Three holes were cored at Site 1266. Hole 1266A was cored continuously to a depth of 344 mcd (295.2 mbsf); the mudline was obtained in Hole 1266B, which was then washed to 253 mcd (220 mbsf) and cored to 367.3 mcd (318.9 mbsf); and Hole 1266C was washed to 57 mcd (62 mbsf) and then cored to 379.4 mcd (333.1 mbsf). The major lithologies recovered include nannofossil ooze, nannofossil chalk, foraminifer-bearing nannofossil ooze, and clay-bearing nannofossil ooze. Minor lithologies include clayey nannofossil ooze, nannofossil clay, and nannofossil-bearing clay. Zeolite-bearing clay and ash-bearing nannofossil clay were present in association with discrete volcanic ash horizons. We have divided this sequence into three units based on lithologic observations and physical property measurements (Table T4; Fig. F6). These observations and measurements are summarized in plots illustrating variation with depth: whole-core MS, natural gamma radiation (NGR), sediment density by gamma ray attenuation, and compressional wave (P-wave) velocity from the multisensor track (Fig. F7); sediment lightness (L*), carbonate content, and chromaticity (Fig. F8); smear slide components (Fig. F9); split-core measurements of grain and bulk density, porosity, and Hamilton Frame P-wave velocity values (Figs. F10, F11, F12); and interstitial water Mn and Fe content (Fig. F13).
The succession of lithologies present at Site 1266 is quite similar to those observed at other Leg 208 sites. Unit I, a foraminifer-bearing nannofossil ooze, represents the upper Miocene through Holocene sequence. This is underlain by Units II and III, which share similar patterns of lithologic variation. Each of these two units comprises a lower nannofossil ooze–dominated facies that grades upward into clay-bearing nannofossil ooze and, periodically, to nannofossil clay. Unit III differs from Unit II in the presence of nannofossil chalk and diagenetic siliceous lithologies including chert and porcellanite in the lower part of the unit.
Unit I is a light gray and white to medium brown foraminifer-bearing nannofossil ooze that is differentiated from underlying units by a greater foraminiferal abundance, low clay concentration, and the common occurrence of size-graded sediment gravity flows (turbidites). The contact with underlying Unit II is distinct, marked by an abrupt transition to pale yellow or light brown nannofossil ooze and a significant decrease in foraminiferal abundance. This lower boundary is coincident with a downcore decrease in sediment L* and increases in chromaticity (b*), MS, and NGR values (Fig. F6). Bulk density and P-wave velocity also decrease, and sediment porosity increases for a short interval below this boundary (Figs. F10, F11A, F12A).
Unit I exhibits stratigraphic variation in color and sedimentary structures. The upper part of Unit I is a cyclic light gray to brown and medium brown foraminifer-bearing nannofossil ooze that alternates on a meter scale. Associated lithologic and sedimentologic features include extensive mottling of the upper part by blebs that appear to be bioturbational or diagenetic features. These 1- to 3-cm-sized pinkish to whitish blebs are distributed throughout the section and are similar to structures observed at other sites and in older units at this site.
The L* of Unit I increases downcore where nannofossil ooze and foraminifer-bearing nannofossil ooze oscillate from white to light gray on a decimeter to meter scale. The unit is marked by distinctive light brown layers of coarse-grained foraminiferal ooze that have sharp erosional bases and diffuse bioturbated upper boundaries (Fig. F14). These horizons range from 3 to 60 cm in thickness and are interpreted as turbidity or gravity flow deposits similar to those described at Site 1262. These occur primarily in the lower part of Unit I and compose <5% of the total unit thickness.
In Unit I, black and dark brown oxides are present as small dispersed granules, concentrated layers (Fig. F14), or discrete nodules. X-ray diffraction analyses of these oxides from Sites 1264 and 1265 (see Fig. F16, in the "Site 1264" chapter and Fig. F13, in the "Site 1265" chapter) identified lithophorite, a mineral composed primarily of Mn oxide. Precipitation of this mineral is controlled by diagenetic oxidation-reduction reactions within sediments that are mediated by variation in organic content and sediment porosity (i.e., burrow margins or discrete concentrations of organic matter). In this sequence, reduction of Mn likely occurs in the finer-grained sediment and oxidation occurs at the interface with sediment of higher porosity where oxidizing conditions exist. These sediments could represent coarser depositional laminae or margins of turbidite layers. Although Mn oxides are common throughout the cored sequence, in Unit I they are frequently present in discrete layers; lower in the Site 1266 sequence, these oxides are observed as dispersed micronodules and granules or as diffuse halos surrounding blebs.
Unit II comprises two major lithologies, a light gray to light brown nannofossil ooze and a dark to medium brown clay-bearing nannofossil ooze. The upper boundary of Unit II is coincident with an abrupt downcore decrease in L* and an increase in both a* (red-green) and b* (blue-yellow) chromaticity (Figs. F8, F15). Magnetic susceptibility and NGR exhibit more gradual transitions from lower and stable values in Unit I to higher and more variable values in Unit II (Fig. F7), reflecting an increase in clay content.
Clay content in Unit II, although variable, generally decreases in abundance downcore. In the upper part of Unit II, the alternation of nannofossil ooze and clay-bearing nannofossil ooze on a decimeter to meter scale is expressed as oscillations in L*, a*, and b* and macroscopically as color cycles (Fig. F8). This alternation is also evident in MS and the NGR variation (Fig. F16). From the upper to lower part of Unit II, the relative abundance of clay-bearing nannofossil ooze progressively decreases as lighter-colored nannofossil ooze becomes the dominant lithology. This change is accompanied by a gradual decrease in MS and NGR and an increase in L* (Fig. F6). The lower boundary of Unit II exhibits a pattern of change in physical properties similar to that observed at the Unit I/II boundary: MS and NGR values increase and become more variable and L* decreases (Figs. F6, F7). Moreover, bulk density increases and is matched by a decrease in sediment porosity (Fig. F10); Mn concentration in interstitial water increases abruptly then decreases to a constant, low level (Fig. F13).
Sediments of Unit II were formed primarily by accumulation of pelagic clay and carbonate, with little sedimentologic evidence of downslope transport by gravity or turbidity flows. Paleontologic observations suggest a significant degree of reworking in both Units I and II. As discussed above, features diagnostic of transport and deposition by density and gravity flows are present in the lower part of Unit I; however, little physical evidence is available in Unit II that demonstrates transport by such a mechanism. The paleontologic evidence for reworking may reflect a process of continuous, although volumetrically minor, transport of surface sediment downslope in response to bottom currents or small-scale gravity flows. It is possible that some of the decimeter- to meter-scale color alternations of upper Unit II could have formed by similar depositional processes. However, given the absence of size grading or other characteristic sedimentary structures, it is difficult to attribute this color and lithologic variation solely to gravity or turbidity flows. Rather, these differences are primarily due to variation in clay content, a feature that could be produced by numerous physical or chemical processes unrelated to downslope sediment transport.
Assuming a constant flux, pelagic clay could be segregated or concentrated by dissolution of carbonate components. In the case of clay-rich sediments of Unit II, carbonate dissolution is a plausible explanation given the nature of accessory phases associated with the clay-rich horizons. For example, volcanic glass is present in small amounts and various states of alteration in most of the lithologies at Site 1266, yet it is a minor to major component in the clayey horizons. These layers also contain abundant hematite and zeolites (primarily phillipsite; e.g., Sample 208-1266C-7H-1, 100 cm), which indicate in situ formation of these minerals from the alteration of volcanic glass. Concentration of clays by dissolution could explain the simultaneous segregation of fine-grained clays and coarser volcanic ash, an association difficult to produce by downslope transport. For these reasons, it is likely that the interbedding of clay-rich and clay-poor lithologies is the result of changes in oceanic carbonate saturation state, rather than some process of sediment transport.
Synsedimentary deformational features, such as inclined bedding and recumbently folded beds, are present in upper Unit II (Fig. F17). These features form through slumping and downslope creep and indicate deformation on a sloped depositional surface. Such observations are compatible with seismic interpretations of "chaotic reflections" in the Oligocene and Miocene and biostratigraphic observations suggesting sediment reworking.
Unit III is dominated by nannofossil ooze, clay-bearing nannofossil ooze, and nannofossil chalk. The boundary between Units II and III is coincident with an increase in MS and NGR, corresponding to an increase in clay content documented by smear slide analysis (Fig. F9). Moreover, sediment L* decreases (Figs. F6, F7), bulk density (BD) and P-wave velocity increase, and grain density and porosity decrease across the Unit II/III boundary (Fig. F10).
The stratigraphic variation in clay content is similar to that observed for Unit II. Clay-bearing nannofossil ooze is the predominant lithology in the upper part of Unit III and is progressively replaced downcore by clay-poor nannofossil ooze. Nannofossil ooze becomes increasingly indurated and grades into nannofossil chalk, which requires that most cores be sawed rather than split with a wire. Chalk abundance increases dramatically below the P/E boundary at 306.77 mcd, where as much as 50% of the sediment is highly indurated.
Minor lithologies of Unit III are clayey nannofossil ooze, nannofossil clay, ash-bearing nannofossil ooze, and ash-bearing clay. Volcanic ash is a minor component throughout the sequence and is locally concentrated as greenish brown clay-rich layers (e.g., Samples 208-1266A-27X-3, 118–121 cm, and 27X-CC, 12–14 cm) or intermixed within clay horizons where alteration of volcanic ash forms authigenic phillipsite, clinoptilolite, and hematite. These ash layers coincide with increases in MS and NGR and are responsible for much of the variability measured in these parameters in the lower part of Unit III.
The lower part of Unit III contains rare silicified horizons that formed by diagenetic replacement of carbonate by SiO2. The bulk of material composing these layers is optically isotropic with the exception of rare nannofossils. These indurated layers are probably composed of opal-CT that has not yet stabilized to chert. As such, these horizons are more appropriately termed porcellanite, given their intermediate stage of lithification and mineral instability. Chert occurs rarely as isolated reddish to pink nodules in Unit III (e.g., Sample 208-1266C-19X-6, 105–110 cm).
The presence of nannofossil clay and clayey nannofossil ooze is likely controlled by carbonate dissolution that concentrates insoluble phases such as clay and volcanic glass. The most notable example of such dissolution events is the P/E boundary interval. This boundary was recovered in all three holes at Site 1266 with varying degrees of drilling disturbance (Fig. F18). The boundary was recovered at 306.77 mcd in Samples 208-1266A-31X-3, 37 cm (271.3 mbsf), 208-1266B-6H-CC, 25 cm (277.75 mbsf), and 208-1266C-17H-3, 112 cm (276.62 mbsf). Hole 1266C provides the most complete section with the least disruption by drilling. It is marked by a precipitous drop in carbonate from 90 wt% beneath the boundary to a ~20-cm-thick interval of clay that is virtually free of carbonate. The clay is a deep dusky red that is characteristic of Fe oxides (commonly hematite). Varying amounts of volcanic glass are associated with the boundary clay, giving rise to ash-bearing nannofossil clay and abundant zeolites derived from in situ alteration of ash. Carbonate content increases over an interval of 50–70 cm above the carbonate-free clay layer and consists dominantly of nannofossils. Bioturbation is nearly absent in the lowermost 20 cm of the boundary clay and then increases in the upper part where it contacts the overlying nannofossil ooze. Based on the overall thickness and MS record, it appears the P/E contact in Hole 1266A is not complete. The lower part of the clay layer was not recovered by XCB coring.
In addition to the P/E boundary sequence, reddish nannofossil clay is present in several intervals in Unit III. These clays are similar in appearance and lithologic structure to the Paleocene–Eocene dissolution interval, although much thinner. They are recognized by increases in MS and NGR (Fig. F7). One distinctly red clay horizon is present above the P/E boundary at ~293 mcd (Samples 208-1266A-30X-2, 53 cm, 208-1266B-5H-5, 87 cm, and 208-1266C-16H-2, 45 cm), is correlative among other Leg 208 sites, and is presently defined as the Chron C24n clay layer. Initial analysis of carbonate content indicates that the degree of dissolution is not as extensive as that in the P/E boundary interval, with a minimum value of ~40 wt% carbonate (Fig. F19). As also illustrated in the records of MS and chromaticity, the basal contacts are gradational rather than sharp, as with the P/E boundary. This and other similar events probably represent shoaling in the depth of the lysocline during the Paleogene.