Styles of basal interaction beneath mass transport deposits

Erosion of the seafloor is often interpreted to be the result of turbidity currents and reflects their frictional and non-cohesive nature. However, evidence of the interaction between sediment gravity-flows and the substrate forming the sea floor has been increasingly reported in the literature. Based on styles of basal interaction with the substrate, we here propose a broad classification of submarine mass movements labelled free- and no-slip flows. Three mechanisms are proposed for free-slip flows during translation of mass movements that are effectively detached from the substrate; hydroplaning, shear wetting, and substrate liquefaction. In contrast, no-slip flows occur where the mass movement is welded to the substrate, and the strain front lies within the substrate itself. In the latter case, flows can erode by pushing forward and/or ploughing into the substrate, often remobilizing sediments that are later incorporated into the flow, a common characteristic shared by many mass transport deposits (MTDs) containing blocks. Additionally, linear track features (e.g. grooves and striations) are described as a consequence of substrate tooling by rigid blocks. Using outcrops in NW Argentina as a detailed case study, we have recorded evidence for penetration of the strain profile into sediments underlying MTDs and categorised the deformation into no-slip basal deformation that may display continuous and discontinuous profiles. Continuous deformation profiles involve the complete deformation of the uppermost layers of the substrate, while discontinuous deformation profiles preserve a undeformed substrate layer between the MTD and the zone of deformed substrate. These features highlight the erosive and deformational nature of MTDs, and can be used as potential kinematic indicators.