Sedimentary Responses of Late Triassic Soft Sedimentary Deformation to Paleoearthquake Events in the Southwest of North China Plate

doi:10.21203/rs.3.rs-1565558/v1

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Sedimentary Responses of Late Triassic Soft Sedimentary Deformation to Paleoearthquake Events in the Southwest of North China Plate

https://doi.org/10.21203/rs.3.rs-1565558/v1

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The tectonic events caused by paleoearthquakes have a good response in sedimentary filling. Outcrops and cores from the Chang-7 Member of the Late Triassic Yanchang Formation, Ordos Basin in Northern China, yield a wide variety of soft sedimentary deformation structures (SSDSs), many of which are laterally extensive for more than 150 km. They include various types of folds, soft sediment liquefaction deformation (liquefied sandstone dyke, liquefied breccia), gravity-driven deformation (load structures, ball-and-pillow structures), hydraplastic deformation (loop bedding, convolute deformation) and brittle deformation (intrastratal faults and stair-step microfaults, microcracks). In most cases, deformation is represented by hybrid brittle-ductile structures exhibiting lateral variation in deformation style. These occur in delta front to semideep to deep lake sands and mudstones (shales). The seismites recognized in outcrops and cores indicate earthquakes with magnitudes between 6 and 8, which are interpreted as a response to splicing collision orogenic events of the South China Block (SCB) and North China Block (NCB) during the Late Triassic period. Systematic study of the spatial and temporal distribution of these seismites improves the understanding of the tectonic context and evolutionary history of sedimentary basements. This study can provide a new perspective on the evolution of tectonic activities in the basin.

Seismic events

Trigger mechanism

Seismites

Chang-7 Member

Ordos Basin

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.

The formation and evolution of the Ordos Basin are controlled by the interaction of three dynamic systems: The Paleo-Asian Ocean, Tethys-Paleo-Pacific Ocean, and Indian-Pacific Ocean (Sun et al., 2006). In the middle Late Triassic, the NCB was dominated by compressional tectonic deformation (Zhang et al., 1995; Sun et al., 2006; Sun et al., 2019). Due to the collision and orogeny of the Qinling orogenic belt, the southern part of the basin formed the edge of the northward thrust nappe fault (Zhang et al., 1995; Sun et al., 2006; Sun et al., 2019). At the same time, affected by the southwest Tethys tectonic domain, a series of strike-slip thrust faults were derived (Zhang et al., 1995; Sun et al., 2006; Sun et al., 2019). The tectonic intensity diminishes from south to north and is spatially heterogeneous, which indirectly controlled the Triassic sedimentary pattern and sequence filling characteristics in the Ordos Basin (Zhang et al., 1995; Zhao et al., 2006; Dong et al., 2015; Sun et al., 2019). Since the Middle Triassic, the Ordos Basin has entered the development period of inland differential subsidence basins (Zhang et al., 2017; Chen et al., 2020). The Indosinian movement opened the continental depression lake basin, which is also the most important structural development stage of the Ordos Basin. Under this structural background, the most developed oil generation system of the Mesozoic was formed (Zhang et al., 2017; Chen et al., 2020).

The Ordos Basin is located in the western part of the NCB. It was a multicycle superimposed craton basin (Fig. 1a) (Zhang et al., 2017; Fu et al., 2018; Wang et al., 2021). The basin is characteristically large, with a gentle slope, shallow water, and multiple provenances (Zhang et al., 2017; Fu et al., 2018; Wang et al., 2021). The Yanchang Formation can be divided into ten subunits based on sedimentary cycles, Chang-10 to Chang-1, which together record a complete cycle of lake development: initial formation and development stage (Chang-10 to Chang-8), peak stage (Chang-7 to Chang-4 + 5) and decline stage (Chang-3 to Chang-1) (Fu et al., 2018). The study area is located in the southwestern Ordos Basin (Fig. 1b-1c). The Chang-7 Member can be subdivided into 3 submembers, named Chang-7₃, Chang-7₂, and Chang-7₁, from bottom to top (Zhang et al., 2017; Fu et al., 2018; Wang et al., 2021).

Figure 1 near here

The 3 outcrops and the cored intervals of 97 exploration wells through the Chang-7 Member of the Late Triassic Yanchang Formation in the Ordos Basin have been analyzed and measured in detail. These intervals were obtained from the PetroChina Changqing Oilfield Company of CNPC China Ltd. Soft-sediment deformation structures only appeared in 64 of the wells. The depth, thicknesses, lithological characteristics, and sedimentary structures were described and photographed, and the SSDSs were described in detail. All the data provide significant information to analyze the sedimentary characteristics of SSDSs and help deduce seismic and tectonic activities.

Outcrops and cores from the Chang-7 Member of the Late Triassic Yanchang Formation in the Ordos Basin yield a wide variety of seismic-related SSDSs. These deformation structural types can be divided into four categories according to their genetic properties: (1) soft sediment liquefaction deformation, (2) gravity driving deformation, (3) hydraplastic deformation, and (4) brittle deformation (Table 1).

Table 1

Genetic types and characteristics of seismites in the Chang-7 Member (Qiao et al., 2007, 2008, 2009; He and Qiao, 2015)
Genetic type	Deformation structure type	Characteristic
Soft sediment liquefaction deformation	Liquefied sandstone dyke	The sand layer in the soft sediment is liquefied and flows to the vein of the adjacent soft sediment layer, and its scale ranges from millimeter level to meter level.
Soft sediment liquefaction deformation	Liquefied breccia	The liquefied sand layer pierces the adjacent argillaceous layer or soft sedimentary layer to brecciate the argillaceous layer or soft sedimentary layer.
Gravity-driven deformation	Load structure	Soft sediments lose balance under the action of earthquakes, move and deform vertically, driven by gravity, and coarse-grained sediments form and sink.
Gravity-driven deformation	Ball-and-pillow	The load body separates from the parent rock to form sand balls and sand pillows.
Hydraplastic deformation	Loop bedding	Under the action of seismic shear stress, hydraplastic sediments are formed by tensile stress.
Hydraplastic deformation	Convolute deformation	Under the action of seismic shear stress, the soft sedimentary layer is formed by plastic sliding.
Brittle deformation	Intrastratal fault and stair-step microfaults	Under the compression force caused by the earthquake, the dislocation caused by the earthquake in the consolidated rock and the sedimentary layer at the top of the stratum that has not been fully consolidated.
Brittle deformation	Microcracks	A series of vertical cracks with irregular fracture surfaces are produced by earthquakes in the consolidated stratum, mainly in the form of tensile cracks.

Table 1 near here

4.1 Soft sediment liquefaction deformation

The pores of saturated soft sediments are filled with water. When subjected to strong earthquake vibration, especially shear force, the originally stable pore water pressure will rise. The rising pressure is the excess pore water pressure, resulting in the liquefaction flow of soft sediments (He and Qiao, 2015; G. Shanmugam, 2016; Verma et al., 2017).

The conditions for liquefaction of soft sediments are described as follows (He and Qiao, 2015): (1) The average diameter of liquefiable particles is between 0.05 and 1 mm, (2) the content of clay particles is less than 10%, (3) the inhomogeneous particle coefficient is less than 10, (4) the sand layers have high porosity, (5) the maximum liquefaction depth is 20 m from the land surface, and the maximum groundwater depth for liquefication is approximately 5 m, and finally, (6) an earthquake magnitude of 5 or greater can induce sediment liquefaction.

Soft sediment liquefaction deformation in the Chang-7 Member is mainly liquefied sandstone dyke and liquefied breccia.

(1) Liquefied sandstone dyke

Seismic action changes the original stable state of sediments in strata, weakens the stress between sediment particles, transfers the stress between sandstone frameworks, and forms excess pore water pressure. The uneven distribution of overpressure leads to rock fracture. Liquefied sandstone moves in the direction of rock fracture or less pressure under the action of stratigraphic pressure, forming liquefied sandstone dyke (Qiao et al., 2007, 2009; Berra and Felletti, 2011; He and Qiao, 2015). Several liquefied sandstone dyke were found in the cores of the Chang-7 Member, including Well N33, Well Z233, Well B442, and Well C96. The observed sandstone dyke exceed 15 cm in vertical extent and are up to 2 cm wide (Fig. 1d, Fig. 2a-2d).

(2) Liquefied breccia

Liquefied breccia is a common seismic deposit. The reason for brecciation is that the liquefied sand layer pierces the adjacent argillaceous layer or other soft sedimentary layers to brecciate the argillaceous layer or the soft sedimentary layer (Guiraud and Plaziat, 1993; Berra and Felletti, 2011; He and Qiao, 2015). Brecciate the argillaceous layer or the soft sedimentary layer indicate that the argillaceous layer or the soft sedimentary layer were overpressured to fluidization during the deformation and damaged by harder silty and sandy units. The formation of these breccias was not associated with slope collapse, fault activity or erosion. The breccia is argillaceous in situ without transportation, and the cement of liquefied breccia is a sand body with liquefaction. They can independently occur as semiplastic mud breccias in situ or appear in association with other ductile deformation structures, such as load structures and liquefied sandstone dyke (Fig. 1d, Fig. 2e-2f).

4.2 Gravity-driven deformation

The soft sediments lose balance under the action of earthquakes; then, they move and deform vertically under the action of gravity and form a load structure. When the load cast continues to sink and breakaway from the mother rock layers, ball-and-pillow structures form (Qiao et al., 2007, 2009; He and Qiao, 2015). Gravity can cause much soft sediment deformation. In the cores of the Chang-7 Member, the seismite types caused by gravity are mainly load structures and ball-and-pillow structures.

(1) Load structure

The static pressure of the unconsolidated sediment layer is destroyed due to shaking or gravity differentiation, and the higher-density sediments sink into the underlying lower-density sediments to form a load structure (Qiao et al., 2007; He and Qiao, 2015). Due to the abnormal compression of the load structure, the overlying sandstone layer is subjected to tensile stress, and the argillaceous sediment penetrates the sandstone layer upward to form a flame structure (Fig. 1d, Fig. 2g-2i). The flame structure is skewed under the action of horizontal shear force, and the direction of the flame points to the downstream direction of the slope (Fig. 2i) (Qiao et al., 2007; Berra and Felletti, 2011).

(2) Ball-and-pillow structure

The ball-and-pillow structure produced by seismic events is different from the ball-and-pillow structure produced by allochthonous differentiation (Qiao et al., 2007, 2008; Berra and Felletti, 2011; He and Qiao, 2015). Under the joint action of density difference, gravity and seismic vibration, the liquefaction and fluidization of argillaceous sediments in the underlying layer lead to the loss of support of overlying sandy sediments to form a ball pillow structure, which is a seismic liquefaction deformation structure induced by strong seismic shear force (Berra and Felletti, 2011; He and Qiao, 2015). The diameters of the ball-and-pillow structures of seismites are between a few millimeters and a few centimeters. Under the influence of earthquakes, sediments show various irregular deformation forms, such as drag, elongation, fragmentation, and tumors (Berra and Felletti, 2011).

The Chang-7 Member ball-and-pillow structure has a thickness of approximately 2 to 6 cm and a width of approximately 4.3 to 8.6 cm (Note: the load structure with a width greater than 1 cm is of seismic origin (Moretti et al., 2007) (Fig. 1d, Fig. 2j-2l).

Figure 2 near here

4.3 Hydraplastic deformation

Hydroplastic deformation occurs in water-rich soft sediments. The seismic shear stress increases the pore water fluid pressure and weakens the support strength between soft sediment particles, but it has not yet reached the degree of complete liquefaction and flow, and continuous deformation without fracture occurs under the action of regional stress (Berra and Felletti, 2011; He and Qiao, 2015). Hydroplastic deformation mainly occurs in situ. The conditions causing hydroplastic deformation are (1) deformation in surrounding soft sediments dragged by liquefaction and thixotropy, (2) sustained and instantaneous stress action, and (3) low-angle slope (Qiao et al., 2007, 2009; Berra and Felletti, 2011; He and Qiao, 2015). The hydroplastic deformation types of the Chang-7 Member are mainly loop bedding and convolute deformation.

(1) Loop bedding

Loop bedding is a ductile deformation structure, which presents as loops or chains (Rodríguez-Pascua et al., 2000). It is the unconsolidated fine-grained thin interbedded sandy argillaceous sediment formed by the tensile stress induced by weak earthquake. These structures consist of bundles of laminae that exhibit local constriction, forming a flattened elliptic geometry 5–50 cm wide and 0.5–30 cm tall. It is a seismic deformation structure formed in a hydrostatic semideep to deep lake environment (Qiao et al., 2007, 2009; Xia and Tian, 2007). Widely distributed loop bedding is found in the upper part of the Zhangjiatan shale in the Chang-7 Member of the Yanhe section (Fig. 1d, Fig. 3a-3c), which shows that the Chang-7 Member in the northeastern part of the lake basin was less affected by earthquakes.

(2) Convolute deformation

When sedimentary layers are not consolidated, convolute deformations form under compression stress and display plastic-flow folds. Convolute deformation occurs in siltstone or silty mudstone, which are composed of laminars or thin soft sediment deformation layers interbedded with undeformed layers. The shape of convolute deformation is generally curved, curled, undulating laminae, or recumbent folds (Qiao et al., 2007, 2009; Berra and Felletti, 2011). Such deformations are widely observed in the Chang-7 Member core and outcrop in the Ordos Basin. Liquefaction convolute deformation (Fig. 1d, Fig. 3d) and wavy lamina (Fig. 1d, Fig. 3e) are found in the Yanhe section, and recumbent folds are found in the core (Fig. 1d, Fig. 3f).

4.4 Brittle deformation

Brittle deformation results in faults and joints that are triggered by seismic activity, and it occurs in semiconsolidated or soft sediments at the top of the sedimentary sequence (Qiao et al., 2007, 2009; Owen and Moretti, 2011). Intrastratal faults, stair-step microfaults and microcracks are typical types of brittle deformation in the Chang-7 Member. Brittle deformations represent earthquakes with magnitudes greater than 6 (Seilacher, 1969).

(1) Intrastratal fault and stair-step microfault

Intrastratal fault and stair-step microfault indicate brittle failure of stiff, perhaps even indurated sediments as a result of elevated stresses (Seilacher, 1969; He and Qiao,2015). Intrastratal faults and stair-step microfaults are formed in the process of sedimentary stratigraphic vibration, mainly in the form of tensional fractures, which can be developed individually or arranged in parallel as stair-step faults, which are limited to intrastratigraphic development, without cutting through the upper and lower rock layers (Seilacher, 1969; Qiao et al., 2007, 2009; He and Qiao,2015). A single fault is often shown as an intrastratal fault, the fracture distance is generally 2–10 mm, and the dip angle is low. Stair-step microfaults are a series of near-parallel faults, the fracture distance is generally 2–5 mm, the dip angle is low, the upper plate is a descending positive fault, and the profile is parallel to the stepped arrangement (Seilacher, 1969; Qiao et al., 2007, 2009; Berra and Felletti, 2011; He and Qiao,2015).

Intrastratal faults and stair-step microfaults were found in the core and field outcrop of the Chang-7 Member. The fault distance of intrastratal faults is generally greater than 1 cm (Seilacher, 1969) (Fig. 1d, Fig. 3g). Stair-step microfaults are distributed in sand-mud interlayers, and the fault distance is generally less than 1 cm. Many stair-step microfaults are arranged in parallel. The length of each microfault is approximately 5 mm; they are dense and steeply dipping, and it is a normal fault (Seilacher, 1969) (Fig. 1d, Fig. 3h-3i).

(2) Microcracks

Microcracks are typical sedimentary structures associated with palaeoseismic events. They are perpendicular to layers, with varied widths from a few to tens of millimeters. Microfractures are formed by the tensile stress and liquefaction of sediments during earthquakes. The microcracks are "V" shaped, which can indicate the top and bottom of the stratum (Qiao et al., 2007, 2009; Berra and Felletti, 2011). Microcracks represent earthquakes with magnitudes greater than 7 (Qiao et al., 2007, 2009; Berra and Felletti, 2011; He and Qiao, 2015). Microcracks related to seismic events are found in sandstone and mudstone of the Chang-7 Member in the Yaoqu section, Well X288, and Well N33 (Fig. 1d,Fig. 3j-3l).

Figure 3 near here

5.1 Earthquake triggering mechanism

Ancient earthquakes occurred in the tectonic background of collisional orogeny, and the strong episodic activity of basins controlling boundary faults was the direct cause of earthquakes (Zhang et al, 1995; Zhao et al, 2006; Chen et al., 2020; Shi et al., 2020). Indosinian movement is one of the most important tectonic movements in South China. During this period, the ancient Pacific Ocean basin began to subduct to the Asian continent (Zhang et al, 1995; Zhao et al, 2006; Chen et al., 2020). During the Indosinian period, the PaleoTethys Ocean was subducted and closed, and the SCB and NCB collided and orogened along the Mian-Lue suture zone, connecting the South China Craton with the North China Craton (Zhang et al,1995; Zhao et al, 2006; Chen et al., 2020). In the middle and Late Triassic, the South China and North China plates were combined, the tectonic deformation and magmatic activity in the Qinling collision orogenic belt were strong, and earthquakes occurred frequently; thus, the Ordos Basin adjacent to the area north of the orogenic belt formed a steeply sloped terrain and semideep to deep lake area on the southwest edge; the records of deformation structure and sedimentation were simultaneously retained (Sun et al, 2019; Chen et al., 2020; Shi et al., 2020). In this process, the earthquake caused by stress adjustment release formed a large number of seismites (Chen et al., 2020).

The formation of seismites is the response of the basin mountain coupling process. The intensity and cycle of the Qinling collisional orogeny not only indirectly controlled the sedimentary pattern and sequence filling characteristics in this period, but also played a special role in the formation of seismites in the basin (Sun et al, 2019; Chen et al., 2020). The tuff at the bottom of the Chang-7 Member is evidence of volcanic activity, and volcanic activity will lead to earthquakes. The zircon age of tuff at the bottom of the Chang-7 Member is 239.3~243±1.3 Ma, belonging to the Middle Triassic Latin stage, and the tectonic activity corresponds to episode I of the Indosinian movement (Sun et al., 2006; Liang,2019). The orogeny and volcanic activity of Qinling Indosinian episode I are the main triggering mechanisms of seismic events in the Chang-7 Member (Figure 4).

Figure 4 near here

5.2 Relationship between the distribution of seismites and seismic intensity

The type of seismites is related to the seismic intensity. Some scholars have proposed using sedimentary structures to judge seismic intensity (Rodrguez-Pascua et al., 2000). Microcracks and stair-step microfaults occur in superstrong seismic events greater than M8. A ball-and-pillow structure is generated by medium earthquakes of Ms 6-Ms 7. Seismic events of Ms 5-Ms 6 will form liquefied convolute deformation and liquefied sandstone dyke in the sedimentary layer, while small seismic events that are less than Ms 5 will form loop bedding and convolute deformation (Rodrguez-Pascua et al., 2000).

Kuribayashi and Tatsuoka (1975) found that there is a clear corresponding relationship between the maximum epicenter distance (R) of liquefaction deformation and seismic grade (M), and the R-M relationship diagram is created. According to the R-M diagram, the maximum epicentral distance of liquefaction deformation is 50 km, and the corresponding earthquake magnitude grade is Ms 7. At 200 km, the earthquake magnitude is Ms 8.

The maximum epicentral distance of liquefaction deformation in the Chang-7 Member of the Ordos Basin exceeds 100 km, so the maximum earthquake level exceeds Ms 7 (Figure 5 and Figure 6).

Figure 5 near here

According to the core distribution of seismites in the Chang-7 Member, different types of seismites developed in semideep to deep lake areas due to different seismic intensities. Loop bedding, disturbed varved lamination, and convolute deformation were developed in areas with weak seismic intensity. Load structures and liquefaction convolute deformation were developed in areas with strong seismic intensity. In the semideep to deep lake area, liquefied sandstone dyke, pillow structures and stair-step microfaults were mainly developed (Table 2). These deformation structural types of seismites match the sequence of seismic intensity maps proposed by Rodrguez-Pascua et al (2000).

There is a close relationship between the magnitude of the earthquake and the extent of the liquefied scope. The higher the earthquake magnitude is, the larger the liquefied scope is. According to the relevant research of Rodriguez-Pascua et al. (2000), when the earthquake magnitude is Ms 8, the liquefied scope can reach 200 km. The seismite distribution characteristics were analyzed by observations of the Chang-7 Member cores and outcrops (Table 2) (Xia and Tian, 2007; Li et al., 2008; Yang et al., 2016; Liang et al., 2019; Gao et al., 2020).

The deposits in which seismites are found in the study area were formed in the environments of a delta front and semideep to deep lake facies. Seismites are most developed in the semideep to deep lake environment, and they are sporadically observed in the delta front environment (Table 2). Seismites induced by earthquakes are not influvial or delta plains because of late-stage erosion (Table 2). Seismites are most developed in the semideep to deep lake environments, and they are sporadically seen in delta front environments (Table 2). Seismites are mainly distributed in the southwestern area of the basin, and there are few seismites in the northeastern area. Therefore, we can infer the development range of seismites in the Chang-7 Member (Figure 6). According to the relationship between the characteristics of seismites and seismic intensity (Kuribayashi and Tatsuoka, 1975), the influence range of seismic intensity is the strongest in the southwestern area and gradually weakens in the northeastern area of the basin. The earthquake occurred in the southwestern part of the basin.

Table 2

Well of seismites developed in the Chang-7 Member (Liang, 2019)

Well	stratum	Depth(m)	R(km)	sedimentary environment	Deformation structure type	data sources
B246	Chang-7₃	2238.6	＞175	Semideep to deep lake	Liquefied sandstone dyke	Li et al., 2008
H60	Chang-7₂	2782.5	＞183	Delta front	Liquefied sandstone dyke
H62	Chang-7₃	2594.31	＞188	Semideep to deep lake	Stair-step microfaults
L51	Chang-7₁	2267.96	＞173	Semideep to deep lake	Microcracks
Li52	Chang-7₂	2294.03	＞137	Semideep to deep lake	Stair-step microfaults
Li52	Chang-7₂	2294.13	＞137	Semideep to deep lake	Convolute deformation
Li52	Chang-7₂	2294.26	＞137	Semideep to deep lake	Convolute deformation
L52	Chang-7₃	2309.1	＞165	Semideep to deep lake	Microcracks
Z20	Chang-7₁	1728.3	＞64	Semideep to deep lake	Intrastratal fault
Y140	Chang-7₁	2211.4	＞107	Semideep lake	Load structure and ball-and-pillow structure
B246	Chang-7₃	2224.2	＞187	Semideep to deep lake	Liquefied sandstone dyke
X44	Chang-7₁	1973.5	＞175	Semideep to deep lake	Liquefied sandstone dyke
Z74	Chang-7₂	2523.5	＞80	Semideep to deep lake	Load structure and ball-and-pillow structure	Xia and Tian, 2007
G9	Chang-7	1767.2	＞125	Semideep to deep lake	Intrastratal fault
Z1	Chang-7	1256.2	＞76	Semideep to deep lake	Liquefied sandstone dyke
Z8	Chang-7₁	1223.5	＞39	Semideep to deep lake	Liquefied breccia
T17	Chang-7₁	1421.8	＞40	Semideep to deep lake	Ball-and-pillow structure
Z331	Chang-7₁	2155.2	＞116	Semideep to deep lake	Stair-step microfaults	Du et al., 2014
M56	Chang-7₂	2284.6	＞98	Semideep to deep lake	Stair-step microfaults
N105	Chang-7₃	1528.3	＞157	Semideep to deep lake	Liquefied sandstone dyke
N33	Chang-7₃	1724.1	＞62	Semideep to deep lake	Stair-step microfaults	Qiu et al., 2013
N39	Chang-7₃	1667.3	＞68	Semideep to deep lake	Stair-step microfaults
X27	Chang-7₂	2044.4	＞92	Semideep to deep lake	Loop bedding
N33	Chang-7₁	1624.1	＞116	Semideep to deep lake	Liquefied sandstone dyke
Z233	Chang-7₃	1797.1	＞68	Semideep to deep lake	Liquefied sandstone dyke
N148	Chang-7₁	1635.8	＞83	Semideep to deep lake	Liquefied sandstone dyke
C96	Chang-7₃	2068.75	＞74	Semideep to deep lake	Liquefied sandstone dyke
C96	Chang-7₃	2080.6	＞147	Semideep to deep lake	Liquefied sandstone dyke
C96	Chang-7₃	2079	＞147	Semideep to deep lake	Liquefied sandstone dyke
C96	Chang-7₃	2067	＞147	Semideep to deep lake	Convolute deformation
L190	Chang-7₁	2246.6	＞147	Semideep to deep lake	Liquefied breccia
C98	Chang-7₁	2036.6	＞156	Semideep to deep lake	Liquefied breccia
B442	Chang-7₁	2146.5	＞144	Semideep to deep lake	Liquefied breccia
N142	Chang-7₃	1711.9	＞169	Semideep to deep lake	Liquefied breccia
X291	Chang-7₂	2010.2	＞78	Semideep to deep lake	Flame structure
N148	Chang-7₂	1719.12	＞124	Semideep to deep lake	Load structure
L301	Chang-7₁	2355.1	＞74	Semideep to deep lake	Ball-and-pillow structure
L258	Chang-7₁	2346.6	＞168	Semideep to deep lake	Convolute deformation
L338	Chang-7₂	2330.9	＞160	Semideep to deep lake	Microcracks
X288	Chang-7₁	2085.9	＞155	Semideep to deep lake	Microcracks
Y1	Chang-7₁	2000.35	＞131	Semideep to deep lake	Stair-step microfaults
Z20	Chang-7₂	1728.3	＞120	Semideep to deep lake	Stair-step microfaults

(R= vertical distance from the well to the bottom of red rectangle study area in Fig. 1)

Table 2 near here

Figure 6 near here

The sedimentary response characteristics of seismites can be divided into soft sediment liquefaction deformation, gravity driving deformation, hydraplastic deformation, and brittle deformation. Soft sediment liquefaction deformation includes liquefied sandstone dyke and liquefied breccia. The characteristics of seismites caused by gravity mainly include load structures and sand ball-and-pillow structures. The hydroplastic deformation type mainly includes loop bedding and convolute deformation. The brittle deformation types are mainly intrastratal faults, stair-step microfaults, and microcracks.

The Indosinian episode I-Qinling regional orogeny is the trigger mechanism of earthquake events. The Chang-7 Member seismites in the Ordos Basin are mainly developed in the semideep to deep lake area. The earthquakes in the southwestern part of the basin are the most active, with the highest earthquake intensity of more than Ms 7, and its influence range gradually weakens from southwest to northeast.

Acknowledgments

Authors thank the Editors and anonymous referees for their helpful comments. This study was supported by the Ministry of Science and Technology (China) (No. 2016ZX0504605-001). Thanks to the approval of core samples from the Xifeng core library of PetroChina Changqing Oilfield Company.

Declaration of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All data generated or analyzed during this study are included in this published article [and its supplementary information files].

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Sedimentary Responses of Late Triassic Soft Sedimentary Deformation to Paleoearthquake Events in the Southwest of North China Plate

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Abstract

Figures

1. Introduction

2. Geological Setting

3. Material And Methods

4. Types And Genesis Of Seismites

4.1 Soft sediment liquefaction deformation

(1) Liquefied sandstone dyke

(2) Liquefied breccia

4.2 Gravity-driven deformation

(1) Load structure

(2) Ball-and-pillow structure

4.3 Hydraplastic deformation

(1) Loop bedding

(2) Convolute deformation

4.4 Brittle deformation

(1) Intrastratal fault and stair-step microfault

(2) Microcracks

5. Discussion

5.1 Earthquake triggering mechanism

5.2 Relationship between the distribution of seismites and seismic intensity

6. Conclusions

Declarations

References

Competing Interests

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