Site 1197 penetrated a 674.9-m-thick sedimentary succession and recovered 10 cm of volcanic basement. The site is located on regional multichannel seismic line MAR07 at shotpoint 3721 and on local grid lines MAR65 and MAR66 at shotpoints 645 and 907, respectively (Fig. F34; also see Fig. F8 in the "Leg 194 Summary" chapter). Site 1197 is positioned ~5 km southeast of the leeward margin of the SMP (Figs. F34, F2). The main purpose at this site was to retrieve a platform proximal record, which documents and dates the platform growth history of SMP. Seismic data indicate that the sedimentary package at Site 1197 can be separated by three major unconformities into three megasequences (B, C, and D) and acoustic basement (Fig. F34).
Because of unstable hole conditions after drilling, downhole logging operations could not be completed at Site 1197. As a result, the time-to-depth conversion for this site was achieved using the shipboard velocity data collected from recovered sediments (see "Core Physical Properties"). The continuity of the velocity values is hindered by the low recovery in part of Site 1197. P-wave sensor velocities also might have a systematic shift to lower values in respect to true formation velocities, as they were not measured under in situ conditions. In order to avoid a significant error in the two-way traveltime (TWT)-to-depth conversion, two prominent physical and lithological surfaces were used as tie points to relate the seismic traveltimes to coring depth. The first horizon is a hardground at 59 mbsf at the base of the hemipelagic drift deposits of lithologic Unit I (see "Lithostratigraphy and Sedimentology"). This first downcore hard surface can be correlated to the high-amplitude reflection of the Megasequence C/D boundary at 536 ms TWT (Fig. F34). The second fix point corresponds to the basalt at the bottom of the hole at 666.6 mbsf, which produces the basement reflection at 1022 ms TWT. The resulting time-depth correlation is shown in Figure F11 in the "Leg 194 Summary" chapter. Because no continuous velocity data are available, no synthetic seismogram was generated.
Megasequence D is characterized by laterally continuous low- to medium-amplitude reflections, which dip gently toward the southeast. They onlap along Megasequence C/D boundary onto underlying Mega-sequence C and form a wedge that disappears upslope, ~5 km to the northwest (Fig. F34). This onlap surface suggests a significant hiatus at the boundary to underlying Megasequence C. Megasequence D infills the topographic relief created by the SMP, indicating the dominance of currents on the sedimentation pattern. No major seismic unconformity can be recognized within Megasequence D.
Megasequence D coincides with lithologic Unit I (0-59 mbsf), which consists of a skeletal grainstone and packstone with planktonic foraminifers deposited in a hemipelagic setting (see "Lithostratigraphy and Sedimentology"). Biostratigraphic data confirm a hiatus at the base of lithologic Unit I from at least 2.2 to 4.8 Ma, but likely even further back into the late Miocene (see "Age Model").
The top of Megasequence C is marked by a high-amplitude reflection that dips to the southeast (~1°-1.5°) and forms a paleoslope topography of the SMP to the northwest (Fig. F34). Reflections within Megasequence C have the same dip as the C/D boundary and show no major intrasequence unconformities. Upslope, some of the reflections can be traced onto the carbonate platform, where they gradually turn horizontal, suggesting a continuous and gradual platform to slope transition, at least for the upper part of Megasequence C. The lower Megasequence B/C boundary is a downlap surface, indicating southeastward progradation of the slope deposits. As a result of this downlap, the lower half of Megasequence C wedges out within the upper 10 km of slope downward from the paleoplatform margin (Fig. F34).
Megasequence C correlates well with the late Miocene lithologic Unit II (see "Lithostratigraphy and Sedimentology" and "Age Model"), which consists of a fine-grained dolomitized skeletal grainstone deposited in a proximal periplatform setting. The high-amplitude reflection at the top correlates to the hardground surface observed in the cores, representing the hiatus at the Unit I/II boundary.
At Site 1197, Megasequence B forms a thick prograding slope sequence with two major seismic facies separated by a time-transgressive boundary (Fig. F34). The upper part of Megasequence B consists of inclined slope reflections that dip toward the southeast. At the base of the slope, these reflections flatten out and form an almost horizontally layered section. This slope-to-plateau system is prograding out over at least 10 km. Because of their high-amplitude signature and their greater thickness than the horizontal open plateau time-equivalent reflections, the slope reflections terminate and downlap at the toe of the slope and form a pseudo-unconformity along the seismic facies boundary. This unconformity, which is the result of beds thinning below seismic resolution, crosscuts timelines, as shown in synthetic seismic sections of similar settings (Schlager, 1992). The slope reflections within Megasequence B display a slightly wavy and hummocky seismic facies in the upper ~150 ms, whereas the underlying plateau reflections yield a cleaner, more continuous seismic reflection pattern.
At Site 1197, Megasequence B coincides with middle to early Miocene lithologic Units III through V. Lithologic Unit III consists of coarse detritus of skeletal grainstones. Unit III corresponds on the seismic data to the top of Megasequence B, displaying a slightly hummocky seismic facies, possibly indicative of the depositional/erosional process linked to the coarse grain size or the observed dolomitization. Underlying Subunit IVA consists of a partly dolomitized, very fine skeletal grainstone deposited in a more distal periplatform setting. The boundary to lithologic Subunit IVB marks the transition between the seismic slope and open plateau facies. Subunit IVB is comprised of similar sediments, as in the slope unit above, but contains a significant amount of clay (5%-40%) that is lacking in Subunit IVA. Surprisingly, despite the difference in bedding angle, no major grain size difference can be observed. Unit V corresponds to the base of Megasequence B and consists of more proximal periplatform skeletal grainstone and rudstone. Because the base of Megasequence B at Site 1197 is partly covered on the seismic data by the first multiple reflections, no detailed seismic facies analyses can be performed.
Acoustic basement at Site 1197 is defined by a strong reflection and a drastic change in seismic facies. Basement is slightly dipping to the southeast and, similar to Site 1198, cannot be traced below the SMP because the overlying platform sediments attenuate the signal and multiples mask most subsurface information.
Acoustic basement at this site is formed by 10 m of volcaniclastic breccia-conglomerate with large angular basalt clasts (lithologic Unit VI) that overlies, at 666.6 mbsf, an olivine basalt likely representing the volcanic basement. The acoustic basement is also masked by multiple reflections so that it is not clear whether the breccia-conglomerate is a laterally continuous unit or just a local feature.