Origin of soft-sediment deformation structures in Nihewan Basin

The Nihewan Basin is a rift basin at the junction of northern Shanxi Province and northwestern Hebei Province in north China. The basin is known for its rich paleontological fossils and ancient human remains. There are also abundant soft-sediment deformation structures (SSDS) in the thick lacustrine sediments. Previously, most SSDS have been interpreted as ice-edge features or ignored entirely. Recently, the authors have carried out several field surveys in the Nihewan Basin and found that many SSDS are sandwiched between normal lacustrine strata at multiple sections. In the excavation pit at the 10th Locality of Maliang Site (ML10), 10 horizontal SSDS layers and two vertically developed geological features have been identified. Based on genesis analysis and related criteria, these features are divided into two categories: cryoturbation-triggered SSDS and earthquake-triggered SSDS. Among them, a special type of ancient ice-wedge pseudomorph (SSDS-8) of ML10 is recognized in the basin for the first time. The other 9 horizontal SSDS are mainly caused by earthquake-triggered liquefaction and slumping. They can be further divided into 14 seismic event layers. These findings indicate that the tectonic activity in the Nihewan Basin is very strong and frequent, and there were cold periods in the geological history of the basin. At the same time, the SSDS with distinct morphological characteristics and stable horizontal distribution in the basin can be used as an important indicator of stratigraphic correlation.