Submarine landsliding and canyon evolution on the northern KwaZulu-Natal continental shelf, South Africa, SW Indian Ocean

Abstract

The morphometric and geomorphological analysis of landslides found within several submarine canyons of varying sizes and morphologies from the northern KwaZulu-Natal continental shelf, South Africa, provides insight into submarine canyon evolution. Six large shelf indenting canyons are recognised (Leven, North and South Leadsman, Diepgat, Wright, and White Sands) interspersed with smaller canyons that occur prominently in the northernmost Mabibi area. Landslides and their surficial attributes were determined from high resolution multibeam bathymetry data. Landslide area, volume, runout length, the slope of the runout, the headscarp slope and height, scar slope, adjacent local unfailed slope gradient, and the depth of each failure are measured and the relationships statistically analysed for each individual submarine canyon. Landslide position and the type of landslide are also documented. The ratio between headscarp height to runout length ( D/L) is assumed as a good proxy for the dynamic rheology of each failure. The largest failures occur in Diepgat, Wright, Leven and Leadsman Canyons. The presence of steeper and larger failure headscarps indicates that the failure rheology was competent in Diepgat and Wright Canyons, whereas failures, particularly in the heads of Leven and Leadsman Canyons are assumed to possess fluidised rheologies based on their small D/L values. A landslide vs. a fluid sapping trigger is postulated for the Diepgat/Wright and Leven/Leadsman landslide sets respectively. The smallest landslides are found in the Mabibi area, and are limited to single retrogressive slumps in the heads of each small canyon. This style of failure is also evident in other smaller canyons that occur between the larger shelf indenting varieties. Headward erosion by small retrogressive failure is assumed to occur, before the canyon thalwegs become oversteepened relative to their walls. Once oversteepened, lateral canyon extension may occur, coupled with increased downslope erosion by sediment flow. The presence of 1) incised inner gorges; 2) hanging or perched slides in the canyon headwalls; 3) terraced slides; and 4) benches in the slide material indicate that axial incision and catastrophic slope clearances during this phase of submarine canyon evolution were prominent. The presence of sea-level notches within the failure scarps indicates that canyons are currently inactive and are unlikely to enlarge further pending relative sea-level fall during the next hypothermal (glacial) period.