Imaging of Cenozoic climatic events and bottom water activities at the northeastern flank of Walvis Ridge: A correlation of seismic data with borehole data measurements

Abstract

The history of sedimentation and current activities at the northeastern flank of Walvis Ridge is investigated by a correlation of multichannel seismic data collected during R/V Meteor Cruise M49/1 in early 2001 with borehole measurements of Ocean Drilling Project (ODP) Leg 208. During Meteor Cruise M 49/1 multi-frequency seismic data were collected using two different seismic sources, in a quasi simultaneous mode in order to get best possible penetration and resolution of the sub-surface structures.Walvis Ridge divides the southeastern Atlantic Ocean into two basins, the Angola Basin to the north and the Cape Basin to the south. It is well known as an impassable barrier for bottom waters which separates Antarctic Bottom Waters (AABW) widely distributed in the Cape Basin from North Atlantic Depth Waters (NADW) in the Angola Basin. The sediments of the northeastern flank of Walvis Ridge are characterized by mostly undisturbed sequences of Cenozoic sediments, with varying thickness of ~150 m at the edge of the Angola Basin and ~450 m at the axis of the ridge.A correlation of seismic data with borehole data was accomplished by calculating synthetic seismograms using GRA density measurements of the cores and an averaged velocity model. This 'simple' event modeling allows to assign ages to the seismic reflectors and to identify key horizons/boundaries like the E/O and the K/T boundaries as well as the ELMO event and deposits of the PETM, representing Cenozoic climatic events. The seismic pattern of these target horizons shows similar properties at the different sites indicating their regional distribution.A special static correction in addition to standard seismic processing steps resulted in high quality high resolution deep sea Watergun images. A comparison between the seismic images and the magnetic core measurements of Leg 208 proofs that Watergun data allows imaging of sub-meter sediment layers like the PETM or the ELMO event. In contrast to the lower resolution GI-Gun data, the critical transitions occur as single horizons, separated from each other by continuous lower amplitude reflector packages on the Watergun data. Several significant seismic reflectors, whose ages were determined using the stratigraphy of Leg 208, can be traced across the entire survey area. The reflectors represent the paleoseafloor of the study area under consideration of the subsidence though neglecting possible tectonic events as they are unknown for this region in the last 60 Ma. Gridding of these horizons leads to a model of the paleoseafloor, which shows that the rate of subsidence of the Angola Basin almost equals the increasing thickness of the sediment column. In addition the model indicates the development of a channel structure across the ridge in the southeast of the study area between 28Ã °S and 30Ã °S, which is possibly connected to an exchange of AABW and NADW between Cape Basin and Angola Basin. Determining the age of this channel by using Leg 208 stratigraphy suggests that current activities have increased since the Middle Miocene. Southwest of the area drilled during ODP-Leg 208, the seismic profiles of Meteor Cruise M 49/1 cross a suggested flow path of AABW between Cape Basin and Angola Basin in a region called Walvis Kom. Walvis Kom is suitable for a bottom water transfer due to its channel like structure. AABW in the Cape Basin might be pushed into the flow path by the Coriolis Force and continues to flow into the Angola Basin. Several toplap structures, contourite-sheeted drifts and sedimentary waves indicate recent bottom water activities in the channel. A combination of bathymetry data and seismic data shows a reduction of the current cross-section along the flow path, which must result in an increasing current velocity explaining less deposition along the path way in direction to the Angola Basin. A simple age model, based on the stratigraphy of Leg 208 at the 28Ã °S, determines the initiation of strong current activities during the Lower Miocene at ~16 Ma. Undisturbed sediment sequences above indicate decreasing current speeds after ~10 Ma during the Upper Miocene. In addition to the assumption that Walvis Ridge builds an impassable barrier for bottom waters with at least two leakages (Walvis Passage and Namib Col), our results suggest two more leakages across the ridge. The initiation of bottom water exchange trough these pathways started during the Miocene. The Miocene is known as an oceanographic dynamic period for the South Atlantic, where for example the Central American Seaway was closed and the recent thermohaline circulation of the South Atlantic developed.