Seismically induced soft-sediment deformation in alluvial fans: mechanisms and implications for geological evolution of the Ordos Basin (China)

Well-exposed soft-sediment deformation structures (SSDS) outcrops are rare and often mistaken for tectonic folds, leading to limited research and underutilisation in sedimentary geology. The Taitongshan section of the Middle-Permian Shihezi Formation in the Ordos Basin, China, provides insights into SSDS, whose spatial distribution and sedimentation reveal information about paleoenvironments, tectonic movements, and seismic events crucial for understanding basin development. This study uses fieldwork and sedimentology to investigate SSDS formation and triggers. Fieldwork identified six siliciclastic facies with SSDS and associated syndepositional structures. Sedimentological interpretations highlighted the connection between seismic activity and the genesis of SSDS, reinforcing their value as proxies for tectonic events in basin evolution studies. SSDS formation is triggered by seismic shaking, initiating liquefaction, erosion, slurry-clump formation, hydroplastic deformation, mass consolidation, and lithification. Rayleigh waves-induced liquefaction in partially-lithified subsurface sediments forms water escape structures. Provenance analysis reveals seismically active, orogenic source areas southwest and southeast of the NCC, with significant uplift and erosion during the Middle-Permian. Key findings indicate that the Middle-Permian braided river facies of the Taitongshan section originated from the Qilian Orogenic Belt, followed by a subsequent provenance shift towards the eastern-North Qinling Orogeny (NQLO), highlighting the dynamic tectonic forces driving rapid provenance shifts, further shaping sediment deposition and advancing basin evolution. Furthermore, the southeastern NCC experienced a more rapid uplift than the southwestern part during that time. These facies were deposited on the distal fan region of a prograding alluvial fan, where seismites likely formed due to active seismicity in the NQLO. The study identifies inter-facies shifts during seismite formation, introducing an innovative concept beyond traditional in-situ genesis models and broadening the understanding of sedimentary dynamics. This critical role of SSDS in sedimentation architecture enhances the understanding of basin evolution in sedimentary geology and provides a framework for interpreting similar structures globally.