Learning From Crushing Failures: Changes in Physical Properties Reveal the Mechanics of Submarine Carbonate Landslides

Calcareous ooze represents the most widespread seafloor sediment, and its elevated water content and high compressibility can lead to the formation of weak layers, thereby increasing slope instability and submarine landslide risk. However, understanding of submarine landslide processes and evolution is hindered by the lack of knowledge of changes in the physical properties of sediments during failure. Here, we address this by documenting failed and unfailed physical properties of a weak layer by integrating two Ocean Drilling Program cores with three-dimensional seismic reflection data from the Exmouth Plateau, offshore northwest Australia. The basal shear zone of two > 40-km-long submarine carbonate landslides was mapped into adjacent undisturbed sediments, where it is characterised by elevated water content and porosity, and recognised as a weak layer. The weak layer coincides with a foraminifera-rich succession capped by a clay-enriched nannofossil-rich zone. Our results suggest that the formation of the weak layer is related to enhanced clay fraction transport into the deep-water basin, due to onshore weathering, forming a low-permeability zone above the water-rich foraminifera-abundant succession. This elevated-clay fraction leads to ‘excess’ water being preserved within chambered foraminifera shells, thus causing a weak layer. When the weak layer acts as the basal shear zone it shows, post-landsliding, markedly lower water content, porosity and foraminifera content. Contractive shearing accounts for these changes in physical properties (dramatic loss in water content, porosity and foraminifera) during failure, through compacting the weak layer, crushing foraminifera and squeezing water out. Water could not escape to the seafloor, causing elevated pore pressure above the weak layer, and mechanically generating a water-rich layer, and/or a water film that lubricated the base of the overriding landslide. Thus, the physical properties of calcareous ooze, including foraminifera percentage and weak layer distribution, need to be considered in future studies to improve submarine slope stability assessments.