Site 1195 penetrated 521.2 m of sediment through seismic Megasequences D, C, B, A, and a previously unmapped carbonate sequence (possibly Eocene in age) at the base of the section (Fig. F3). This site was chosen for the continuous nature of the seismic reflections, which indicate a nearly complete Neogene sedimentary section. This section is expected to provide a higher resolution chronostratigraphy than what could be produced from locations proximal to the Marion Plateau carbonate platforms (Fig. F7 in the "Leg 194 Summary" chapter). Site 1195 is located at the intersection of multichannel seismic lines MAR15 (shotpoint 3505) (Fig. F3) and MAR04 (shotpoint 3928) (Fig. F2), located ~70 km east of the Northern Marion Platform and 60 km north of the Southern Marion Platform (Fig. F2 in the "Leg 194 Summary" chapter).
Three check shots between 70 and 120 mbsf provide an accurate traveltime-depth link between the seafloor and the upper section of the hole. No further check shots are available below because deteriorating conditions did not allow lowering the tool further downhole (see "Downhole Measurements"). Consequently, the time-to-depth conversion for the lower section of this site was partly calculated by integrating shipboard velocity data collected from cores using the P-wave sensor method (see "Core Physical Properties"). The resulting traveltime to depth conversion was fixed at the depth of the basal carbonate unit at 521 mbsf, which corresponds to a strong reflection just above basement at 1076 ms two-way traveltime (TWT). Using shipboard velocity data and bulk density from a first successful logging run, a synthetic seismogram was calculated that is superimposed and compared to line MAR15 and displayed at the right on Figure F32. A normal polarity zero-phase wavelet of 80 ms length was statistically extracted from the seismic data of line MAR15 to generate the seismogram.
The seismic facies of Megasequence D (550-712 ms TWT; 0-123 mbsf) (Fig. F3) is characterized by laterally continuous low to moderate amplitude reflections showing large-scale convex-upward mounded geometry that is typical for drift geometries. A relatively strong reflection near 602 ms TWT marks the top of a drift package, which thickens to the northeast. Above 602 ms TWT, the drift package, which potentially migrated from the southwest, is onlapping onto this reflection, indicating a hiatus across this unconformity (Fig. F3). The reflections in Megasequence D have low to moderate amplitude differences that are weaker than those below 602 ms TWT. The Megasequence C/D boundary, at the base of Megasequence D, occurs at 123 mbsf (712 ms TWT) and is marked by a conformable low-amplitude reflection (Fig. F3). The age model for this site provides an age of 7.22 Ma for the base of Megasequence D (see "Age Model").
Megasequence D correlates well with the late Miocene to Pleistocene hemipelagic sediments of lithologic Units I, IIA, and IIB (0-123.9 mbsf) (see "Lithostratigraphy and Sedimentology"). The sediments within these units are predominantly unconsolidated packstone and grainstone. Physical properties data from Site 1195 show a shift to less variable velocity values downcore at the base of lithologic Unit I, which correlates well with the prominent reflection at 602 ms TWT. The base of Megasequence D (lithologic Unit II/III boundary) corresponds to the presence of a 15-m-thick interval of glauconite-rich layers indicative of low sedimentation rates. In addition, this horizon can be seen in physical properties data as a shift to lower P-wave velocity.
Megasequence C (712-824 ms TWT; 123-230 mbsf) is characterized by laterally continuous, generally low-amplitude reflections that dip gently to the northeast (Fig. F3). To the southwest of the site, two higher amplitude reflections can be seen at 719 and 730 ms TWT. These reflections decrease in amplitude to the northeast. The Megasequence B/C boundary at the base of Megasequence C is present at 230 mbsf and is characterized as a conformable low-amplitude reflection that has been dated as 11 Ma (see "Age Model").
Megasequence C correlates with the upper part of lithologic Subunit IIC, which is a skeletal wackestone to packstone/grainstone deposited in a hemipelagic environment. Physical properties data from the Megasequence C interval show reduced variability in density, magnetic susceptibility, and natural gamma radiation (see "Core Physical Properties") that are reflected in the seismic data by the low-amplitude nature of the reflections. An exception to the reduced variability data set is the prominent velocity peak that appears near 210 mbsf. This peak correlates well with a relatively high-amplitude reflection that can be seen on the seismic profiles at 796 ms TWT. The horizon marking the base of Megasequence C coincides with the top of a glauconite-rich interval at the base of lithologic Subunit IIC (230-255 mbsf). It also corresponds to the upcore change in facies from a distal periplatform to a hemipelagic depositional environment.
Megasequence B (824-983 ms TWT; 230-408 mbsf) consists of laterally continuous reflections, which show a gradual dip to the northeast. Reflections in Megasequence B have low amplitudes in the interval above 827-909 ms TWT and higher amplitudes below. Megasequence Boundary B/A at the base of Megasequence B occurs at 408 mbsf and is seen as a conformable, low-amplitude reflection with an age of 19.8 Ma (see "Age Model").
Megasequence B encompasses the lower part of lithologic Subunits IIC, IIIA, and the upper portion of IIIB. The sediments deposited in this interval are predominantly skeletal mudstone to grainstone characterized by a downcore increase in terrigenous concentration. A scoured horizon that marks the base of lithologic Unit II does not correlate to a significant reflection in the seismic data. However, physical properties data from this megasequence show increased variability in most data sets relative to the intervals above. The depth at which a change in reflection amplitude occurs (827 m) correlates well with the base of Subunit IIIA that marks a change from a greater amount of terrigenous sediments in Subunit IIA to a more carbonate-rich depositional system with increased dolomite concentration.
Megasequence A (983-1076 ms TWT; 408-517 mbsf) is characterized by low-amplitude reflections within two packages showing mounded drift geometries. At Site 1195, these packages are separated by a prominent unconformity within Megasequence A at 1040 ms TWT. The amplitude of this reflection increases to the northeast of the site.
Megasequence A encompasses the lower part of lithologic Unit III and all of Unit IV. The boundary between these two units is correlated with the prominent seismic unconformity between two sediment drift packages in Megasequence A (Fig. F3). Sedimentologically, this boundary is characterized by a downcore change from a light gray grainstone with rare quartz deposited in a distal periplatform environment (Unit III) to a greenish gray wackestone with high glauconite and quartz content deposited in a proximal periplatform environment (Unit IV) (see "Lithostratigraphy and Sedimentology" and "Biostratigraphy and Paleoenvironments"). This change in depositional environment between the two drift sequences may reflect a change in sedimentation source.
At the base of Site 1195 (517.5 mbsf), 20 cm of well-indurated, yellowish brown skeletal grainstones of possibly Eocene age were recovered. This sequence is observed on the seismic data to be a thin veneer of sediment overlain by a high-amplitude reflection at 1081 ms TWT and underlain by acoustic basement (Fig. F3). This reflection can be traced laterally for a short distance before it onlaps onto a basement high to the southwest of the site. To the northeast, this horizon continually thins below seismic resolution and merges with the basement (Fig. F3).