Soft-Sediment Deformation Structures Related to Slumping in Lower Cretaceous Turbidite in Lingshan Island, Shandong Province

The origins of the soft-sediment deformation structures outcropped in Lingshan Island are diverse, and that the trigger of the deformation structures in the lowermost part of the Dengta section is in dispute. This contribution studied the forming processes, trigger agents and geological significance of the slump and the included structures combining yield observation and stereographic projection methods. The results show that Dengta section mainly consists of turbidite sequence. The deformed horizon is sandwiched between undeformed layers and triggered by earthquake. 4 decollement surfaces occurred internally, dividing the slump into 5 units which were different from each other in shortening and deformation degree. The main type of soft-sediment deformation is fold which display thickened fold hinge in sandstone, and layer thickening and thinning occur occasionally, both are deformations of liquefied sand under the action of driving forces. The folded sand layer and mud layer became isolated from pore-fluid communication with the sea-floor during slumping, in consequence they became temporarily overpressured, allowing the liquefaction of sand to take place. The fold axes are mainly oriented SSW-NNE, and the axial planes mainly dip to SEE (120°), both indicating the flows coming mainly from SEE. This result is consistent with the regional paleocurrent measurements from the sole marks in turbidites. Therefore, the folds in slump that develop in the subaqueous slope system have significance in indicating paleocurrent direction.