Ocean Drilling Program, United States

Walvis Ridge, located in the eastern South Atlantic Ocean, is one of the few known locations where it is possible to recover undisturbed, complete, and possibly expanded Paleogene sediments over a wide range of water depths. The main objective of Ocean Drilling Program (ODP) Leg 208, realized on outer Walvis Ridge in the summer of 2003, was recovery of intact composite sequences of "critical" transitions like the Cretaceous/Tertiary (K/T) boundary, Eocene/Oligocene (E/O) boundary, Paleocene/Eocene Thermal Maximum (PETM), and the Elmo event. Meteor Cruise M49/1, in early 2001, was the main seismic presite survey for Leg 208. The high-resolution multichannel seismic system of the University of Bremen was used to identify drill sites at outer Walvis Ridge that promised recovery of undisturbed, complete, and possibly expanded Cenozoic sequences. Based on these new seismic data, six sites were drilled at water depths between 2500 and 4755 m on the northeastern flank of Walvis Ridge during Leg 208. To ground truth the seismic record, synthetic seismograms were calculated from closely spaced core logging density measurements of Leg 208 cores. The high quality of both seismic and core logging data allows good correlation between synthetic seismograms and recorded seismic data. These results allow identification and characterization of target reflection horizons like the E/O boundary, Elmo horizon, PETM, and K/T boundary in seismic images and dating of individual reflectors. Total sediment thickness in the survey area varies between 200 and 530 m, and the study area can be separated into three zones that have different deposition conditions. In general, seismic data show a decrease in total sediment thickness with increasing distance from the ridge crest, but lateral variations in sedimentation rates were found across the entire survey area. Especially close to the crest of the ridge, these lateral variations in sedimentation rates might indicate the influence of bottom water currents on sedimentation at the outer Walvis Ridge.