How are giant clasts incorporated into submarine landslides and how do they change flow dynamics?

Abstract:

Giant clasts (>10,000 m³) are common in many submarine landslide deposits. Here, we use submarine landslide deposits exposed in the field (Karoo Basin, South Africa and Neuquén Basin, Argentina) and imaged in high-resolution reflection seismic data (offshore Colombia and offshore Brazil) to investigate how giant clasts were entrained and transported in the parent flows, and how they changed longitudinal flow behaviour. Based on: (i) facies/seismic facies similarities between clast and substrate; (ii) evidence for basal scour at a scale comparable to the clasts; and (iii) the spatial association of scours and clasts, we are able to discriminate between clasts derived from the substrate during flow run-out rather than from headwall degradation. Highly sheared basal surfaces are well-developed in distal locations, sometimes extending beyond the deposit. This points to deformation and weakening of the substrate ahead of the flow, thus providing a potential mechanism to entrain large clasts on low gradients in the absence of basal flow turbulence. Scours and ramps on the basal shear surface, which are associated with concentrations of large clasts, commonly occur where bathymetric changes are abrupt (confinement and/or steepening) or where more easily eroded substrate are encountered (e.g. sand-prone channel-fills). Substrate entrainment bulks the flow volume along during flow evolution, thereby increasing mass and momentum. Our outcrop and subsurface examples point to a complicated interplay between flow evolution, bathymetry, and substrate geology, which combine to control the development of the basal shear surface. Improved understanding of these interrelationships will help to refine the estimates of the volumes and runout distances, and therefore geohazard potential, of submarine landslides.