The main objective of the project is stratigraphy and correlation between seismic and well data, which is based on pattern recognition of reflector sequences in the first place. Identification of the critical transition in the entire survey area is of great importance for this approach. As demonstrated above, we can identify the critical transitions in the seismic data at the locations of the Leg 208 drill sites, demonstrating that our relatively simple event modeling is sufficient for this task. Seismic characteristics for each target horizon at Leg 208 drill sites were studied.
The Miocene Bolivina Acme Event is a continuous reflector with strong amplitudes within Neogene sediments. Depending on the thickness of the Neogene sequences, it is surrounded by sediments with lower amplitudes (Sites 1264 and 1265) or is part of a package with strong amplitudes (Sites 1262, 1266, and 1267).
The E/O boundary in core samples is marked by a step increase in magnetic susceptibility (Shipboard Scientific Party, 2003), which suggests a change of the physical properties of the sediments. In seismic data the E/O boundary is represented by a sharp transition from a package of sediments imaged as reflectors with strong amplitudes to a package of sediments imaged with lower amplitudes at all sites except at Site 1266, where possibly synsedimentary slumping during the Oligocene disturbed the sedimentation pattern.
The Elmo horizon is represented as a strong, continuous, mostly isolated reflector surrounded by sediments with lower amplitudes. The Elmo horizon, which is characterized by a 30- to 50-cm-thick layer with 10% higher density than the surrounding sediments, includes a drop in calcium carbonate content (Shipboard Scientific Party, 2003). It has a strong impedance contrast resulting in a continuous reflector with high amplitudes.
The PETM is also characterized by low carbonate content and 10% higher density. Because of a surrounding sediment with strong amplitudes, the 50- to 80-cm-thick clay layer (Shipboard Scientific Party, 2003) cannot be resolved as an individual reflector with GI gun data. Therefore, the PETM is not imaged as an individual reflector, but sediments at the depth of the PETM are characterized by strong amplitudes at all sites except for Site 1264, which was not drilled to PETM depth.
The K/T boundary at Sites 1262 and 1267 occurs as the first reflector of a reflector package with strong amplitudes and a sharp transition immediately above basement. The basal contact of this clay layer is also sharply defined by an increase the in magnetic susceptibility data.
Most of the sediments at Walvis Ridge are characterized by undisturbed sediment sequences, but based on changing deposition conditions, varying basement structures, and/or influences of bottom water currents, it is possible to separate the study area into three zones (Fig. F1). Zone 1 covers the northwestern part of the study area, that is the beginning of Angola Basin. Basement is partly faulted, as shown on profiles running parallel to the axis of the ridge, which results in a rough basement topography (Fig. F4) with small basins filled with pelagic sediments. These sediments are generally undisturbed except some debris flow or slump structures in the younger sections. The shallow-dipping basement parallel to the flank of the ridge is mostly not faulted. Zone 1 is characterized by the smallest sediment thickness of ~150 to 200 m.
Zone 2 covers the flank of the ridge (Fig. F1). Sediment thickness increases toward the ridge axis. Basement is smoother compared to Zone 1, which probably indicates less tectonic activity. The sedimentation pattern is more disturbed than in Zone 1 because of the flank's slope of 0.7° (Figs. F10, F11). A few channel structures at the flank of the ridge appear in a small part of Zone 2 (Figs. F1, F10, F11).
In Zone 3, sediment thickness reaches 450 m. Adjacent to some smaller channels with a width of 1–2 km (Fig. F9), there are larger channel structures with widths up to some tens of kilometers, indicating a large zone influenced by bottom water currents (Fig. F7). The thickness of Neogene sediments is significantly decreased in this area, whereas older sediment packages show a similar thickness as the surrounding areas. Hence, confined bottom currents occurred mainly in the Neogene. These changing deposition conditions offered the possibility to drill Sites 1263, 1264, and 1265 within a few kilometers with varying thickness of Neogene sediments (Figs. F7, F8, F9). No major faults were identified along the axis of the ridge, suggesting a mostly undisturbed sedimentary sequence.