DESCRIPTION OF SITES

Six sites were drilled at water depths between 2500 and 4770 m on the northeastern flank of Walvis Ridge during Leg 208 (Fig. F1). The main objective of Leg 208 was to recover intact composite sequences of "critical" transitions in the Paleogene (Shipboard Scientific Party, 2003). This paper concentrates on these transitions, namely the following:

  1. Eocene/Oligocene (E/O) boundary (35.7 Ma): this sharp transition is marked by a step increase in magnetic measurements and a step decrease in color lightness (Shipboard Scientific Party, 2003).
  2. Chron C24n Elmo event (53.2 Ma): this event is characterized by a drop in calcium carbonate content in the sediment (Scientific Shipboard Party, 2003) and is characterized by a density contrast of ~10% in comparison to the surrounding sediments. The layer was reached at five of the six sites and has a thickness of ~30 cm (Shipboard Scientific Party, 2003).
  3. Paleocene/Eocene Thermal Maximum (PETM, 55 Ma): isotope records suggest that at ~55 Ma the deep ocean and high-latitude surface waters warmed by 4°C and 8°C, respectively (e.g., Kaiho et al., 1996). The PETM at Leg 208 sites is a clay layer without carbonate content. The thickness of the layer varies from 50 cm at the deepest site to 80 cm at shallower sites. Like the Elmo horizon, the density of the PETM increases ~10% (Shipboard Scientific Party, 2003).

Additionally, it is possible to identify the Miocene Bolivina Acme event (18 Ma), which represents a foraminiferal assemblage dominated by small, smooth, thin-walled bolivinids (Smart and Murray, 1994), at all sites and the Cretaceous/Tertiary (K/T) boundary (66.4 Ma) at the two deepest sites, Sites 1262 and 1267.

Identification of critical transitions on seismic data was done by correlating the synthetic seismograms. Figure F3 shows data examples for Sites 1262, 1264 1266, and 1267. Because of the high quality of both data sets, the reflection patterns of the synthetic seismograms fit well with the seismic traces. This "event modeling" offers the possibility to assign characteristic layers of the drill sites to seismic reflectors. The depth values of the target horizons described below are calculated from seismic traveltime, including the simplified linear velocity model used for calculation of synthetic seismograms. Scaling the resolution of the top and base of the target horizons (such as the PETM and Elmo) on a meter to submeter scale is not possible with a GI gun (~150 Hz) as a seismic source. For this task a wavelet with a length not smaller than two times the thickness of the layer is needed (Badley, 1985). However, the fit of the reflection pattern, together with the knowledge of the physical properties of each event (Scientific Shipboard Party, 2003), allows identification of seismic reflectors in which each event must be included. Because of changing density values (e.g., the increase of 10% for the Elmo horizon and the PETM), those reflectors are mostly characterized by stronger amplitudes.

Synthetic seismograms for Sites 1263 and 1265 are missing because of spurious data values in the GRA density values, where synthetic seismograms do not fit with the seismic data. Hence, identification of target horizons at both sites is based on correlation between Leg 208 sites by tracing the target horizons along the seismic profiles. Table T1 summarizes depths of the target horizons for each Leg 208 site.

Site 1262

Site 1262 (Fig. F4), located on Profile GeoB01-035 at a water depth of ~4755 m, is the deepest site of the drilling transect, situated at the edge of Angola Basin. The seismic profile is characterized by an undulating topography with a number of sedimentary ridges located on top of basement highs and small sedimentary basins in between. Several debris flows and/or slumps were imaged as transparent zones in the sedimentary basins. Total sediment thickness is between 150 and 220 m (Fig. F4). Site 1262 is located in one of the small sedimentary basins. The upper ~100 ms TWT is characterized by continuous reflectors with high amplitudes. A debris flow or slump is located in this depth interval directly west of the site. The upper ~100 ms TWT represents Neogene and Eocene sediments. The Miocene Bolivina Acme event is found in this interval at ~53 mbsf and can be characterized by a reflector with high amplitudes. At the depth of ~100 ms TWT, the seismic pattern changes with a clear decrease of amplitude at the E/O boundary at a depth of ~78 mbsf. Seismic data below ~100 ms TWT show interlayering of weak and strong reflector packages, including the Elmo horizon and the PETM, down to the basement at ~300 ms TWT. Both clay layers are characterized by strong amplitudes. The Elmo horizon at ~115 mbsf appears as a single distinct continuous reflector, whereas the PETM at ~139 mbsf appears as the first reflector of a package of several reflectors with strong amplitudes. With a drilling depth of 209 mbsf it was possible to recover all critical transitions, including the K/T boundary, which is characterized by a strong amplitude reflection close to the crystalline basement at a depth of ~195 mbsf.

Site 1267

Site 1267, located on Profile GeoB01-030, is situated close to the edge of Angola Basin at a water depth of 4355 m (Fig. F5). The profile runs perpendicular to the ridge axis in a northwest-southeast direction. Sediment thickness in the northwest part of the profile is ~340 m. Site 1267 is located close to a basement high, which results in mostly parallel and undisturbed but inclined reflectors. Upper Maastrichtian sediments, which were the oldest sediments drilled during Leg 208, were reached at a drilling depth of 329 mbsf. The interval 0–150 ms TWT consists of Quaternary and Neogene sediments and includes a package of disturbed reflectors embedded with parallel reflectors with strong amplitudes. The Miocene Bolivina Acme is the first reflector of a package of stronger reflectors at ~111 mbsf. The E/O boundary at ~128 mbsf is a strong continuous reflector. In contrast to Site 1262, underlying reflectors also show high amplitudes. The Elmo horizon at ~179 mbsf is characterized by a continuous distinct reflector with strong amplitudes surrounded by sediments with lower amplitudes. The PETM at ~231 mbsf is embedded within a package of parallel reflectors with average amplitudes. At Site 1267, the K/T boundary was reached at a drilling depth of ~298 mbsf. The reflector representing the K/T boundary is located only ~30 m above the crystalline basement and shows strong amplitudes. Because of the proximity of the K/T reflector to basement with its rough surface, the reflector is not continuous along the profile.

Site 1266

Site 1266, in a water depth of 3798 m, is also situated on Profile GeoB01-30 at the flank of the ridge (Fig. F6). It reaches upper Paleocene sediments at a drill depth of 334 mbsf. Sediments of Profile GeoB01-030 in the vicinity of Site 1266 along the flank of the ridge have a thickness of ~370 m, which is only slightly more than those at Site 1267. The northwest part of the profile is characterized by mostly undisturbed and parallel reflectors, but in the direction of the ridge with decreasing water depth the pattern becomes more and more disturbed. Close to Site 1266 an unconformity occurs that results from a Neogene hiatus. Amplitudes of seismic reflectors at Site 1266 vary with depth, although only a few intervals with low amplitudes exist. The Miocene Bolivina Acme at ~115 mbsf is a continuous reflector with high amplitudes within a larger package of strong reflectors. The E/O boundary at ~207 mbsf is a transition from high amplitudes to a package of reflectors that are slightly disturbed and have lower amplitudes. The boundary is not as sharp as at Site 1262, for example (Fig. F4). The Elmo horizon at ~270 mbsf and the PETM at ~306 mbsf are parts of packages with strong amplitudes. The package including the PETM is the lowermost sediment package that overlies the crystalline basement. The package consists of disturbed high-amplitude reflectors.

Site 1265

Site 1265 is located on Profile GeoB01-048, which is almost parallel to the axis of the ridge in a northeast-southwest direction (Fig. F7). The profile shows increasing sediment thickness up to ~450 m with decreasing water depth. Reflectors are mostly undisturbed and parallel except in a big channel structure in the center of the profile where Quaternary and Neogene sediments thin out. The reflectors of the flank of the channel are thinned out, probably because of nondeposition. Additionally, the disturbed sedimentation pattern of older sediments (e.g., Eocene) in the center of channel likely indicates changing current activities. Expanded Neogene sediments shown in the northeast part of Profile GeoB01-048 are part of a morphologic high in which the shallowest site, Site 1264 (see below), was drilled. The morphologic high is characterized by the thickest sediments sequence at Walvis Ridge (up to 450 m). Site 1265 was drilled at the edge of the channel, where Quaternary and Neogene sediments are thin and older sediments are not disturbed by proximity to the channel. At a water depth of 3060 m and a drilling depth of 321 mbsf, Site 1265 reaches upper Paleocene sediments. The Miocene Bolivina Acme event within the uppermost 100 ms TWT at ~88 mbsf is characterized by a strong continuous reflector that is disturbed or absent in the center of the large channel. The E/O boundary at ~192 mbsf is situated in the interval between ~200 and 300 ms TWT. The boundary is not as sharp at other sites, but still a small change from higher to lower amplitudes beneath is notable. The boundary is not continuous, especially in northwest direction, where sedimentation was possibly disturbed by variable current activities. The Elmo horizon at a depth of ~241 mbsf in the interval of ~300 to 400 ms TWT is not conspicuous because of only a small increase of the reflector amplitude. The PETM at ~315 mbsf is again part of the package with higher amplitudes but is not resolved as one continuous reflector.

Sites 1263 and 1264

Sites 1263 and 1264, located on Profile GeoB01-046 (Fig. F8), were drilled close to the ridge flank where sediment column thickness varies from ~360 m at Site 1263 to ~450 m at Site 1264. Akin to Profile GeoB01-048 (Fig. F7), the profile shows an channel with thinned out reflectors at the flank and an expanded Quaternary and Neogene section. The western flank of the channel is not shown on the profile. The expanded section of Quaternary and Neogene sediments shown in the eastern part of Profile GeoB01-046 are part of the morphologic high, which also appears on Profile GeoB01-048.

Site 1263, at a water depth of 2717 m, was drilled into an apparent bottom water channel where Quaternary and Neogene sediments are absent or thin and thus Paleogene sediments are shallow in depth. The upper ~100 ms TWT represents thinned out Quaternary and Neogene sediment sequences. It includes the Miocene Bolivina Acme at ~44 mbsf, which appears as a continuous reflector with high amplitudes. The parallel to subparallel reflectors below ~100 to ~300 ms TWT show high amplitudes and good continuity. At ~100 mbsf the E/O boundary appears as a sequence of continuous reflectors with high amplitudes. Reflectors below ~300 ms TWT start to undulate with depth. This unit shows varying amplitudes and includes the Elmo event as a strong reflector at ~265 mbsf. The site reaches Paleocene sediments, including the PETM, which is surrounded by reflectors with high amplitudes at ~335 mbsf.

Site 1264 was drilled into an expanded section of Neogene sediments on the morphologic high close to the ridge flank. The location of the site is the crosspoint of Profiles GeoB01-046 (Fig. F8) and GeoB01-031 (Fig. F9). The morphologic high, illustrated on both profiles, is characterized by parallel layered sediment sequences. The sediments are disturbed by a slump or debris flow deposition shown in the south of Profile GeoB01-031 and smaller channels in the younger sediments in the north of the profile.

Site 1264, in a water depth of 2505 m, reaches Oligocene sediments at 282 mbsf. The parallel layered sediments represent a high-resolution data set of Quaternary and Neogene sediments with a pattern of varying strong and weak amplitudes in which the Miocene Bolivina Acme appears at ~159 mbsf as a continuous reflector with strong amplitudes.

NEXT