Upper Slope Canyon Formation documented in 3D Seismic Images along the West Coast of Africa

Abstract

Abstract This paper describes three examples of upper slope environments along the West Coast of Africa. Images of Ivory Coast, Equatorial Guinea, and Namibia are derived from newly acquired 3D seismic data. Several variations in depositional style are seen in these images. Linear erosional canyons dominate the western slope of Ivory Coast. These canyons have maintained their positions through an extended period of geological time. Southern Equatorial Guinea displays both erosional and depositional features. Erosional gullies and confined fan systems are present near the shelf edge slope break. Meandering channel-overbank and unconfined fan systems appear in outboard locations. Northern Namibia"s upper slope is generally smooth, concave, and nearly featureless. A large scale, minimal relief collapse feature breaks this smooth surface. Examination of the variables controlling the upper slope environments in these three areas indicates that similar physical processes result in unique modifications of each area. Introduction During the years 2000-2001 three 3D seismic surveys were acquired in the offshore, upper slope regions of western Ivory Coast, southern Equatorial Guinea, and northern Namibia. These seismic data are excellent quality and extend over 5000 square kilometers previously covered by only sparse 2D surveys. The continuous nature of 3D data, the large aerial extent of the data, and improvements in mapping techniques allow insights into recent submarine geological processes that are generally inaccessible by other methods. The three study areas cover similar present day water depths ranging from 200 to 3000 meters. However, the tectonic settings and sediment accumulations are quite variable. Ivory Coast and Equatorial Guinea are both strongly influenced by transform faults associated with seafloor spreading. Yet, Equatorial Guinea is also influenced by mobile salt. Namibia exhibits more typical passive margin characteristics, but lacks evidence of salt. Tertiary sedimentation in the three areas is also diverse. Comparing and contrasting the interpreted sedimentary character of these areas with their known environmental and tectonic settings results in further understanding and delineation of the processes controlling sedimentation and the resulting stratigraphic sequences in upper slope environments. Study Area Location and Scope The study area includes portions of the offshore slope of three countries in Sub-Saharan Africa. The most northerly study area is in offshore Cote D"Ivoire. At the westernmost offshore boundary of the country lies Block 112, where data includes approximately 1200 square kilometers of 3D data (Figure 1). The nearest well control for calibration is approximately 200 kilometers to the east. These wells have been tied to the 3D data using a previously existing 2D seismic grid. The secondstudy area is located at the southern boundary of offshore Equatorial Guinea and includes 3100 square kilometers of 3D seismic data (Figure 1). Well calibration is much closer in this case, less than 20 kilometers. Northernmost Namibia provides the third and most southerly study area location. The 3D seismic survey in this area extends over 700 square kilometers (Figure 1). Well control includes two wells approximately 200 kilometers to the south and two DSDP cores to the northwest.