Three-dimensional seismic reflection data are used to investigate the geometry, scale, and distribution of structures within large clasts (megaclasts) contained within a Tertiary mass transport deposit, Santos Basin, offshore Brazil. Normal faults and folds are observed within the megaclasts; the folds are typically best developed toward either the frontal or lateral margins of the clasts. The highly variable map-view orientation of these structures, their relative ages, and their relationship to the geometry of the basal shear surface indicate that the structures developed during both the motion and arrest of the parent flow. This study indicates that megaclasts may be deformed despite the associated flow being cohesive and lacking turbulence. Deformation is related to local differential shear within the viscous body of the flow and mechanical interaction of megaclasts with the basal shear surface.