Evolution of flow cells within a mass-transport complex: Insights from the Gorgon Slide, offshore NW Australia

Mass flows evolve longitudinally during emplacement, but can also vary laterally by forming discrete flow cells with different rheological states separated by shear zones. Despite being documented in many field and subsurface studies, the initiation, translation, and cessation of the flow cells remain unclear. We use five, high-quality post-stack time-migrated (PSTM) 3D seismic reflection datasets to investigate the evolution of flow cells in a seabed mass transport complex (MTC), the Gorgon Slide, on the Exmouth Plateau, offshore NW Australia. The slide originated from a 30 km-wide, NE-SW trending headwall scarp that dips steeply (c. 30o) seaward, and was translated to the NW over a basal-shear surface that deepens downslope (up to 500 m below seafloor). The slide is dominated by chaotic seismic facies with discrete packages of coherent reflectors, which is interpreted as a debrite that carried megaclasts (c. 0.05-1 km-long) derived from the headwall domain. The morphology and orientation of the basal-shear surface focused the pathway of the slide, resulting in clustering of megaclasts in proximal parts of the translation domain. The megaclasts cluster became an obstacle to flow, which resulted in the formation of two flow cells (Cells A and B) separated by a longitudinal shear zone. The interaction between the two cells is recorded by sinuous shear bands within, and ridges on the top surface of, the slide. Along the longitudinal shear zone, the shear bands and ridges of Cell A were dragged downslope, due to Cell A impeding the movement of Cell B. This interaction suggests that Cell B travelled faster, and/or further, than Cell A, due to the absence of any flow obstacles. The abrupt cessation of Cell A is recorded by positive seabed relief, whose amplitude decreases updip. The transport processes of the Gorgon Slide show how entrainment and abrasion of megaclasts induced velocity perturbations during emplacement causing: (i) changes to the flow rheology, and (ii) initiation and cessation of flow cells. A better understanding of how flow cells evolve during MTCs transport may help to refine modelling of the potential impact of MTCs on submarine structures (e.g. pipelines, cables, etc.).