Through the study of Miocene Monterey shale, American geologists found obvious progressive fracture characteristics and proposed the concept of seismites (Seilacher, 1969). The fractures are induced by seismic action caused by mudstone that has not undergone compaction in a hydrostatic basin (Seilacher, 1969). Since Seilacher (1969) proposed the concept of seismites, many studies have been devoted to this topic, and significant progress has ensued: (1) The identification markers and vertical sequence of seismic rocks has been proposed (Cita et al., 1984; Mutti et al., 1984; Spalleta et al., 1984; Seilacher et al., 1984). (2) The genetic mechanism of sediment liquefaction deformation caused by seismic activities has been analyzed (Roep et al., 1992; Owen, 1996; Frank et al., 1998; Rodriguez-Pascua et al., 2000; Rossetti et al., 2000; Takahama et al., 2000; Kullberg et al., 2001; Knaust et al., 2002; Montenat et al., 2007; Fortuin et al., 2008; Van Loon, 2009; Ettensohn et al., 2011; Owen and Moretti, 2011; Qiao et al., 2011; He et al., 2014; Tian et al., 2014). (3) Clear, seismites exist in various rock types, such as claystone, coarse sandstone to fine sandstone, grainstone and shale, and they have also been reported in evaporite (Jones and Omoto, 2000; Rossetti and Goes, 2000; Bachmann and Aref, 2005; Mazumder et al., 2006). (4) The age of seismite formation is from meso Neoproterozoic to modern (Mazumder et al., 2006). (5) The development environment of seismites includes lacustrine facies, fluvial facies, deep-sea basin, inland basin, beach, foreshore, transition zone, continental shelf, and pelagic sedimentation (Moretti et al., 1999; Rossetti et al., 2000; Jewell et al., 2004; Bachmann and Aref, 2005; Gerhard et al., 2005; Rodríguez-López et al., 2007; Montenat et al., 2007; Qiao et al., 2008; Van Loon, 2009; John et al., 2011; Owen and Moretti, 2011; Waldron and Gagnon, 2011; He et al., 2014; Törő and Pratt, 2016). (6) Recently, the research on the structural characteristics, sequence, development mechanism, and scientific significance of SSDSs has strengthened (Montenat et al., 2007; Qiao et al., 2008; Van Loon, 2009; Ettensohn et al., 2011; Owen and Moretti, 2011; Qiao et al., 2011; He et al.,; 2014; Tian et al., 2014; Van et al., 2014; Bryant et al., 2016; Liu et al., 2016; Ko et al., 2017; Owen, 2017; Verma et al., 2017; Törő and Pratt, 2016; Liang, 2019; Zeng et al., 2019; He et al., 2021; Hou et al., 2020; Meng et al., 2021).
The definition of seismites is extended from the deformation structure caused by seismic events to the deformation structure caused by seismic or seismic-induced tsunamis, turbidity currents and other events (Cita and Lucchi, 1984). These deformation structures mainly include (1) deformation structures in the sedimentary layer that are directly formed by seismic vibration (such as annular layers and irregular convolute stratification), (2) the overall transportation of sediments due to geological events caused by earthquakes (such as turbidity current events), and (3) sediment homogenization induced by seismic events. Seismites are sediments with SSDSs caused by seismic events or other events induced by earthquakes, which are typical representatives of sedimentary rocks generated by structural events (Seilacher, 1969; Cita and Lucchi, 1984).
Seismites are symbolic products of paleoearthquakes and paleoenvironmental impacts. They not only provide a reasonable dynamic explanation for the mechanism of tectonic evolution but also provide a scientific basis for restoring the strong activity of basin boundary faults on a smaller time scale (Montenat et al., 2007; Ettensohn et al., 2011; Bryant et al., 2016; Rossetti et al., 2017).
The Ordos Basin evolved into an inland depression basin in the Mesozoic (Zhang et al., 2017; Fu et al., 2018; Wang et al., 2021). A large amount of seismites were preserved in the Chang-7 Member of the Yanchang Formation in the Late Triassic, which is an excellent place to study the sedimentary response of paleoseismic events in the depression lake basin (Xia et al., 2007; Li et al., 2008; Du et al., 2014; Tian et al, 2015; Zhang et al., 2017; Fu et al., 2018; Wang et al., 2021).
Previous researchers have systematically studied seismites in the Triassic Yanchang Formation in the Ordos Basin (Xia et al., 2007; Li et al., 2008; Du et al., 2014; Tian et al, 2015; Li et al., 2021). Tian et al. (2012, 2014) expounded the superposition relationship between seismite sand and other genetic sand. Although the sedimentology of the Yanchang Formation in the Ordos Basin has been studied extensively, only a few have recorded the presence of seismites (Xia et al., 2007; Li et al., 2008; Du et al., 2014; Tian et al., 2015). The study of the formation mechanism, distribution range, and seismic grade of seismites in the Chang-7 Member remains weak.
The study of seismites in the Chang-7 Member is helpful to restore the tectonic activity history of the basin, and has important scientific value for the Triassic Ordos Basin tectonic evolution and basin mountain coupling process (Yang et al., 2016; Liang et al., 2019; Chen et al., 2020). We studied the deformation structure using host lacustrine deposits. This study allowed us to (1) determine the distribution range of seismites in the study area; (2) deduce the most likely triggering mechanism; and ultimately (3) interpret the seismic grade at that time. Our study shows that understanding the deformation features of the Chang-7 Member and the physical conditions under which these features formed can enhance interpretations of the evolution of the Ordos Basin and enable the refinement of the paleoenvironmental setting and the paleotectonic history of the Ordos Basin.