Asymmetrical soft-sediment deformation structures triggered by rapid sedimentation in turbiditic deposits (Late Miocene, Guadix Basin, southern Spain)

Soft-sediment deformation structures in Tortonian turbiditic deposits of the Guadix Basin (southern Spain) have been described. The most common structures are asymmetrical pillow structures and elongated sets of load-casts. The structures are metric in scale and have been interpreted as the result of liquefaction and/or fluidization processes triggered by the rapid sedimentation of single high concentration turbidites. Final morphology of soft-sediment deformation structures is related to two main driving force systems: unstable density gradient and lateral shear stress. The latter is probably induced by the downslope component of the sediment weight. The asymmetry of deformational structures (in horizontal and vertical cross-section) allows a clarification of the relationship between morphology of deformation and direction of lateral shear stress: this relationship seems ambiguous and confused in the literature. The interpretations both of deformation mechanism and trigger agent have been supported with: - field analyses; - calculations on the liquefaction processes induced by rapid sedimentation; - qualitative models in laboratory.