwest. The conformable reflections of the slumped interval coincide with the top of Subunit IB and the potential slump interval between ~70 and 60 mbsf (Fig. F29). Thus, the observed slump structures seen in the cores could be the upslope equivalent of the larger-scale disturbed intervals, although the layering along the seismic line and the local site survey grid indicate horizontal bedding. Hemipelagic sediments directly overlie a hardground surface at 201 mbsf, coinciding with the Megasequence C/D boundary. The surface represents a hiatus from 4.5 to 7.2 Ma (see "Age Model").
The top of Megasequence C is marked by a high-amplitude reflection that dips to the northwest, away from the platform escarpment. The Megasequence C/D boundary is defined by toplap below the boundary and downlap above, resulting in an unconformity. This change in reflection dip across the boundary indicates a change in transport direction between Megasequences D and C. This geometry led to the interpretation that the C/D boundary marks the end of platform shedding from the SMP and, thus, the timing of platform drowning and the subsequent onset of current-controlled hemipelagic deposition in Megasequence D. Close to the escarpment, the seismic facies is chaotic to transparent, with numerous small-scale, erosional high-amplitude cut and fill patterns (Fig. F29). These geometries are typical for channelized lower-slope deposits (Anselmetti et al., 2000). Farther away from the platform, the reflection geometry of Megasequence C becomes more regular, indicating a reduced influence of platform-derived gravity flow deposits. Site 1198 is located roughly at the transition between these two seismic facies within Megasequence C. Only the strongest of the intra-Megasequence C erosional surfaces reaches basinward of Site 1198. Already 3 km northwest of Site 1198, or ~9 km away from the platform, continuous low-amplitude reflections indicate that sediments within Megasequence C form a generally continuous succession.
The top of Megasequence C coincides with the top of lithologic Unit II (see "Lithostratigraphy and Sedimentology"), which is mostly of late Miocene age (see "Age Model"). The contact between lithologic Units I and II (201 mbsf) is recovered in the cores as a 10-cm-thick layer of dark phosphatic nodules. The occurrence of these nodules correlates well with the high-amplitude reflection character of C/D and its interpretation as an unconformity as well as the hiatus in the chronostratigraphy (see "Age Model"). Megasequence C correlates with Subunits IIA and IIB. Based on their high content of neritic constituents, Unit II sediments are interpreted as periplatform deposits (see "Lithostratigraphy and Sedimentology"). Subunit IIA consists of fine-grained packstone to grainstone, whereas Subunit IIB has a much coarser carbonate lithology that was deposited in a more proximal periplatform setting, such as a reef talus (see "Lithostratigraphy and Sedimentology"). The lithology of Subunit IIB correlates well with the chaotic seismic facies seen in Megasequence C. The base of Megasequence C is placed using the time-depth correlation at 320 mbsf.
At Site 1198, the Megasequence B/C boundary does not have a strong seismic signature. The upper 100 ms TWT of Megasequence B displays a facies similar to Megasequence C: chaotic to transparent close to the escarpment and more regular and coherent further from the platform (Fig. F29). Below that interval, a series of coherent high-amplitude reflections extend to the southwest underneath the platform edge where they are masked by the transparent platform facies and multiples. West of the platform, reflections in the lower part of Megasequence B display variable amplitudes with few erosional geometries. The lowermost reflection above the basement downlaps onto its surface to the northwest and in turn is onlapped from the northwest, indicating a laterally restricted unit overlying basement. Seismic Megasequence A cannot be traced to this area.
The top ~100 ms of Megasequence B corresponds to middle Miocene Subunit IIC, which consists of fine-grained carbonates rich in neritic components deposited in a periplatform setting similar to Subunit IA. Thus, periplatform carbonates occur above and below the Megasequence B/C boundary and record the shedding of neritic components from the nearby carbonate platform into the bathyal environment during the middle Miocene, as well as during the late Miocene (see "Lithostratigraphy and Sedimentology"). The lower part of Megasequence B consists of the hemipelagic lithologic Unit III, characterized by a downcore increase in carbonate content from ~50 wt% at the top to ~95 wt% at the bottom, and of lithologic Unit IV (504-513 mbsf). The top of lithologic Unit III coincides with the high-amplitude reflections within Megasequence B that can be traced from the distal areas to the platform margin (Fig. F29). Unit IV consists of floatstone to rudstone, with rhodoliths and abundant large benthic foraminifers, and is capped by a phosphatic hardground surface. This interval can be correlated to the seismic data as the lowermost sequence overlying the basement reflection, which is onlapped by the hemipelagic lithologic Unit III.
Acoustic basement at Site 1198 is defined by a strong reflection and a drastic change in seismic facies. It is slightly dipping to the northwest and cannot be traced below the SMP, as the overlying platform sediments attenuate the signal and mask most subsurface information. Basement at this site, reached at 513 mbsf, consists of an olivine basalt.