Maximum depth of penetration: 321 mbsf
Oldest formation: late Paleocene
Time on site: 6.9 days (1410 hr on 08 April–1130 hr on 15 April)
Site 1265 (proposed Site WALV-9B) is located along the northwestern flank of Walvis Ridge just a few hundred meters beneath the summit (Figs. F16, F23). At 3.08 km water depth, this site represents the shallow middepth site of the Leg 208 depth transect. Situated above the level of the CCD throughout the Cenozoic, Site 1265 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 complete recovery of the sedimentary section with minimal coring disturbance, a requirement that is best met by using 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.
Four holes, offset ~20–30 m from each other, were cored at Site 1265 using the APC and XCB coring systems to recover a 321-m-thick section of upper Paleocene to Pleistocene nannofossil ooze and chalk. Hole 1265A was cored with the APC from the mudline to 286 mbsf and with the XCB to 321 mbsf. Hole 1265B was cored with the APC from the mudline to 252 mbsf, where mechanical problems prevented further penetration and required the recovery of the drill string. Hole 1265C was cored with the APC from 185 to 204 mbsf, where the same problem as in Hole 1265A occurred and the drill string had to be recovered again. Hole 1265D was cored with the XCB from 248 to 270 mbsf and by the APC to 275 mbsf to recover the P/E boundary. Total nominal core recovery for the site was 95%. Using MS and color reflectance (red/blue) data, cores from the four holes were aligned by depth shifting and representative intervals were spliced to create a nearly continuous stratigraphic section for the uppermost Paleocene to Pleistocene, extending from 0 to 316 mcd. Two small (<1 m) gaps exist in the lower Eocene interval. Cores from the upper Paleocene, where recovery was low, were depth shifted by extrapolating a constant growth rate of 14% from the correlated interval. The total length of the composite is 359 mcd.
The sediments recovered at Site 1265 are predominantly composed of nannofossil ooze, foraminifer-bearing nannofossil ooze, foraminifer-nannofossil ooze, nannofossil-foraminifer ooze, clay-bearing nannofossil ooze, and a foraminifer- and clay-bearing nannofossil ooze (Fig. F24). The sequence has been divided into two lithologic units and several subunits. Lithologic Unit I (0–55.8 mcd) is composed of upper Miocene to Pleistocene foraminifer-bearing nannofossil ooze and nannofossil ooze. Lithologic Unit II (55.8–359.1 mcd) is composed of upper Paleocene to middle Miocene nannofossil ooze with rare intervals of foraminifer-bearing nannofossil ooze and clay. Unit II is further subdivided into three subunits. Subunit IIA (55.8–192.7 mcd) consists of lower Oligocene to upper Miocene nannofossil ooze and foraminifer-bearing nannofossil ooze. Subunit IIB (192.7–248.5 mcd) is lower to upper Eocene nannofossil ooze, with occasional intervals of foraminifer-bearing nannofossil ooze. The contact between Subunits IIA and IIB represents the E/O boundary and is marked by a step increase in MS and step decrease in color lightness. Subunit IIC (248.5 – 359.1 mcd) is a Paleocene to lower Eocene nannofossil ooze and foraminifer-bearing nannofossil ooze.
The P/E boundary clay layer was recovered with mixed success. In Hole 1265A, the entire clay layer was recovered intact in Core 208-1265A-29H (317 mcd), the last full-stroke APC of the hole. In Hole 1265B, the APC became stuck in the core barrel at 251.7 mbsf (~288.7 mcd), just ~30 m above the target interval. In Hole 1265C, during our attempt to spot core the lower Eocene and upper Paleocene sediment, the core barrel again became stuck, this time at 204 mbsf (~229.7 mcd). After several XCB cores were taken in Hole 1265D, we returned to APC to core the boundary interval. A partial stroke (4.8 m) penetrated the top of the clay layer and recovered 50 cm in the core catcher. A second APC, fired from 2 m above the bottom of the hole, advanced <1 m and recovered an additional 10 cm of the clay layer.
Based on biostratigraphy, the sequence appears to be relatively complete over the upper Paleocene and lower Eocene, upper Oligocene and lower Miocene, and upper Pliocene. The middle Eocene is condensed, and unconformities spanning ~7 and ~3 m.y., respectively, are present at the lower/middle and middle/upper Eocene boundaries (Fig. F25). An unconformity spanning ~2–3 m.y. is present at the Miocene/Pliocene boundary. Sedimentation rates range from 10 to 25 m/m.y. in the upper Paleocene and lower Eocene, from 4 to 12 m/m.y. in the upper Eocene to middle Miocene, and from 8 to 14 m/m.y. in the Pliocene–Pleistocene. Downslope reworking of microfossils, particularly foraminifers, is common in the middle Eocene through Pliocene sequences.
A number of "bioevents" is recorded in Site 1265 that are also recorded at the other Leg 208 Sites. These include an early Miocene Bolivinid abundance acme at 85–87 mcd, an event recorded elsewhere in the Atlantic and Indian Oceans. Several discrete Braarudosphaera layers are also present in the upper Oligocene between 154 and 161 mcd. These layers, which are prominent throughout the south Atlantic Ocean, particularly at sites along the margins, are recorded at precisely the same stratigraphic level in each of the Leg 208 sites.
The moderate sedimentation rates and generally high carbonate content of the Site 1265 sediments are consistent with the relatively shallow depth of the site. During the Paleogene, the site apparently remained above the lysocline depth with the exception of several brief periods in the latest Paleocene and early Eocene, including the P/E boundary. The latter is marked by an abrupt contact between upper Paleocene nannofossil ooze and lower Eocene dusky red, zeolite- and nannofossil-bearing clay that grades upcore into nannofossil ooze. The minimum carbonate concentration at this "intermediate" depth site is ~30 wt%, whereas minimum values near 0 wt% characterize the deeper and shallower Sites 1262 and 1263, respectively. Either carbonate was bioturbated into the first centimeters above the contact at Site 1265 or the earliest Eocene is not represented at Site 1265. 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 this layer at 318.81 to 318.86 mcd. This coincides with the brief appearance of excursion planktonic foraminifer taxa and slightly precedes an abundance shift in nannofossils from Fasciculithus to Zygrhablithus.
Site 1265 sediments record pervasive bedding cycles as expressed in the MS, color reflectance, and other high-resolution core logging data. 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 Sites 1262 and 1263, which should permit high-resolution correlation of units.