6. Site 12651

Shipboard Scientific Party2


Site 1265 (proposed Site WALV-9B; seismic line GeoB 01-048, common depth point [CDP] 3854) is located in a broad channel at the base of a slope extending westward down from the crest of Walvis Ridge (Figs. F1, and F2, both in the "Leg 208 Summary" chapter; Fig. F1). The site is at an upper bathyal water depth of 3083 m, ~48 m deeper than Deep Sea Drilling Project (DSDP) Site 529 located 15 km to the southeast where a 417-m-thick sequence of Maastrichtian to Pleistocene sediment was recovered (Moore, Rabinowitz, et al., 1984). The Site 529 sequence was poorly recovered, particularly the Paleocene interval, because of rotary drilling. Slumps were encountered at this site in the upper Paleocene, in the Oligocene–Miocene, and in the lower Pleistocene intervals. The upper lower Oligocene through Pleistocene interval (0–160 meters below seafloor [mbsf]) of the Site 529 sedimentary record consists of homogeneous foraminifer-nannofossil and nannofossil ooze. The upper Paleocene to upper lower Oligocene unit (160–284 mbsf) consists of nannofossil and foraminifer-nannofossil oozes and chalks. This unit lies on top of the lowermost interval, which extends to the bottom of the hole at 417 mbsf. The oldest sediments are Maastrichtian in age.

Our main objective for this site was the recovery of undisturbed sediments recording critical intervals in the early Cenozoic, specifically the Paleocene/Eocene Thermal Maximum and the period of global cooling and growth of polar ice caps across the Eocene/Oligocene (E/O) boundary into the earliest Oligocene (early Oligocene Glacial Maximum). We planned to recover 100% of the sedimentary section in multiple holes to make it possible to establish a cyclostratigraphy and develop an astronomically tuned timescale. We aimed to fully document events across the critical intervals as well as short climate fluctuations during the early Eocene Climatic Optimum. Site 1265 is located in water ~350 m deeper than that at Site 1263, the shallow anchor of the Leg 208 depth transect, and thus will provide constraints on intermediate water chemistry.

Site 1265 is located in an area where a continuous and expanded sequence of upper Paleocene and Eocene calcareous nannofossil ooze could be cored using the advanced piston corer (APC), and we aimed to avoid slumps. It is situated near a small bathymetric high that is underlain by thick sequences of Neogene and Paleogene sediment (Fig. F2). The sequences are separated by a prominent reflector, RO/M, which is just below the Oligocene/Miocene (O/M) boundary (Fig. F3). Two distinct reflectors in the lower sequence, RP/E and RK/P , represent the Paleocene/Eocene (P/E) and Cretaceous/Paleogene (K/P) boundaries, respectively. The upper sequence thins considerably in a northeast direction from the high toward the channel, although the thickness of the lower sequence as judged by the distance between RP/E and RK/P remains about the same. As a result, the depth of the P/E boundary, estimated to be at 270 mbsf (velocity = 1.8 m/ms; 300 ms two-way traveltime [TWT] below seafloor) was just within the effective working range of the APC.

Coring in four holes at Site 1265 yielded a 313-m-thick sequence of upper Paleocene through Holocene calcareous sediments with condensed clay-rich intervals in the middle to upper Eocene and upper Miocene (see the "Leg 208 Summary" chapter). The APC system was used to the depth of the P/E boundary (~274 mbsf) in at least two holes (1265A and 1265D). The extended core barrel (XCB) system was deployed when the APC system reached refusal depth, which varied from hole to hole. The upper Paleocene to lower Eocene and upper Eocene to lower Oligocene are expanded and stratigraphically complete. Despite significant reworking, lithologic cycles in the cores spanning the E/O boundary can be correlated with cycles in other cores of the depth transect. The P/E boundary clay layer was recovered fully intact in one hole.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 208IR-106