Cenozoic bottom current sedimentation in the Cape Basin, South Atlantic

The Cape Basin is located at a central location with respect to variouswater masses. The coldest and densest of these water masses is theLower Circumpolar Deep Water.The location and topography of the Agulhas Ridge effectively blocks itsnorthward flowing branches and deflects them into a south-westerndirection to follow the bathymetric contours of the Agulhas Ridge wheresuspended sediments accumulate in contourite drifts.Several hundred metres of sediments have accumulated since the onsetof Antarctic Bottom Water intrusion into the basins of the South Atlantic.A present day like bottom current following Agulhas Ridge contours existssince an erosional event in the Early Oligocene took place which markeda prominent change in sedimentation.Strong and varying currents in the Early Oligocene formed a sheet likedeposition at high sedimentation rates.The fluctuating types of sediments produce strong seismic reflections.It was followed by a very stable unidirectional bottom current with mainlymuddy sediments. This resulted in the formation of a more than 200 kmlong, approx. 30 km wide elongate contourite drift which shows fewinternal reflections on the seismic data. An increase of well-defined unitsis found after a period of non-deposition in the Middle Miocene.Still the build-up of the drift structure continued, but at places erosion isfound, especially in Quaternary sediments and probably in relation toglaciation cycles

Tying seismic data to geologic information from core data: an example from ODP Leg 177

The integration of seismic data with core data provides ground-truth toa structural interpretation of seismic data.The most important difficulty that arises in an integration effort isthe correct translation between the different scales of the core dataand the seismic data.In the absence of check-shots, detailed knowledge of the velocitystructure at the drilling locations is required, either from downholelogging measurements, velocity analysis of the seismic data, or directmeasurements on core samples.Three of the seven drill-sites during ODP (Ocean Drilling Program) Leg177 in the south-eastern Atlantic were located on the Agulhas Ridge andconnected through eight seismic profiles.Synthetic seismograms created from velocity and density measurements onselected core samples generally show a good agreement with real seismicdata with respect to amplitude and waveform, whereas timing of theevents is troublesome.The use of velocity profiles with inaccurate sections along cores, afalse depth scale due to recovery problems, and inaccuracies in thepositioning during both seismic and coring operations are the mainshortcomings of this method.The main reflectors identified on seismic data correspond to hiatuses orperiods of reduced sedimentation rates, and correlate well with densityvariations.In this way the cored data provide a calibration tool for the overallgeological interpretation of the seismic sections