Three holes were cored at Site 1262. Hole 1262A was cored to a depth of 182.4 mcd (162.0 mbsf); Hole 1262B was cored to 232.9 mcd (209.7 mbsf); and Hole 1262C was washed to 97.8 mcd (90.0 mbsf) and cored to 236.9 mcd (213.0 mbsf). The major lithologies recovered include nannofossil ooze, foraminifer-bearing nannofossil ooze, foraminifer-nannofossil ooze, clay-bearing nannofossil ooze, clayey nannofossil ooze, and clay. Minor lithologies include volcanic ash–bearing nannofossil ooze and hematite-bearing clay. Combining measurements of MS, natural gamma radiation (NGR), and L* with smear slide analysis, we have divided this sequence into three major lithostratigraphic units, with two units further subdivided based on minor lithologic variation (Table T4). In addition to the description of these lithostratigraphic subdivisions, we provide descriptions of the P/E and K/P boundary lithologies.
Unit I consists primarily of nannofossil ooze that varies slightly in the relative abundance of associated skeletal components, organic content, and clay content (Fig. F6). MS and NGR are low, and L* is high, reflecting a high average carbonate content of 89 wt% (Figs. F7, F8, F9). The color varies throughout from brown and very pale brown to light gray in 10- to 50-cm bands and likely reflects differences in clay, organic content, and carbonate production or preservation. Other than these features, bedding is largely absent. The boundaries between color bands are generally diffuse, reflecting moderate to pervasive bioturbation. In the upper part of this unit, oscillations in sediment color may record the variable dissolution of carbonate during glacial–interglacial cycles of the Pliocene–Pleistocene (i.e., typical Atlantic-type carbonate cycles). These decimeter-scale cycles are well expressed in signals of both MS and L* (Fig. F10A). Porosity decreases, whereas bulk density and compressional wave (P-wave) velocity increase slightly downcore in Unit I (Figs. F11, F12A, F13A), reflecting sediment consolidation. In general, densities and P-wave velocities determined with the MST correlate well with the discrete sample measurements (Figs. F12B, F13B), indicating that the cores have not been severely disturbed during the splitting process. In addition, porosity is negatively correlated with bulk density, whereas P-wave velocity and grain density are positively correlated with bulk density (Fig. F12C, F12D, F12E).
Unit II consists of upper and lower clay-rich subunits separated by a middle subunit of nannofossil ooze. Subunit IIA is composed of clay, ash-bearing clay, and nannofossil clay; Subunit IIB is an intermediate interval of nannofossil ooze and clayey nannofossil ooze; Subunit IIC is a thick interval of clay, ash-bearing clay, and nannofossil clay.
The most common lithology of Subunit IIA is a dark gray to brown clay, containing variable amounts of nannofossils, altered volcanic ash, and rare foraminifers (Fig. F6). The generally uniform color of this unit limits determination of bioturbation, but where color gradients exist, bioturbation appears moderate to extensive. Scattered oxide grains (2–25 µm), zeolites, and volcanic ash and shards are present locally as accessory components. MS and NGR values are high, and carbonate is virtually absent (Fig. F7).
The contact between Unit I and Subunit IIA is marked by sharp breaks in physical properties that reflect the change from a carbonate- to clay-rich lithology. Bulk density, grain density, carbonate content, L* (Fig. F14A), and seismic velocity (Figs. F11, F13A) decrease downhole as the nannofossil ooze grades into carbonate-poor clay (Fig. F14A, F14B). The correlation of bulk density with porosity, grain density, and P-wave velocity sensor (PWS3) measurements with the Hamilton Frame probe (Fig. F12C–E) suggests that sediment bulk density is controlled primarily by the degree of compaction. The chromaticity ratio (a*/b*) increases dramatically at the boundary as a result of relative clay abundance (Fig. F14C).
Subunit IIB is light brown to gray nannofossil ooze and foraminifer-bearing nannofossil ooze alternating with medium brown to gray clayey nannofossil ooze. This subunit is higher in nannofossil, carbonate, and sediment L* and lower in MS and NGR than Subunits IIA and IIC (Figs. F6, F7). Subunit IIB has slightly higher bulk density and P-wave velocity than Subunit IIA (Figs. F8, F12A, F13A), resulting from a higher carbonate content. Some foraminifer-rich intervals have sharp, irregular basal contacts with the underlying clay and fine upward into nannofossil ooze (Fig. F15). In contrast to the sharp basal contact, the upper contact of these carbonate intervals is bioturbated, forming a gradual contact with overlying, relatively dark nannofossil clay. We interpret these sequences as fine-grained carbonate turbidites in a clay-dominated sedimentary regime. This interpretation is supported by biomarkers of Paleocene and Eocene age for the basal coarser-grained sediments compared to Oligocene–Miocene biostratigraphic ages for finer-grained sediment above and below.
The E/O boundary, although not clearly defined at this site (see "Eocene/Oligocene Boundary Interval (75–81 mcd)" in "Biostratigraphy"), occurs in an interval containing the base of Subunit IIB and the upper part of Subunit IIC. Section 208-1262B-9H-4 (Fig. F16) highlights upcore lithologic transition of clay-rich lithologies characterized by high MS and NGR and low L* (Subunit IIC) to calcareous foraminifer- and nannofossil-rich lithologies characterized by low MS and NGR and high L* (Subunit IIB). This lithologic transition represents our best approximation of the E/O boundary.
Subunit IIC is lithologically similar to Subunit IIA and is composed primarily of dark gray to brown clay with intervals of light brown nannofossil ooze. A prominent 2-cm volcanic ash layer was recovered at 85.25 mcd in intervals 208-1262A-8H-5, 6–8 cm (71.2 mbsf), and 208-1262B-10H-1, 93–94 cm (77.83 mbsf). This ash layer contains abundant clear volcanic glass shards, suggesting a felsic volcanic origin; some have been altered to clays by devitrification and weathering. The ash layer is marked by a large spike (625 instrument units) in the MS record (Fig. F7). Subunit IIC is distinguished from Subunit IIB above and Subunit IIIA below by its higher MS and NGR values and lower carbonate content and L* (Figs. F7, F8, F9, F12A).
Unit III is a carbonate-rich lithology with low MS and NGR signals, similar to lithostratigraphic Unit I (Fig. F7). We have divided Unit III into two subunits based on the abundance of clay and volcanic ash (Fig. F6). Subunit IIIA is a nannofossil ooze with very minor clay and ash components; Subunit IIIB shows a progressive downcore increase in clay and ash. Although petrographically gradual, this increase in clay is associated with a distinct increase in the MS and NGR values (Figs. F7, F8) and a decrease in sediment L* (Figs. F7, F9, F14A).
Subunit IIIA is a 75-m-thick interval of relatively pure nannofossil ooze (92.4 wt% carbonate) with low MS and NGR values and high L* (Fig. F7). The subunit has lower porosity and higher bulk density and P-wave velocity than Units I and II (Figs. F11, F12A, F13A). Color oscillates from light gray brown to medium gray on the decimeter to meter scale and covaries with changes in MS (Fig. F10B). Based on the gradational nature of most contacts between these colors and the occasional preservation of burrow traces, we interpret this interval as moderately bioturbated. Moreover, large rounded or irregular pinkish white blebs (1–3 cm in diameter) are present throughout this unit. These blebs, which may be associated with burrowing, often have a halo of small brown to black opaque grains interpreted to be Mn oxides and Fe oxides based on reflected-light observations in smear slides. Pyrite was not commonly observed in these sediments, which is in agreement with interstitial water chemistry that shows little evidence of sulfate reduction (see "Geochemistry").
Subunit IIIA consists almost entirely of nannofossil ooze with the exception of a thin clayey horizon at ~117.3 mcd and 30–40 cm of clay-rich sediment directly above the P/E boundary (Figs. F17, F18). The layer at 117.3 mcd comprises ~20 cm of reddish clay-bearing nannofossil ooze containing abundant fragments of volcanic ash. This horizon exhibits a distinctive positive spike in MS and chromaticity a*.
The P/E boundary was recovered in three holes at ~140.2 mcd (Sections 208-1262A-13H-6, 50 cm, at 122 mbsf; 208-1262B-15H-3, 74 cm, at 127.84 mbsf; and 208-1262C-5H-4, 95 cm, at 127.95 mbsf) and is marked by an abrupt contact between nannofossil ooze and dusky red hematite and ash-bearing clay (Fig. F18). Bioturbation is nearly absent in the 10 cm above the contact and then increases as sediment grades into nannofossil ooze above. Nannofossil-rich horizons in the underlying Paleocene sequence are firmer and approach chalk, whereas Eocene sediments are uniformly soft and unlithified. The P/E boundary is interpreted as a horizon formed in response to a severe dissolution event at Site 1262 (4753 m water depth) as expressed by a decrease in carbonate content (see "Geochemistry"), a decrease in L*, and increases in NGR and MS (Figs. F18, F19). The structure of the MS signal is nearly identical for all three holes (Fig. F20).
Subunit IIIB has a greater abundance of clay than Subunit IIIA and exhibits a greater range of minor lithologies, including nannofossil ooze, foraminifer-bearing nannofossil ooze, clay-bearing to clayey nannofossil ooze, foraminifer- and nannofossil-bearing clay, and ash-bearing clay. The upper part of Subunit IIIB is typically very uniform, lacks the thin layers (5–10 cm) of light-colored nannofossil oozes that are sporadic in Subunit IIIA, and is generally darker (perhaps reflecting the increased clay content). MS, NGR, and P-wave velocity are higher in Subunit IIIB than in Subunit IIIA (Figs. F7, F13A). Variations in sediment lightness are negatively correlated with MS and display distinct cyclicity (Fig. F10C), representing a potential sedimentologic response to orbital forcing.
In Subunit IIIB, clay content increases downhole where the major lithology, a medium brown clayey nannofossil ooze and ash-bearing clay, is interbedded and intermixed with a reddish brown nannofossil ooze directly above the K/P boundary. We recovered the K/P boundary from two holes at ~216.6 mcd (Sections 208-1262B-22H-4, 137 cm, at 195.53 mbsf and 208-1262C-13H-2, 68 cm, at 195.68 mbsf). The boundary marks an abrupt lithologic transition from underlying carbonate-rich, clay-bearing nannofossil ooze with foraminifers and nannofossil-bearing and foraminifer-bearing clays above (Fig. F21). Across the boundary, clay content, MS, and NGR values increase (Figs. F6, F7).