Maximum depth of penetration: 334.2 mbsf
Oldest formation: late Paleocene
Time on site: 5.6 days (1405 hr on 15 April–0445 hr on 21 April)
Site 1266 (proposed Site WALV-10F) is located along the northwestern flank of Walvis Ridge (Figs. F16, F26). At 3.8 km water depth, this site represents one of the middepth sites of the Leg 208 depth transect. Situated near the level of the CCD throughout the Cenozoic, Site 1266 was expected to yield a stratigraphically continuous and expanded sequence of upper Paleocene and lower Eocene pelagic nannofossil ooze. The primary objective was to recover sedimentary sections with sufficient resolution to characterize high-frequency changes in bottom water chemistry and circulation at shallow bathyal depths during several of the key climatic events of the Paleogene including the Eocene–Oligocene transition, the EECO, and the PETM. Ideally, this requires recovery of the complete sedimentary section with minimal coring disturbance; a requirement that is best met with the APC. To this end, the site was located in a broad channel at the base of a slope extending down from the ridge crest. Because the Neogene is relatively condensed here, the key Paleogene target intervals are at subbottom depths of <300 mbsf.
Three holes, offset ~20–30 m from each other, were cored at Site 1266 using the APC and the XCB coring systems to recover a 334.2-m-thick section of upper Paleocene to Pleistocene nannofossil ooze and chalk. Hole 1266A was cored with the APC to 217.1 mbsf and with the XCB to 298.6 mbsf. Hole 1266B was cored with the APC from the mudline to 7.6 mbsf. The hole was then drilled ahead to 220 mbsf without coring to save time, and the remainder of the hole (220–321 mbsf) was cored with the XCB. Hole 1266C was washed down to 62 mbsf and cored with the APC from 62 to 192 mbsf. From 192 to 245 mbsf the hole was washed again, and a single XCB core was taken from 245 to 254.5 mbsf, followed by three APC cores from 254.5 to 282.7 mbsf, which recovered the P/E critical boundary interval. Finally, the hole was completed with the XCB to 334.2 mbsf. Total nominal core recovery for the site was 98%.
Using MS and color reflectance (red/blue) data, cores from the three holes were aligned by depth shifting. A splice of representative intervals was created for the uppermost Paleocene to lowermost Eocene, the interval that contains the P/E boundary. Because the strategic decision not to double core the Pliocene–Pleistocene and upper Eocene and because of poor overlap between cores from Holes 1266A and 1266C in the Oligocene to Miocene interval, recovery of a complete section was not achieved in those intervals and cores from the Pleistocene down to lowermost Eocene were depth shifted, assuming a constant mcd growth rate of 15%.
The sediments recovered at Site 1266 are comprised predominantly of nannofossil ooze, foraminifer-bearing nannofossil ooze, foraminifer-nannofossil ooze, nannofossil-foraminifer ooze, clay-bearing nannofossil ooze, and foraminifer- and clay-bearing nannofossil ooze (Fig. F27). The sequence has been divided into three lithologic units. Lithologic Unit I (0–76.8 mcd) is composed of uppermost Miocene to Pleistocene foraminifer-bearing nannofossil ooze and nannofossil ooze. The unit yields frequent turbidites marked by distinctive light brown layers of coarse-grained foraminifer ooze that have sharp boundaries and gradational tops. Lithologic Unit II (76.7 to 214.7 mcd) is composed of Oligocene to uppermost Miocene clay-bearing nannofossil ooze and nannofossil ooze and clay. Lithologic Unit II shows clay horizons that are a function of carbonate dissolution, and inclined and folded layers are indicative of synsedimentary slumping. Unit III includes upper Paleocene to Oligocene clay-bearing nannofossil ooze, nannofossil ooze, nannofossil chalk, and nannofossil clay. The chalky intervals are in the lower part of Unit III, mainly below the P/E boundary. Volcanic ash is a minor component throughout the sequence.
As at Sites 1263 and 1265, recovery of the P/E boundary clay layer met with mixed success. The XCB recovered only a partial clay layer in Hole 1266A (Core 208-1266A-31X), presumably because the cutting shoe rotation tends to grind away sediment at the interface of stiff clay and soft calcareous ooze. Therefore, the APC was used in the subsequent two holes. In Hole 1265B, the APC core catcher got stuck in the clay layer (Section 208-1266B-6H-CC). In Hole 1265C, the APC achieved a full stroke and recovered the entire clay layer in Section 208-1266C-17H-3.
Biostratigraphy indicates that the calcareous microfossils generally show moderate preservation. All microfossil groups indicate extensive reworking throughout the section except for the interval surrounding the P/E boundary, the prime target of Site 1266. Despite all the reworked specimens, two unconformities were recognized, one with a duration of 1.2 m.y. in the upper Miocene and the second one spanning ~37 to ~47 Ma, a large part of the middle Eocene. Sedimentation rates range from 6 to 24 m/m.y. in the upper Paleocene to lower Eocene, <4 m/m.y. in the middle Eocene, and from 4 to 8 m/m.y. in the upper Eocene to Pleistocene, not including the upper Miocene unconformity (Fig. F28).
At the approximate middepth of the Leg 208 transect, Site 1266 is critical for constraining the timing of shifts in ocean carbonate chemistry. The moderate sedimentation rates and generally high carbonate content of the upper Paleocene–lower Eocene sediments are consistent with the results at Sites 1262 and 1267. During the Paleogene, the site apparently remained above the lysocline depth with the exception of at least two brief periods. The first "pink" clay-rich layer is present in the upper Eocene interval at ~293 mcd (carbonate content = ~40 wt%). The lower Eocene "pink" layer falls in zone NP11, and has now been identified in each of the lower Eocene sequences recovered on Walvis Ridge. This ~10-cm-thick layer clearly represents a regional bedding horizon and exhibits several lithologic characteristics of the P/E boundary layer. The second prominent clay layer is the P/E boundary clay at 306.4 mcd. The P/E boundary is marked by an abrupt contact between upper Paleocene nannofossil ooze and lower Eocene dusky red zeolite- and nannofossil-bearing clay that grades upwards into nannofossil ooze. The carbonate content is 0 wt% at this "intermediate" depth site. Moreover, the P/E boundary interval is marked by sharp increases in MS and a decrease in color lightness. The benthic foraminiferal extinction event occurs just below the base of the clay layer (306.8 mcd). This coincides with a major shift in nannofossil abundances from Fasciculithus to Zygrhablithus.
The Site 1266 sediment record displays pervasive bedding cycles as expressed in the MS, color reflectance, and other high-resolution core logging data. The entire Site 1266 sequence shows cycles with different wavelengths. The Miocene sequence exhibits light gray to brown foraminifer-bearing nannofossil ooze that alternates with medium brown foraminifer-bearing nannofossil ooze on a meter scale. The lightness record shows cycles on a decimeter to meter scale. The lower Eocene, in particular, is characterized by pronounced decimeter- to meter-scale bedding cycles. The variance is concentrated in three frequency bands. The shorter cycles have a frequency close to that of the orbital precession, whereas the longer oscillations have a frequency close to the 100- and 400-k.y. eccentricity cycles. Both the frequency and amplitude of the bedding cycles are similar to those observed at Sites1262, 1263, and 1265 and should permit high-resolution correlation of units.