Site 998 is located in 3190.7 m of water on a plateau-like feature on the Cayman Rise, bordered by the Yucatan Basin to the north and the Cayman Trough to the south. Drilling during ODP Leg 165 recovered a largely complete lower Eocene to Pleistocene sedimentary section composed of ooze, chalk, clay, limestone, turbidites, and ash layers (Sigurdsson, Leckie, Acton, et al., 1997). Penetration at Site 998 reached 904.8 mbsf, and the hole was logged from ~180 to 880 mbsf. SCS reflection Line EW9417-13 crossed Site 998 at Shotpoint 348.
Some intervals from the synthetic seismogram do show excellent agreement with the SCS data. Reflectors from SCS Line EW9417-13 that correlate with the synthetic seismogram include those at 4280, 4355-4405, 4475, 4480, 4500, 4515, 4600-4615, 4630, 4730, 4750, 4765, and 4940-5020 ms (Fig. 18). Figure 18 also illustrates the age-depth relationship of the section cored at Site 998.
The synthetic seismogram permits correlation of the seismic data with core lithologies recovered during drilling. Several lithologic units were defined for ODP Site 998 (Sigurdsson, Leckie, Acton, et al., 1997). Unit I consists of nannofossil ooze with foraminifers and clays interbedded with graded foraminifer ooze and volcanic ash layers. Subunit IIA contains clayey nannofossil mixed sediment interbedded with turbidites and ash layers. Subunit IIB has nannofossil ooze with foraminifers and clays interbedded with turbidites and ash layers. Subunit IIC contains clays with nannofossils to nannofossil mixed sediment interbedded with turbidites and ash layers. Unit III contains nannofossil chalk with clays, interbedded with foraminifer chalk with clay, altered volcanic ash layers, and chert. Unit IV has calcareous volcaniclastic mixed sedimentary rock interbedded with altered volcanic ash with carbonates.
Figure 19 illustrates the core lithologies and lithostratigraphic units that elicit responses in the SCS data that are distinct from units and lithologies above and below them. The interval from 30 to 120 mbsf correlates with a zone of lower amplitude and less continuous reflections. This interval is also associated with an interval of increased turbidite thickness compared to the underlying section. Lithostratigraphic Subunits IIB and IIC correlate with a zone of high frequency, high amplitude, and highly continuous reflections. Unit IV is marked by a set of low-frequency, high-amplitude, moderately continuous reflections.
In addition to the correlation of these units with the seismic data, several other lithologic intervals appear to correlate with the seismic data. Chert layers present in the core at 250, 350, 680, 730, 800, and 880 mbsf appear to correlate with moderate- to high-amplitude, moderately continuous reflectors. Unfortunately, chert layers at 465, 495, 520, 570, and 605 mbsf do not appear to correlate with any significant seismic reflectors. The lack of a seismic response may be a result of thin chert layers. Turbidite-rich intervals from 30 to 120, 180 to 260, 470 to 510, and 610 to 680 mbsf correlate with zones of lower amplitude, less continuous reflectors compared to intervals above and below the turbidite-rich intervals. Aside from the chert and turbidite-rich intervals, the relatively homogenous sediment recovered at Site 998 does not have significant variations in the physical properties that might cause a change in seismic response.