Soft-sediment deformation structures related to volcanic earthquakes of the Lower Cretaceous Qingshan Group in Lingshan Island, Shandong Province, East China

Abstract The study on soft-sediment deformation structures (SSDS) of Lingshan Island has been one of the hot topics of sedimentology researches in China in recent years, and SSDS developed in turbidite system in the Laiyang Group are widely known by domestic researchers. However, few studies were conducted on the SSDS in fan delta system in the Qingshan Group, Lingshan Island. This study analyzes the classification and characteristics of SSDS especially their lithofacies association and lithologic characteristics through field outcrops investigation and thin section analysis as well. A conclusion was acquired that the paleoenvironment was a fan delta system with occurrence of several volcanic eruptions, where the water became gradually shallower. The SSDS types in the Qingshan Group includes load and flame structure, ball and pillow structure, water-escape structure, hydroplastic deformation structure, plastic sandstone breccia structure, volcanic drop stone and V-shaped ground fissure mainly caused by volcanic earthquakes of three types: (1) seismic waves, (2) gravity and inertia effect of pyroclastic flows, (3) instant differential air pressure; which is different from slumping and tectonic earthquakes occurred in the Laiyang Group. In addition, with the lithofacies association analysis between pyroclastic flow and SSDS beds, a distribution model of SSDS related to volcanic earthquakes can be established: SSDS types changed gradually with their distance further away from the volcanic activity core. Brittle deformation which was common in the proximal zone disappeared gradually; liquefied and plastic SSDS continued to dominate in the medial zone; and slightly liquefied SSDS were developed in the distal zone. Meanwhile, the scale and size of SSDS is negatively correlated with the distance of SSDS depositional locations from the volcanic vent.