Soft-sediment deformation structures in the Wangshi Group of the southern Tan-Lu Fault Zone, eastern China: implications for the Late Cretaceous tectonic activities and seismic events

A variety of soft-sediment deformation structures (SSDSs) were identified in the Late Cretaceous Wangshi Group, which is situated in the South Malingshan Hill in the southern segment of the Tan-Lu Fault Zone (TLFZ), eastern China. The SSDSs mainly formed in the piedmont-diluvial, alluvial, and lacustrine stratum in the Gaoshan-Tancheng graben basin bounded by the normal fault systems within the TLFZ. In this study, the morphological features and formation mechanisms of SSDSs have been investigated, and potential linkage between the formation of SSDSs and regional tectonic activities has been inferred. The results showed that the types of SSDSs observed in this area consist of the liquefied sand veins, liquefied sand dykes, liquefied breccias, diapirs, convolute laminations, soft boudinage structures, load and flame structures, ball-and-pillow structures, seismic folds, seismic slumps, syn-sedimentary faults, seismic fissure fillings and flowing structures of thixotropic soft clay. The analysis of SSDSs reveals that their deformation mechanisms can be attributed to the liquefaction, fluidization, thixotropy, plastic and brittle deformations within sandy, silty, and clay alternating stratigraphic systems, driven by gravitational instabilities due to reversed density gradients, uneven loading, slope-related gravitational forces, and shear and lateral stresses. Considering the characteristics of SSDSs and geological setting, seismic events and fault activities are proposed as the most plausible triggers for SSDSs, indicating that at least 19 seismic events of magnitude M ≥ 5 have occurred in the Late Cretaceous period. The study also provided significant insight into the declining trend in tectonic and seismic activities during the Late Cretaceous period in this region.