Evolution of seafloor pockmarks along the Northern Orange Basin, offshore South Africa

Seafloor pockmarks and other erosional morphologies were discovered in deep-waters (800 - 2400 m) offshore Northern South Africa based on the analysis of a high-resolution 3D seismic reflection dataset. These non-random pockmarks have different morphologies with individuals appearing as circular, elongated, and crescentic. The pockmarks have diameters and depths ranging between 0.2 - 2.8 km and 10 - 130 m respectively. Interestingly, the spatial distribution of these seafloor pockmarks appears to be preferentially aligned along the axis of buried turbidite channel systems and other buried erosional structures mapped within the near seafloor succession (up to 250 ms TWT below the seafloor). For this segment of the Orange Basin, fluids driving pockmarks formation are either derived from a shallow source associated with contourites or the turbidite channels (pore water and or biogenic gas). However, a dual fluid source may be operational involving the former and fluids derived from a deeper thermogenic source related to the CenomanianTuronian source rocks. The complex fault systems associated with the Cretaceous Megaslide complex in the study area may serve as conduits for channeling thermogenic gas to the shallower units. These fluids are either sequestered and redistributed in the coarser components of the channels or contourites and later focused to the seafloor for expulsion resulting in the formation of the pockmarks. However, the discovery of crescentic pockmarks and scours in the study area, is evidence for post-formation modification of the initial pockmark morphologies by operational deep-water bottom currents. This new evidence of pockmarks from this segment of the South Atlantic margin has implications for understanding the complex interplay between subsurface fluid plumbing and bottom current activities over geological time scale during basin evolution. Also, It suggests such fluid expulsion features may be more widespread than currently mapped in the Orange Basin. This has implications in understanding their potential role in the South Atlantic marine ecosystems and global climate change in terms of the expulsion of climate forcing gases.