Three holes were drilled at Site 1264. Hole 1264A was drilled to a depth of 316.5 mcd (280.9 mbsf); Hole 1264B was drilled to a depth of 316.1 mcd (283.2 mbsf). One core was taken in Hole 1264C to a depth of 3.1 mcd (3.0 mbsf) to recover the mudline. The major lithologies include nannofossil ooze, foraminifer-bearing nannofossil ooze, foraminifer nannofossil ooze, and nannofossil foraminifer ooze. Clay-bearing nannofossil ooze and foraminifer-bearing nannofossil chalk are present as minor lithologies. Ash-bearing nannofossil ooze and oxide-bearing nannofossil ooze are present as accessory lithologies. Based on visual core descriptions, color reflectance, smear slide descriptions, MS, and compressional wave (P-wave) velocity measurements, the sediments at Site 1264 are divided into two lithostratigraphic units, with the lower unit further divided into two subunits (Table T4; Fig. F7). Physical property data obtained from whole rounds, split cores, and smear slide data support the lithostratigraphic subdivisions (Figs. F8, F9, F10, F11, F12, F13, F14). The boundaries between lithostratigraphic units are gradational; therefore, boundary depths are approximate. Bioturbation appears rare to moderate but may not be visible in some intervals because of subtle color contrast. Measurements of natural gamma radiation (NGR), grain density, bulk density, and porosity show only minor variations downhole (Figs. F8, F11). Homogeneity of the sediments is also reflected by minor downhole variation in grain density and P-wave velocity. Bulk density and porosity are well correlated, indicating that bulk density variations are controlled by porosity (Fig. F12). In these carbonate-rich sediments, L* is not controlled by the carbonate content (Fig. F9) but, rather, by other factors. For example, at Site 1264, chromaticity a* covaries with the Mn concentration of interstitial water, implying that the quantity of trace components such as Mn oxides is exerting a strong influence on color (Fig. F14).
Unit I consists of pale brown nannofossil foraminifer ooze, foraminifer nannofossil ooze, and foraminifer-bearing nannofossil ooze. Based on the smear slide descriptions, the foraminifer content decreases downhole from 80% to 10% (Fig. F10) and nannofossil content shows a complementary increase. The sediment L* and carbonate content increase downhole in accordance with the higher nannofossil content (Fig. F9). The average carbonate content is 94.6 wt%. Unit I exhibits a downhole change in color from pale brown to very pale brown, as indicated by decreasing color reflectance values on both the a*-axis (red-green chromaticity) and b*-axis (yellow-blue chromaticity). In addition to color change, MS, NGR, porosity, and P-wave velocity values all slightly decrease downhole through Unit I (Figs. F7, F8).
Unit II is characterized by high nannofossil content >75%. Dominant lithologies include nannofossil ooze and foraminifer-bearing nannofossil ooze. Clay-bearing nannofossil ooze and nannofossil chalk are reported as minor lithologies. Volcanic ash and oxides are present throughout and are occasionally abundant enough to form accessory lithologies such as ash-bearing nannofossil ooze and oxide-bearing nannofossil ooze. Density profiles show little variation except for three abrupt steps in grain density and porosity at ~135, ~165, and ~235 mcd (Fig. F11). Dramatic changes in interstitial water Fe and Mn concentrations suggest a diagenetic origin for these features. Downhole variations in chromaticity values a* and b* exhibit profiles similar to the interstitial water Mn content (Fig. F14). The white lithology of Subunit IIA is characterized by high Fe concentrations in the interstitial waters, whereas interstitial waters of the brownish colored Subunit IIB are characterized by high Mn concentrations.
Subunit IIA is composed of white nannofossil ooze with light bluish-gray bands alternating at a centimeter to decimeter scale (Fig. F15). Foraminifer-bearing nannofossil ooze is present as a minor lithology. The color banding likely reflects either differences in the concentration of oxides related to sediment diagenesis or a variation in clay content resulting from the balance of carbonate production vs. preservation. Contacts between colored bands and the white ooze range from sharp to slightly diffuse, and the contacts are sometimes sharp at the base and gradual at the top. In general, the banding is well preserved with only minor to moderate disruption by the drilling process. Visible bioturbation features are rare, pointing to either sediment homogeneity or postdepositional diagenetic color changes that obscured the bioturbational features. Despite obvious color oscillations in Subunit IIA, the lithologic composition and physical properties are relatively invariant. Nannofossil content is consistently >80%. The resulting high carbonate content (average = 96.1 wt%) is reflected by negative MS values (Figs. F7, F15). MS, sediment L*, and chromaticity a* and b* values are less variable than those in either Unit I or Subunit IIB. Despite the overall constancy of sediment properties, quasi-periodic oscillations are common within this subunit (Fig. F15). At the highest frequency, individual centimeter-scale gray bands may represent short-term orbital (precessional) or even suborbital cycles. Clusters of gray bands on a decimeter scale as expressed by the L* oscillations in Figure F15 may be related to precessional or obliquity cycles. Lower-frequency oscillations in the MS values are very low and not reliable for frequency analysis in this interval.
Subunit IIB is characterized by an alternation of nannofossil ooze and foraminifer-bearing nannofossil ooze on a decimeter to meter scale. Compositional changes are accompanied by color variations; slightly darker intervals are foraminifer rich. L* generally decreases downhole. Within the upper part of this subunit (117–190 mcd), a color change from light gray to pale brown is accompanied by increases in MS, NGR, and chromaticity (a* and b*) values (Figs. F8, F9). Disseminated oxide is present in trace amounts throughout this interval and persists downcore. Oxide-enriched zones are represented in several ways within the core: gray banding, gray halos surrounding mostly pale pinkish brown–colored blebs, and black concretions of oxides. X-ray diffraction analysis of Sample 208-1264A-26H-5, 25 cm, indicates that these oxide concretions comprise calcite and the Mn oxide, lithiophorite (Al, Li)MnO2(OH)2 (Fig. F16).
The interval from 190 to 220 mcd is dominated by pale brown foraminifer-bearing nannofossil ooze. MS, NGR, and chromaticity a* and b* values all peak in this interval (~210 mcd), whereas the mean carbonate content exhibits a subtle drop to a low average of 92.3 wt% (Fig. F9). Below 220 mcd, alternation of foraminifer-bearing nannofossil ooze and nannofossil ooze continues on a decimeter to meter scale. The color of this interval changes slightly from very pale brown within the foraminifer-rich layers to pinkish gray in the nannofossil-dominated sediments. The observed alternation of color in Subunit IIB may reflect cyclic production/preservation oscillations on orbital timescales (Fig. F17).
After the sharp decrease at ~220 mcd, MS, NGR, gamma ray attenuation bulk density, and chromaticity a* values begin to increase, whereas L* and b* values show weak trends downhole (Figs. F8, F9). This may reflect a change in the terrigenous sediment component rather than in carbonate preservation. Between 295 and 316.5 mcd, MS and NGR values decrease abruptly (Fig. F8).