Polygonal faults-furrows system related to early stages of compaction - upper Miocene to recent sediments of the Lower Congo Basin

A new polygonal fault system has been identified in the Lower Congo Basin. This highly faulted interval (HFI), 700+/-50 m thick, is characterized by small extensional faults displaying a polygonal pattern in plan view. This kind of fracturing is attributed to volumetric contraction of sediments during early stages of compaction at shallow burial depth. 3-D seismic data permitted the visualization of the progressive deformation of furrows during burial, leading to real fractures, visible on seismic sections at about 78 m below seafloor. We propose a new geometrical model for volumetrical contraction of mud-dominated sediments. Compaction starts at the water-sediment interface by horizontal contraction, creating furrows perpendicular to the present day slope. During burial, continued shrinkage evolves to radial contraction, generating hexagonal cells of dewatering at 21 m below seafloor. With increasing contraction, several faults generations are progressively initiated from 78 to 700 m burial depth. Numerous faults of the HFI act as highly permeable pathways for deeper fluids. We point out that pockmarks, which represent the imprint of gas, oil or pore water escape on the seafloor, are consistently located at the triple-junction of three neighbouring hexagonal cells. This is highly relevant for predictive models of the occurrence of seepage structures on the seafloor and for the sealing capacity of sedimentary cover over deeper petroleum reservoirs.