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Physical Properties, Downhole Measurements, and Core Logging
Downhole Logging

Two holes were logged during Leg 198: Hole 1207B with the triple combination tool (triple combo) and FMS-sonic tool strings, and Hole 1213B with the triple combo. Collecting data from the OAE1a black shale interval near the base of the Aptian was one of the major objectives of the leg. This sedimentary interval is clearly represented as peaks in the natural gamma radiation logs at both of the logged sites (Fig. F44). Most of the gamma radiation comes from uranium adsorbed onto the organic matter, and potassium and thorium are also high, indicating the presence of clay in the sedimentary rocks. The gamma radiation peak at OAE1a is much stronger than the other peaks in the log; other OAEs are either absent or are weaker than OAE1a on Shatsky Rise.

The Formation Micro-Scanner (FMS) resistivity images from Hole 1207B reveal the form, thickness, and depths of the chert horizons that formed the bulk of the recovered core below 230 mbsf (Fig. F45). Between 210 and 375 mbsf the chert layers occur on average every 83 cm and have an average thickness of 9 cm. The cherts typically appear as layers rather than nodules. Low core recovery in chalk/chert alternating sequences has been the subject of much discussion, and the image logs from Hole 1207B provide important data to develop better strategies for core recovery in chalk/chert sequences.

Synthetic seismograms were constructed from density and velocity data from both logs and core physical properties measurements. These reconstructions enabled the core and logs to be correlated with the seismic section and, hence, enabled ages to be assigned to the seismic reflectors.

Physical Properties

Physical properties data from Leg 198 show variation both with depth below the seafloor and with geographical location. The Northern and Central Highs of Shatsky Rise (Sites 1207 and 1208) have similar sedimentary histories dominated by major unconformities that are of ten's of millions of years in duration. The unconformities are manifested in the physical properties data as large peaks in magnetic susceptibility, P-wave velocity, and a downhole increase in bulk-density (both GRA and discrete measurements) (Figs. F12, F16). At Site 1207 a peak in natural gamma radiation is also apparent due to the presence of a manganese nodule proximal to the level of the major unconformity.

The Southern High of Shatsky Rise is characterized by more continuous sedimentation and shorter-duration hiatuses relative to the Northern and Central Highs. The physical properties data from Sites 1209, 1210, 1211, and 1212 suggest that compaction and dewatering, although important processes in the upper part of the sedimentary column, cannot explain all the trends observed (Figs. F20, F26, F30, F34). The Eocene–Cretaceous sediments are notable in that they appear to be underconsolidated with respect to their age and burial depth, suggesting that primary conditions, such as sediment composition, may be controlling the degree of lithification.

At Sites 1207, 1213, and 1214, a variety of lithologies of Cretaceous age, including chert, porcellanite, radiolarite, chalk, and limestone, were recovered. These sedimentary rocks exhibit a wide range of average P-wave velocities, from ~2200 m/s in the chalk and limestones to ~4760 m/s in the cherts (Fig. F12). Porcellanite and radiolarite have average P-wave velocities between this range. The high degree of variability of P-wave velocities in the Lower Cretaceous has significant implications for seismic data interpretation on Shatsky Rise.

Composite Depths and Cycle Stratigraphy

The recovery of complete sediment sections of APC-cored intervals was crucial to fulfilling the primary paleoceanographic objectives of Leg 198. Coring of multiple Holes at Sites 1209, 1210, 1211, and 1212 ensured the recovery of the complete (except unconformities) Cenozoic sedimentary record. Consequently, composite records of MST-derived physical properties and color reflectance data were produced for this time interval. These data compilations are unique in that previous drilling of Paleogene and Cretaceous sediments in the western Pacific failed to recover the complete and undisturbed sequences that are necessary to identify and characterize high frequency sedimentary cycles. As a result, little was understood about the influence of orbital and other periodic forcing on pre-Neogene sedimentation in the Pacific. The high-quality double and triple APC cores recovered during Leg 198 have the potential to remedy this situation. Most such sedimentary intervals exhibit pervasive lithologic cycles throughout the Cenozoic including those identified across several key transitions (Fig. F46, F47). Systematic changes in cycle amplitude and frequency are consistent from site to site, suggesting that these changes reflect regional paleoceanographic processes. The cycle packages (in all physical properties) are sufficiently distinct to allow for detailed correlation between sites (Fig. F47).

The most distinct cycles in terms of color variation and other physical properties occur in the upper Neogene. These are best represented by the total color reflectance (L*) records from Sites 1208 and 1209 as plotted along with orbital obliquity and eccentricity for the intervals 0–2 and 3–5 Ma (Fig. F48). The Pleistocene–Holocene color data at both sites exhibit the "classic" asymmetric glacial to interglacial cycle pattern. The interglacials are characterized by carbonate-rich, light-colored nannofossil oozes with clay, whereas the glacials are characterized by clay- and diatom-rich, dark-colored clayey nannofossil oozes or nannofossil clays. The transitions are mostly gradational, although several glacial to interglacial contacts are sharp. Preliminary biostratigraphic age constraints suggest that the dominant cycle frequency over the last 0.7 m.y. is near that of the 100-k.y. eccentricity cycle (Fig. F48). From 0.7 to 2.6 Ma, the dominant period shifts toward a higher frequency close to that associated with the 40-k.y. obliquity cycle. Throughout the last 2.6 m.y., the cycle amplitudes in reflectance remain remarkably similar between the Southern and Central Highs, although the mean total reflectance is higher on the Southern High. Climate-driven variations in opal and carbonate production and preservation, and in clay fluxes are responsible for these changes.

The lower Pliocene prior to the onset of Northern Hemisphere glaciaiton at 2.6 Ma is also characterized by regular lithologic cycles. As in the late Pliocene and early Pleistocene, the wavelengths indicate a dominant period close to 40 k.y. However, the cycle amplitude in this period is noticeably reduced, particularly at sites on the Southern High. The reduction in the high-frequency cycle amplitude is accompanied by an apparent increase in a low frequency cycle amplitude. In the Site 1209 color reflectance record (over the period 3 to 5 Ma), for example, there appears to be a long wavelength oscillation with a period of roughly 1.0 to 1.25 m.y. Comparison with the derived orbital curves (Laskar, 1990) suggests that this cycle may be in phase with the long period 1.25-m.y. cycle of obliquity (Fig. F48).

In the Paleogene, the sedimentation rates are sufficiently low (~3 to 5m/m.y.) over most of the Southern High that it is difficult, if not impossible, to identify cycles associated with obliquity and precessional-scale forcing. Nevertheless, prominent periodic to quasi-periodic color (light–dark) and magnetic susceptibility cycles occur throughout the upper Paleocene to late Eocene at Sites 1209, 1210, 1211, and 1212. The variations represent subtle changes in carbonate and/or Fe oxide content. The mean wavelength of the highest amplitude oscillations indicates a cycle frequency in the approximate range of the 100-k.y. eccentricity cycle. These cycles in turn exhibit an amplitude modulation with a frequency range suggestive of 400-k.y. eccentricity forcing. If so, these data would be consistent with observations elsewhere of a dominant response to precession and eccentricity forcing during the ice-free Paleogene prior to the late Eocene appearance of ice sheets on Antarctica.

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