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 th e Guadix Basin (southern Spain) have been described. The most common struct ures are asymmetrical pillow structures and elongated sets of load-casts. T he structures are metric in scale and have been interpreted as the result o f liquefaction and/or fluidization processes triggered by the rapid sedimen tation 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 str ess. The latter is probably induced by the downslope component of the sedim ent weight. The asymmetry of deformational structures (in horizontal and ve rtical cross-section) allows a clarification of the relationship between mo rphology of deformation and direction of lateral shear stress: this relatio nship seems ambiguous and confused in the literature. The interpretations b oth of deformation mechanism and trigger agent have been supported with: - field analyses; - calculations on the liquefaction processes induced by rap id sedimentation; - qualitative models in laboratory.