Submarine canyon evolution of the Southwest Cape continental margin.

Abstract

Submarine canyons are diverse geomorphological systems that are characterised by a wide variety of geomorphic and sedimentary processes. The complexities that arise during their evolution reveal changes in the tectonic and eustatic setting that actively sculpt continental margins. Newly acquired high-resolution bathymetry reveals 15 submarine canyon systems, most of which were previously undocumented, and a large fluid seep/pockmark field off the Cape continental margin of the west coast of South Africa. These are hosted in the Orange Basin, South Africa’s largest gas producing basin. High resolution 2D seismic reflection and borehole data were used to establish a general seismic stratigraphy in which eight units are defined (Seismic Facies 1 – 8). Five key unconformity bounding surfaces are delineated (surfaces A – E) and related to major fluctuations in sea-level. Surface A marks the Albian sedimentation in the basin, B defines the Turonian – Conacian boundary and is imprinted by the first palaeo-canyons of the area, C characterises the Maastrichtian – Danian boundary and correlating with another episode of canyon formation, D marks the Palaeocene – Eocene surface, above which an assumed Oligocene canyon system was formed, and E defines the Mid-Miocene unconformity correlating to a pulse of uplift of the hinterland and further canyon incision. The modern-day canyons observed from multibeam bathymetry are suspected to have initiated in the Pliocene. The contemporary canyon morphologies vary, with many canyon features yet to be described in the literature. These morphologies are broadly classed into linear, sinuous, hooked and shelf-indenting types. Pockmarks are situated in close proximity to the sinuous, hooked and shelf-indenting canyon types and were quantified using hydrological extraction techniques to a total of 2219. These pockmarks represent the terminus of stratigraphic fluid migration from an Aptian gas reservoir, evidenced in the form of blowout pipes and brightened reflectors. Various pockmark morphologies are exhibited including circular, elongate, crescentic, composite and stringed-types. This pockmark morphological diversity is explained through localised bottom current controls which modify a point-sourced circular pockmark to establish the more complicated morphologies. The morphometric analyses of the canyons suggest contemporaneous down- and upslope eroding paradigms, that later were dominated by the influences of vertical fluid flows and gas seepage. It is proposed that fluid flow plays a key role in establishing the morphological variability of canyons

along the Cape continental margin. Vertical fluid migration within the study area has the potential to mobilise sediments, evidenced by the occurrences of blowout pipes, pockmarks and neighbouring mass wasting deposits. The youngest (or most immature) canyons are considered to be the linear-types, produced by the amalgamation of intra-slope rills and with a notable absence of fluid flow features. Succeeding these are the sinuous canyons, their sinuous form dictated by the spatially irregular control of fluid flow on the sea-floor stability. The hooked canyons are defined by their arcuate heads and dense pockmark associations, suggesting further fluid flow interaction around the canyon head, producing erosion patterns associated with neither up, nor downslope mass wasting. A single shelf indenting/breaching canyon is observed. This is considered the most mature canyon system. The meandering mid-components of this canyon formed by fluid-interactions, however slumping may have been of sufficient magnitude to have extended beyond the pockmark fields, the canyon head thus gaining access to sediment flows from the upper slope. The head then subsequently retrogressed beyond the shelf-break to its present position. This thesis provides the first opportunity for a glimpse into seafloor fluid venting and escape features from the South African margin and how they affect canyon morphologies.