Evacuation and infill of a composite submarine slide scar

Submarine slide scars are ubiquitous across continental margins; they are frequently observed on the modern seafloor and within seismic data, but due to limited spatial extent are rarely identified at outcrop. Detailed... [ view full abstract ]

Submarine slide scars are ubiquitous across continental margins; they are frequently observed on the modern seafloor and within seismic data, but due to limited spatial extent are rarely identified at outcrop. Detailed understanding of the infill of such common features is vital in constraining the effect of their morphology on subsequent sediment routing and trapping patterns across continental slopes. This study documents an exceptionally well exposed succession of mass transport deposits (MTDs) and turbidites that accumulated above a submarine slide scar.
This study focuses on a 3 km long and 150 m high outcrop of the Permian Ecca Group in the Laingsburg depocentre, Karoo Basin, South Africa. Thirteen measured sections through the full outcrop as well as numerous detailed shorter sections were collected and key surfaces were walked out to establish a stratigraphic framework. An incisional basal surface cuts >100 m into regional basin floor deposits. Palaeocurrent analysis and mapping of the infilling stratigraphy suggests that the outcrop is a rare example of infill above a lateral margin of a major slide scar. The infill of the slide scar shows significant stratigraphic and lateral variability. Based on abrupt facies changes, stratal terminations and erosional bounding surfaces 16 genetic units have been identified within the in place deposition and the fill. The MTD infill within the slide scar range in scale and morphology, including small (metre scale) and larger (10s of metres scale) slump deposits and debrites that incorporate megaclasts of underlying basin-floor mudstones that are several 10s of metres high and 100s of metres long. A rugose upper surface to the MTDs is initially onlapped by thick graded turbidites suggesting confinement of turbidity currents. The thickness of graded tops decreases stratigraphically as the erosional relief was healed. Overall, the infill demonstrates a transition from slide scar confined mass flows to unconfined flow as the slope equilibrium was re-established. The entrainment of megaclasts from the underlying basin-floor succession, and the deep erosion, suggests steepening of a southern basin margin during a phase of major seascape degradation and healing.
Overall this outcrop allows the examination of bed-scale interaction of MTDs and subsequent turbidity currents during the infill of a seismic-scale submarine slide scar. Application of the results of this work to subsurface data sets will provide better understanding of transport and depositional process within submarine slide scars.