Four holes were drilled at Site 1263. Hole 1263A was cored to a depth of 400.7 mcd (340.1 mbsf); Hole 1263B was cored to 396.6 mcd (337.1 mbsf); Hole 1263C was cored to 339.9 mcd (289.1 mbsf); and Hole 1263D was cored to 337.3 mcd (286.6 mbsf). To expedite recovery of E/O and P/E boundary sections, Hole 1263B was washed to a depth of 53.3 mcd (46.0 mbsf) prior to coring. Hole 1263C was washed to 103.1 mcd (90.0 mbsf) and cored to 136.7 mcd (118.4 mbsf) to fill a recovery gap, and then it was further washed to 225.3 mcd (193.0 mbsf) to recover the P/E boundary. Hole 1263D was washed to 318.5 mcd (272.0 mbsf) to recover the P/E boundary. Physical property measurements were performed on whole-core and working-half sections from Hole 1263A; compressional wave (P-wave) velocity and moisture and density (MAD) measurements were made only on working halves that appeared less disturbed. As a result, sampling was biased toward indurated sections in XCB cores.
The major lithologies recovered at Site 1263 include nannofossil ooze, chalky nannofossil ooze, foraminifer-bearing nannofossil ooze, and clay-bearing nannofossil ooze. Minor lithologies and accessories include foraminifer nannofossil ooze, ash-bearing nannofossil ooze, clayey nannofossil ooze, limestone, chert, and clay-bearing ash. The degree of induration varies from ooze to chalky ooze in the deeper part of the sequence. Combining measurements of MS, natural gamma radiation (NGR), and L* with smear slide and visual analysis, we classified this sequence as a single lithostratigraphic unit composed of three subunits that are differentiated by minor variations in foraminifer content; MS, NGR, and L* values; and carbonate content (Table T4; Fig. F8). Additional observations of physical properties and Mn and Fe contents are summarized in Figures F9, F10, F11, F12, F13, F14, and F15.
Unit I, representing the entire section recovered at Site 1263, predominantly consists of nannofossil ooze. Variations in measured physical character, the degree of induration, and the relative abundance of skeletal components, organic matter, and clay (Figs. F8, F11) allow division into three subunits. MS and NGR values are low throughout the section, reflecting an overall high carbonate content (average = >90 wt%) (see "Geochemistry"). A downcore reduction in porosity and increases in bulk density and P-wave velocity reflect normal consolidation trends for homogeneous sediments (Figs. F12, F13). In general, densities and sound velocities determined with whole-core multisensor track (MST) measurements correlate well with discrete sample measurements, indicating the absence of severe disturbance during core splitting (Fig. F14).
Subunit IA is 99.1 mcd thick and grades downcore from foraminifer-nannofossil ooze to nannofossil ooze, with low MS and high L*. The uppermost 1.3 m of the subunit is enriched in foraminifers (>25%) associated with a sandy grain-size texture, which may be the result of sediment winnowing by bottom currents (e.g., Moore, Rabinowitz, et al., 1984). The grain size decreases downsection in conjunction with the increase in nannofossil percentage. In the upper 10 m of Subunit IA, color oscillates between light brownish gray and light gray. From 10 mbsf, the sediment is a uniform very pale brown; light gray oscillations start at 27 mbsf. Much of the section is mottled by large (1–6 cm in diameter) irregular pinkish white blebs that are lithologically similar to the surrounding sediment. These blebs may reflect variations in sediment diagenesis controlled by porosity differences associated with bioturbation and/or localized concentrations of organic matter. Additional evidence of bioturbation is not apparent in this interval, which could simply be the result of the homogeneous color of the sediments.
In Section 208-1263A-4H-1, a sharp, angular contact at ~80 cm (25.9 mcd) separates slightly foraminifer enriched sediment above from darker blebless sediment below (Fig. F16). This contact is coincident with downhole increases in MS and bulk density and decreases in porosity and L*; it may represent a slump or upper Neogene hiatus at Site 1263 (see "Biostratigraphy").
The basal contact of Subunit IA, located at 99.1 mcd, is marked by a gradational change toward slightly lower L*. This contact is in the E/O boundary interval (86–104 mcd) (see "Biostratigraphy"). Above the contact, red-green-blue color and MS values decrease slightly, whereas L* increases slightly (Fig. F17).
The upper boundary of Subunit IB is marked by a downhole decrease in L* and an increase in MS. Petrographically, this subunit is more diverse than Subunits IA and IC, comprising intervals of all major lithologies recovered at Site 1263 and displaying an overall downhole decrease in foraminifer abundance and increase in clay fraction and induration. Centimeter-scale chalky intervals are first observed at ~157 mcd and increase in frequency downhole. Color varies from very pale brown and light gray to white; the middle of the subunit is marked by meter-scale light–dark oscillations which may cycle at the 100-k.y. eccentricity band (see "Age Model and Mass Accumulation Rates"). Furthermore, MS throughout this interval appears to cycle on a decimeter scale, suggesting possible forcing by orbital precession (Fig. F18). This signal weakens downhole as the abundance of volcanic ash increases (Fig. F18B). Irregularly shaped white blebs with halos of small, dark, opaque grains, interpreted in smear slide examination to be Mn oxides and Fe oxides, decrease in abundance downhole (Fig. F19). Halos are particularly pronounced between 135 and 145 mcd. Volcanic ash is first observed at 155.7 mcd, and chert nodules are first observed at 231.1 mcd. The grain density record of Site 1263 is marked by an interval of increased values between 200 and 270 mcd. This corresponds to a characteristic interval of high Mn values just below the absolute peak in Mn interstitial water concentrations (see "Geochemistry").
Peaks in MS and NGR values and a drop in L* coincide with a 6-cm interval of reddish nannofossil clay at 295.9 mcd; this feature likely corresponds to a similar clay-rich layer recovered ~22 m above the P/E boundary at Site 1262 (Chron C24n clay layer). Roughly 2 m above the red interval at Site 1263, a darker clay-enriched horizon coincides with a series of less pronounced peaks (Fig. F20).
A marked downhole decrease in MS, a local minimum in carbonate content, and the onset of a gradual increase in NGR values define the upper boundary of Subunit IC. The 82.7-m-thick sequence is dominated by massive homogeneous nannofossil ooze and chalk with rare centimeter-scale intervals of partially silicified limestone. Subunit IC is almost entirely calcareous with the exception of 50–60 cm of sediment immediately above the P/E boundary. This boundary was recovered intact in two of four holes at ~335.6 mcd (Sections 208-1263C-14H-2, 149 cm, at 285.4 mbsf and 208-1263D-4H-1, 34 cm, at 285.0 mbsf) and is marked by a sharp contact between grayish brown ash-bearing clay above and light gray nannofossil ooze below. Portions of the clay-rich interval were recovered from Holes 1263A and 1263B. Bioturbation is not present in the 30 cm above the contact but is clearly visible upsection as sediment grades back into nannofossil ooze and chalk; it is not observed farther upsection because of color uniformity. The clay-rich interval is characterized by low carbonate content (see "Geochemistry") and L* and by high MS and NGR values (Figs. F21, F8). Working-half P-wave velocity measurements were taken at a 1-cm sampling interval on Section 208-1263C-14H-2, revealing a stepped downhole increase in sound velocity from ~1600 to ~1800 m/s just above the contact. Below this peak, velocity returns to ~1600 m/s over a 1-m interval (Fig. F14C).