The structure and kinematics of substrate entrainment into high concentration sandy turbidites: a field example from the Gorgoglione “flysch” of southern Italy.

Sandy turbidites commonly show evidence for significant dynamic coupling with their substrate. The resulting deformation can be described using structural kinematic methods, linked to palaeoflow indicators, to better understand the links between flow and entrainment processes. A field example from the syn-orogenic Gorgoglione Flysch, a succession of upper Miocene turbidites deposited into a deforming array of thrust-top basins in the southern Apennine thrust belt, Italy, is described. The succession contains metre-scale packages of alternating sandy turbidites and shales but is notable for containing > 100 m thick, massive sandbodies. These are structureless apart from sporadic horizons of aligned mud clasts. Commonly, the substrate beneath the massive sandbodies is deformed, with minor folds and thrusts verging in the direction of palaeoflow determined from tool marks and flutes at the base of these sandbodies. Structural studies from the base of a selected massive sandbody have identified that the substrate mud has been injected upwards, with flames sheared over in the direction of palaeoflow. Thus the substrate has deformed and become entrained during emplacement of the massive sandy body. At some locations, the substrate can be traced into the overlying deposit, with substrate clay beds becoming boudinaged and entrained into the sandbody. Analysis of the orientation of the mud clasts indicates that this bed disruption and incorporation into the sandy massive-bed turbidite was an organized, viscous process. These features indicate the significant shear stress was partitioned out of the flow and onto the substrate. The incorporation and disruption of substrate into the sandbody suggest that post-disruption strains increase upwards – implying that displacement gradients increased into the flow. These behaviours, showing variations in strain partitioning between the flow and its substrate, are explored in terms of evolving flow dynamics and substrate rheology.