Paul J. Valle¹, John G. Gjelberg², Susanne Sperrevik¹

(1) University of Bergen, Bergen, Norway
(2) Norsk Hydro ASA, Bergen

ABSTRACT: Impact of Halokinesis on Depositional Systems and Channel Architecture in The Lower Congo Basin, Offshore Angola

In the present study we investigate the interaction between varying seafloor topography and sub-marine sedimentary systems from the Oligocene sequence Lower Congo Basin offshore Angola, West Africa. The basin has been influenced by salt tectonics from the early Cretaceous (Aptian time). Since the late Cretaceous the basin acted as a deep marine sedimentary basin with mass flows as the major depositional process for coarse-grained sediments. Excellent 3D seismic data and cored exploration wells have enabled us to map the spatial distribution of the coarse-grained deposits during the Oligocene. The mapping shows that these sediments are present in stacked channel deposits.

We differentiate between two expressions of salt tectonic influence on the coarse-grained Oligocene deposits:

1. Path of mass flows: By comparing paleo-flow directions obtained from seismic attribute mapping and trends of coeval structures, our study demonstrates that changing seabed topography strongly impacted upon the direction of sediment transport through time. As a consequence the seabed topography led to shut off of coarse-grained sediment supply for major parts of the basin during specific time intervals.

2. Vertical stacking patterns of sub-marine channel systems: Seismic sections indicate highly variable stacking patterns for the channel deposits, such as 1: deposition in deeply incised channels and canyons 2: erosional and aggradational systems where channels juxtapose or erode/truncate each other and 3: systems with sheet-like geometry and non-erosive bases. Our study indicates that there is a connection between evolving salt structures and the "choice" of vertical stacking pattern.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado