Evolution of seafloor pockmarks along the Northern Orange Basin, Offshore South Africa: Interplay between fluid flow and bottom current activities

Abstract

Pockmarks are pervasive geomorphologic features identified along continental margins resulting from fluid expulsion on the seafloor. However, the understanding of the underlying geological mechanism/control in relation to their evolution, distribution, and morphology is limited, especially along data-starved continental margins such as the Northern Orange Basin. Analysis of a high-quality 3D seismic reflection data reveals at least 50 individual pockmarks, two channel-like depressions and several irregular depressions in water depth ranging between 800 m and 2400 m. Morphologically, the pockmarks are circular, elongated, comet-like and crescentic in shape, with diameters and depths ranging between ∼0.2 - 2.8 km and ∼10 - 130 m, respectively. Preferential alignment of these pockmarks on the seafloor in relation to the axis of underlying turbidite channels, erosional morphologies and mass transport complexes portray a genetic relationship. The slope architecture hints at the possibility of both deep and shallow fluid source driving pockmark formation. Under this scenario, deep thermogenic gas derived from Cretaceous source rocks migrated along fault systems associated with the Late Cretaceous Megaslide complex to the overburden. The fluids are stored/redistributed in contourite and turbidite channels and subsequently focused toward the seafloor under an increased pore pressure regime. Yet, the fluids may be either solely biogenic gas or heterogeneous, incorporating biogenic components and pore-water derived from the channels and dewatering of the contourites. Importantly, the discovery of crescentic and elongated end-member pockmark morphologies indicate post-formation sculpting of the initial pockmark morphologies by bottom currents. The discovery of these deep-water pockmarks opens the possibility that such fluid escape features may be more widespread than currently documented in the Northern Orange Basin. This has implications in understanding of the petroleum system here and their potential role in the South Atlantic marine ecosystems and global climate change in terms of the expulsion of climate forcing gases.