Miocene Seismic Stratigraphy and Deepwater Depositional Systems, Offshore Angola: Interaction of Salt Tectonics and Slope Sedimentation

The interaction of submarine gravity flows with evolving salt-related topography on continental margins has been the focus of many detailed studies, but they are largely based on analysis of single structures or individual channel systems.Styles of interaction are dependent on the nature of the flow (e.g. volume, frequency & duration) vs characteristics of the topography (e.g. geometry, relative orientation & growth history) and include: diversion, confinement, deflection and blocking of gravity flows.We document the interaction of lobe-channel-levee complexes (LCLCs) and salt-related structures at regional-scales (c. 7,500 sq km) from offshore Angola throughout the Miocene epoch in order to provide greater insight into local vs regional controls on deepwater sedimentation in areas subject to salt tectonics.

We use interval isochron variations and cross-sectional lap-out relationships to reconstruct the evolution of salt-related structures (e.g. growth faults, salt walls) throughout the study area. Deepwater depositional systems and component depositional elements have been imaged using combinations of attributes, including spectral decomposition. Synthesis of these two workflows allows us to reconstruct the tectono-stratigraphic evolution of the study area throughout the Miocene. Although the interactions between growth topography and LCLCs are highly variable, three end-member interactions can be identified:

1. Large-scale confinement of LCLCs for > 70 km within laterally continuous, elongate minibasins bounded by salt walls

2. Tortuous corridors associated with partial confinement of flows by salt diapirs, resulting in significant changes in direction of flow >10 km (diversion) and local changes in flow path of LCLCs around fold segments < 5 km (deflection)

3. Ponding of flows in enclosed salt withdrawal minibasins, with aggradation of deposits in the depocentres, and subsequent spill basinward into the next minibasin (fill & spill)

The resultant tectono-sedimentary response of LCLCs is dependent on degree of maturity of salt diapirs and faults relative to timing of deposition. Where structural maturity predates flow deposition, Type 1 interaction is observed, with multiple transitions between Type 2 and Type 3 where structural growth is coeval with sedimentation. Long-term pinning of sediment entry points and fairway location coupled with structural maturity vs timing of deposition results in predictable sediment dispersal patterns during the Miocene.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California