Sites 1196 and 1199 penetrated the SMP. The drowned and current-swept top of this platform edifice forms the modern seafloor. Site 1196 is positioned in a water depth of 304 m at the intersection of regional seismic lines MAR07 and MAR70 at shotpoints 2808 and 178, respectively (Fig. F8 in the "Leg 194 Summary" chapter). Site 1199 is positioned in 316 m of water on line MAR20 at shotpoint 1341. Figures F72 and F2 display line MAR20 with the margin-parallel projected location of Site 1999. The two sites can be correlated along lines MAR70 and MAR20 as shown in Figure F73.
The main goal at these sites was to drill a complete section of the SMP into underlying basement in order to investigate the platform growth history, which can be linked to the slope and plateau record of other Leg 194 sites. Site 1996 penetrated to a depth of 672.2 through a thick platform section into a thin siliciclastic unit, but did not reach basement. Site 1999 is located ~3 km from Site 1996 and penetrated a 419.5-m-thick succession of platform sediments.
The irregular, scoured hard top of the platform (Fig. F72) yields a very high impedance contrast with the water column that scatters most of the acoustic energy from the seismic source. Consequently, only very limited subsurface information is available from underneath the platform. Two sites drilled on the upcurrent and downcurrent flanks of this platform (Sites 1198 and 1197, respectively) provide the seismic sequence stratigraphic framework that may be correlated with the platform subsurface.
Downhole seismic measurements at Site 1196 were performed at 12 check shot stations between 94 and 523 mbsf, which provided an accurate time-depth correlation that is used to calibrate the integrated downhole velocity curve (see "Downhole Measurements"). Velocity information was provided by the downhole sonic log data and, in the upper ~80 mbsf, where the pipe in the hole prevented downhole measurements, by P-wave velocity measurements on cores (see "Core Physical Properties"). The resulting traveltime-to-depth correlation is shown in Figure F11 in the "Leg 194 Summary" chapter. No check shot or sonic log is available for Site 1199, and the Site 1196 velocity-depth trend was used instead.
The velocity data, together with downhole density data, was used to calculate a seismic impedance curve for generation of a synthetic seismogram (Fig. F74). A normal polarity, zero-phase wavelet of 80-ms length was statistically extracted from the seismic data of line MAR20 and used for convolution of the downhole impedance curve and calculation of the synthetic seismogram.
The top of the SMP platform is characterized by a high-amplitude reflection that scatters and attenuates almost all seismic energy. The seismic facies is, consequently, almost transparent. In most areas, however, one broad low-frequency reflection can be traced at ~100 ms TWT below the platform top. This reflection does not mimic the seafloor morphology. Its topography is highly irregular and thus suggestive of a karstic surface (Fig. F72). It also forms an elevated rim at the upcurrent platform margin that is neither a multiple nor a velocity effect of the overlying platform top. The reflection overall dips to the southeast beyond the downcurrent slope of the platform and appears to correlate with the seismic Megasequence B/C boundary (Site 1197) (Fig. F2). The intraplatform reflection could thus be the platform equivalent of B/C and separate the middle Miocene growth phase of the SMP from the late Miocene growth phase.
Sites 1196 and 1199 sediments consist mostly of shallow-water carbonates, which overlie a thin siliciclastic unit representing the flooding of the basement during the latest Oligocene and the earliest Miocene. Large parts of the section are dolomitized (see "Lithostratigraphy and Sedimentology"). For the upper part (0-~170 mbsf) of the litho-column, both sites seem to have very similar sedimentary records, whereas differences increase deeper in the platform. Of prime interest for seismic stratigraphy is the origin of the low-frequency intraplatform reflection at ~100 ms TWT. Using the time-depth curve from the check shots, 100 ms is the equivalent of ~110 mbsf. In both cores, this depth coincides with a zone, displaying numerous indications of subaerial exposure (caliche crusts, vadose silt, and very indurated cemented lithology). In addition, drilling operations revealed several holes that also are indicative of an extensive karst system and just below that depth. In both holes, a depth of 110-130 mbsf matches the transition between lithologic Subunits IA through ID. Subunits IA and IB are rhodolithic dolostones that overlie a reefal interval (Subunit IC), which is only partly dolomitized and overlies another dolomitic floatstone unit. Evidence of exposure occurs above and below Subunit IC. The position of the intraplatform reflection at approximately the same depth was confirmed by the synthetic seismogram, which also shows a high amplitude at ~120 mbsf (Fig. F74). Consequently, the low-frequency reflection can likely be correlated with those exposure indications between 110 and 130 mbsf. However, as a result of the broad reflection character, the seismic pick of the boundary is not very accurate. In addition, it is biostratigraphically not proven whether this platform horizon truly separates the middle Miocene SMP from an overlying late Miocene growth phase. Alternatively, the middle to late Miocene boundary could be located slightly deeper at 163 mbsf, as discussed in "Downhole Measurements".