Soft-sediment deformation structures induced by rapid sedimentation in Early Cretaceous turbidites, Lingshan Island, eastern China

Abstract

Turbidites of the Early Cretaceous Lingshandao Formation on Lingshan Island, eastern China, show numerous soft-sediment deformation structures. The turbidites accumulated in a rift basin plain on the distal part of a low-energy fan, together with autochthonous fine-grained sediments that settled from suspension. Occasionally, sets of both types of sediments slumped down the basin slope. The turbidites were studied in two sections. Paleocurrent directions, measured from sole marks at the base of the sandy turbidites, indicate a slope from SE to NW; this is consistent with the dip direction of the axial planes of the major slump folds. Among the numerous types of soft-sediment deformation structures that are present, two types are of special interest because of their specific genesis, viz. rip-up clasts and dome structures. It appears that rapid sedimentation must be considered as the main trigger behind these deformations: the rapid accumulation of particularly the sandy turbidites resulted in overloading of the underlying beds and then consequently in an increase in their pore-water pressure that induced liquefaction and fluidization, which triggered the formation of the rip-up clasts and dome structures. The downslope movement of freshly deposited turbidites after slope failure over the inclined sedimentary surface induced shear stresses, which resulted in the complication of the rip-up clasts and the preferred orientation of some dome structures. Both the dip directions of the axial planes of some of the dome structures and microfolds within the rip-up clasts are consistent with the paleocurrent direction indicated by the sole marks and the slump folds.